Superelectrophilic tetrakis(carbonyl)palladium(II)- and -platinum(II) undecafluorodiantimonate(V), [Pd(CO)4] [Sb2F11]2 and [Pt(CO)4][Sb2F11]2: Syntheses, physical and spectroscopic properties, their crystal, molecular, and extended structures, and density functional calculations

588

J. Am. Chem. Soc. 2001, 123, 588-602

Superelectrophilic Tetrakis(carbonyl)palladium(II)- and -platinum(II)

Undecafluorodiantimonate(V), [Pd(CO)₄][Sb₂F₁₁]₂and

[Pt(CO)₄][Sb₂F₁₁]₂: Syntheses, Physical and Spectroscopic Properties,

Their Crystal, Molecular, and Extended Structures, and Density

Functional Calculations: An Experimental, Computational, and

Comparative Study

Helge Willner,*^,†Matthias Bodenbinder,^‡Raimund Bro1chler,^‡Germaine Hwang,^‡

Steven J. Rettig,^‡James Trotter,^‡Britta von Ahsen,^†Ulrich Westphal,^†Volker Jonas,^§

Walter Thiel,^§and Friedhelm Aubke*^,‡

Contribution from the Fachbereich 6, Anorganische Chemie, Gerhard-Mercator-UniVersita¨t GH Duisburg,

Lotharstrasse 1, D-47048 Duisburg, Germany, Department of Chemistry, 2036 Main Mall, The UniVersity

of British Columbia, VancouVer, British Columbia, V6T 1Z1, Canada, and Max-Planck-Institut fu¨r

Kohlenforschung, Kaiser-Wilhelm Platz 1, D-45470 Mu¨lheim an der Ruhr, Germany

ReceiVed June 29, 2000

Abstract: The salts [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, are prepared by reductive carbonylation of Pd[Pd(SO₃F)₆],

Pt(SO₃F)₄or PtF₆in liquid SbF₅, or HF-SbF₅. The resulting moisture-sensitive, colorless solids are thermally

stable up to 140 °C (M ) Pd) or 200 °C (M ) Pt). Their thermal decompositions are studied by differential

scanning calorimetry (DSC). Single crystals of both salts are suitable for an X-ray diffraction study at 180 K.

Both isostructural salts crystallize in the monoclinic space group P2₁/c (No. 14). The unit cell volume of

[Pt(CO)₄][Sb₂F₁₁]₂is smaller than that of [Pd(CO)₄][Sb₂F₁₁]₂by about 0.4%. The cations [M(CO)₄]²⁺, M )

Pd, Pt, are square planar with only very slight angular and out-of-plane deviations from D_4hsymmetry. The

interatomic distances and bond angles for both cations are essentially identical. The [Sb₂F₁₁]^-anions in [M(CO)₄]-

[Sb₂F₁₁]_2,M ) Pd, Pt, are not symmetry-related, and both pairs differ in their Sb-F-Sb bridge angles and

their dihedral angles. There are in each salt four to five secondary interionic C- -F contacts per CO group. Of

these, two contacts per CO group are significantly shorter than the sum of the van der Waals radii by 0.58 -

0.37 Å. In addition, structural, and spectroscopic details of recently synthesized [Rh(CO)₄][Al₂Cl₇] are reported.

The cations [Rh(CO)₄]⁺and [M(CO)₄]²⁺, M ) Pd, Pt, are characterized by IR and Raman spectroscopy. Of

the 16 vibrational modes (13 observable, 3 inactive) 10 (Pd, Pt) or 9 (Rh), respectively, are found experimentally.

The vibrational assignments are supported by DFT calculations, which provide in addition to band positions

also intensities of IR bands and Raman signals as well as internal force constants for the cations. ¹³C NMR

measurements complete the characterization of the square planar metal carbonyl cations. The extensive

characterization of [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, reported here, allows a comparison to linear and octahedral

[M(CO)_n][Sb₂F₁₁]₂salts [M ) Hg (n ) 2); Fe, Ru, Os (n ) 6)] and their derivatives, which permit a deeper

understanding of M-CO bonding in the solid state for superelectrophilic cations with [Sb₂F₁₁]^-or [SbF₆]^-as

anions.

Introduction

1831¹is considered to be the first organo transition metal

compound. Three platinum(II) carbonyl chlorides, among them

cis-Pt(CO)₂Cl₂, discovered by P. Schu¨tzenberger in 1868,^2,3are

the first transition metal carbonyl complexes. Their isolation

predates the synthesis of the first homoleptic metal carbonyl,

Square planar coordination compounds of platinum group

metal ions (groups 9 and 10) with a d⁸electron configuration

(Rh(I), Ir(I), Pd(II), and Pt(II)) containing π-acceptor ligands

are of both historical relevancy and considerable practical

importance today.

Two important milestones in the development of transition

metal chemistry involve platinum(II) complexes. Zeise’s salt,

K[PtCl₃(π-C₂H₄)]‚2H₂O, synthesized in 1827 and reported in

4

Ni(CO)₄by 22 years. The nature of the bonding in these

complexes and their molecular structures however were recog-

nized much later; for example, the molecular structure of cis-

Pt(CO)₂Cl₂was reported only a few years ago.⁵

(1) Zeise, W. C. Annu. ReV. Phys. Chem. 1831, 21, 497.

(2) Schu¨tzenberger, P. Bull. Soc. Chim. Fr. 1868, 10, 188.

(3) Schu¨tzenberger, P. C. R. Hebd. Seances Acad. Sci. 1870, 10, 134.

(4) Mond, L.; Langer, C.; Quincke, F. J. Chem. Soc. 1890, 749.

(5) Bagnoli, F.; Belli Dell’Amico, D.; Calderazzo, F.; Englert, U.;

Marchetti, F.; Herberich, G. E.; Pasqualetti, N.; Ramello, S. J. Chem. Soc.,

Dalton Trans. 1996, 4317.

* Corresponding authors. H. Willner: Telephone: 49-203-379-3309.

Fax: 49-203-379-2231. E-mail: willner@uni-duisburg.de. F. Aubke: Tele-

phone: 604-822-3817. Fax: 604-822-2847. E-mail: aubke@chem.ubc.ca.

^†Gerhard-Mercator-Universita¨t GH Duisburg.

^‡The University of British Columbia.

^§Max-Planck-Institut fu¨r Kohlenforschung.

Published on Web 01/04/2001

[Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂

J. Am. Chem. Soc., Vol. 123, No. 4, 2001 589

There are a number of applications for square planar

complexes. In homogeneous catalysis of various carbonyl-

ation-, hydrogenation-, formylation-, and hydroformylation

processes, a number of square planar d⁸complexes are used as

catalysts, often on an industrial scale. Two prominent examples

are RhCl(tpp)₃, “Wilkinson’s catalyst”,⁶and trans-Ir(CO)Cl-

(tpp)₂, “Vaska’s compound”,⁷tpp ) P(C₆H₅)₃. In solid-state

chemistry and material sciences, stacked square planar com-

atm). Initial claims of the generation of [M(CO)₆]²⁺cations, M

) Fe, Os,³²by the same method have been retracted.³⁴In view

of the failure of these and other synthetic attempts³⁵and guided

by the prevailing view on metal-CO bonding, the Dewar-

Chatt-Duncanson concept^36-38with a strong emphasis on the

π-back-bonding contributions to the M-CO bond,³⁹metal

carbonyl cations with metals in oxidation states of +2 and +3

have been viewed for a long time as incapable of existence.⁴⁰

An alternate synthetic approach, the CO addition to metal

salts of weakly coordinating anions,⁴¹has so far not generated

any polyvalent homoleptic carbonyl cations.⁴²It is also noted,

that CO addition to form such molecular CO-adducts is

frequently reversible at ambient conditions.^43-45

A new synthetic methodology, the use of various super-

acids^46,47as reaction media in carbonylation and solvolysis

reactions, provides a viable pathway to a wide range of

homoleptic metal carbonyl cations,^44,45,48among them many

superelectrophilic cations,⁴⁹with ionic charges of +2 and +3.

Superacids, which are extensively used to generate and stabilize

a wide range of carbocations,⁵⁰have found in the past only

occasional use in transition metal chemistry.^15,47The superacids

employed by us are the Brønsted superacids anhydrous HF⁵¹

and HSO₃F,^52-55the strongest protonic acids known,⁴⁶the Lewis

superacid SbF₅,^56,57the strongest molecular Lewis acid,^58,59and

the conjugate Brønsted-Lewis superacids, which have the

highest Brønsted acidities,^46,47HF-SbF₅^46,47,60,61and HSO₃F-

SbF₅^46,47,62known as “magic acid”.⁴⁶Isolable, thermally stable

salts are frequently formed with the superacid anion [Sb₂F₁₁]^-,⁶³

which forms by self-assembly during reactions in SbF₅, HF-

SbF₅, and surprisingly also in HSO₃F-SbF₅.^46,62

plexes formed by tetracyanoplatinate(II) anions, [Pt(CN)₄]^2-

,

with various cations^8,9or neutral halo tris(carbonyl)iridium(I)

compounds, Ir(CO)₃X, X ) Cl, Br,^8,10,11are, after oxidative

doping, one-dimensional electrical conductors.^8,9,12,13

Even though cis-Pt(CO)₂Cl₂is known since 1868, its square

planar coordination geometry⁵is limited in metal carbonyl

chemistry^14-17largely to halo carbonyl derivatives of various

metal ions from the 4d and 5d series (Rh(I), Ir(I), Pd(II),

Pt(II))^14-21with a d⁸configuration and to coordination com-

pounds, which have a limited number of CO ligands. Of the

halo carbonyl complexes, those of palladium and platinum have

been widely studied,^18-24mainly by Calderazzo and his

group.^22-24For homoleptic typical carbonyls^14-17and highly

reduced carbonylates,²⁵the square planar coordination geometry

is unknown. All known tetracarbonyl species are invariably

tetrahedral,^14-17,25including Pd(CO)₄and Pt(CO)₄, which exist

in inert gas matrices only.²⁶

The first reported homoleptic carbonyl cations^27,28are of the

[M(CO)₆]⁺type, M ) Mn, Tc, Re, with [M′X₄]^-as counter-

anions, M′ ) Al, Fe; X ) Cl, Br, reported by E. O. Fischer et

al.^29,30and W. Hieber et al.^31-33in the early 1960s. They form

by halide abstractions from M(CO)₅X, M ) Mn, Tc, Re; X )

Cl, Br, with Lewis acids such as AlX₃and FeX₃, X ) Cl, Br,

at high temperatures (∼90 °C) and high CO pressures (∼300

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77, 1869.

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590 J. Am. Chem. Soc., Vol. 123, No. 4, 2001

Willner et al.

All of these, with the exception of [Ir(CO)₆]^{3+ 78}

are structur-

Following the generation of linear [Au(CO)₂]⁺_(solv)in HSO₃F

solution⁶⁴by reductive carbonylation of Au(SO₃F)₃^65,66in 1990

and the subsequent conversion of the intermediate Au(CO)-

SO₃F⁶⁴into the thermally stable salt [Au(CO)₂][Sb₂F₁₁] by

solvolytic carbonylation in SbF₅,⁶⁷the same synthetic approach

has been applied to the reductive carbonylation of Pd[Pd-

(SO₃F)₆]^68,69and Pt(SO₃F)₄,⁷⁰respectively. In complete an-

alogy to the gold(I) carbonyl system, the solvated cations

[M(CO)₄]²⁺_(solv), M ) Pd, Pt, form initially in HSO₃F, but the

fluorosulfates cis-M(CO)₂(SO₃F)₂, M ) Pd, Pt, are eventually

isolated.⁷¹The molecular structures of cis-M(CO)₂(SO₃F)₂, M

) Pd,⁷²Pt,⁷³are known. The [Pt(CO)₄]²⁺cation is isolated from

HSO₃F in the initial stages of the reductive carbonylation of

Pt(SO₃F)₄as [Pt(CO)₄][Pt(SO₃F)₆].⁷⁴The self-ionization ion

[Pt(SO₃F)₆]^2-of the diprotic conjugate superacid HSO₃F-

Pt(SO₃F)₄,⁷⁰is structurally characterized as cesium salt.⁶³In

liquid SbF₅in a CO atmosphere both cis-M(CO)₂(SO₃F)₂, M

) Pd, Pt, are easily converted to [M(CO)₄][Sb₂F₁₁]₂, M ) Pd,

Pt. The syntheses and the vibrational spectra in the CO-

stretching region are reported in a preliminary communication.⁷⁵

The square planar coordination geometry of the cations is easily

deduced from their vibrational spectra.

,

ally characterized by single-crystal X-ray diffraction with

[Sb₂F₁₁]^-as counteranion.^{77,78,80,82-85}In all instances significant

interionic C- -F contacts are observed, resulting in extended

molecular structures. The salts [M(CO)₆][Sb₂F₁₁]₂, M ) Fe, Ru,

Os, are readily converted to their related [SbF₆]^--salts, whose

molecular structures are obtained as well.^80,82

The cation [Pt(CO)₄]²⁺is found with three different coun-

teranions, [Sb₂F₁₁]^-,⁷⁵[Pt(SO₃F)₆]^2-74and [PtF₆]^{2- 86}

while

,

71,72

[Pd(CO)₄][Sb₂F₁₁]₂⁷⁵and the precursor cis-Pd(CO)₂(SO₃F)₂

appear to be the only true cationic polycarbonyl derivatives of

palladium(II),²⁰with the possible exception of thermally unstable

Pd(CO)₂R₂, R ) C₆F₅, C₆Cl₅.⁸⁷An early claim of the synthesis

88

of cis-Pd(CO)₂Cl₂has been repudiated,⁸⁹and the reported

crystal structure and spectroscopic properties of “[Pd(µ-Cl)-

(CO)₂]₂”⁹⁰are evidently identical in all respects to those of well-

known [Rh(µ-Cl)(CO)₂]₂.^91,92

The objectives of this study are: (i) to summarize and to

evaluate a number of alternative synthetic routes to both

[Pd(CO)₄]^{2+ 93}and [Pt(CO)₄]²⁺salts,^86,93(ii) to investigate the

thermal behavior of [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, in detail,

(iii) to report the crystal, molecular and extended structures of

[M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt. Square planar homoleptic metal

carbonyls are, as discussed, exceedingly rare.^14-17,25Only very

recently the structural characterization of square planar

[Rh(CO)₄]⁺has been achieved with either [1-Et-CB₁₁F₁₁]^{- 94}

or [Al₂Cl₇]^{- 95}as counteranion, (iv) to analyze the complete

vibrational spectra of the cations [M(CO)₄]²⁺, M ) Pd, Pt, and

[Rh(CO)₄]⁺(D_4h). Such an analysis has been reported so far

only for the homoleptic carbonyl cations [Au(CO)₂]⁺(D_4h)⁶⁷

and [Fe(CO)₆]²⁺(O_h)⁸⁰, (v) to perform density functional

calculations of [M(CO)₄]²⁺, M ) Pd, Pt, which are extended

to include in addition to the known cation [Rh(CO)₄]^{+ 94,95}the

probable cations [Co(CO)₄]⁺and [Ir(CO)₄]⁺, as well as the

hypothetical cations [Ni(CO)₄]²⁺, [Au(CO)₄]³⁺, and [Hg-

(CO)₄]⁴⁺. The calculations address the molecular structures,

vibrational assignments and atomic charge distribution for square

planar [M(CO)₄]ⁿ⁺cations. Thus far, theoretical calculations of

existing homoleptic carbonyls have been limited to octahedral

species^83,96-100and linear d¹⁰ions.^101-104

Subsequently a number of additional, highly unusual super-

electrophilic metal carbonyl cations are obtained as [Sb₂F₁₁]^-

salts from SbF₅or HF-SbF₅as reaction media. They include

linear [Hg(CO)₂]^{2+ 76,77}

,

the first, and so far only, thermally stable

homoleptic carbonyl cation formed by a post-transition metal,⁴⁸

[Ir(CO)₆]^{3+ 78}

the first tripositive metal carbonyl cation, [Fe-

,

(CO)₆]²⁺, the first dipositive cation formed by a 3d metal,^79,80

octahedral [M(CO)₆]²⁺, M ) Ru, Os,^81,82the monochloro

pentacarbonyl cations [M(CO)₅Cl]²⁺, M ) Rh, Ir,^78,83and the

group 6 hexafluoro antimonato(V) cations [W(CO)₆(FSbF₅)]^{+ 84}

and cyclic [{Mo(CO)₄}₂(µ-F₂SbF₄)₃]^{+ 85}as part of a polymeric

chain.

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Strauss, S. H. J. Am. Chem. Soc. 1999, 121, 11920.

(95) von Ahsen, B.; Bach, C.; Ko¨ckerling, M.; Willner, H.; Aubke, F.

Manuscript in preparation.

(79) Bley, B.; Willner, H.; Aubke, F. Inorg. Chem. 1997, 36, 158.

(80) Bernhardt, E.; Bley, B.; Wartchow, R.; Willner, H.; Bill, E.; Kuhn,

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Soc. 1999, 121, 7188.

(96) Ehlers, A. W.; Ruiz-Morales, Y.; Baerends, E. J.; Ziegler, T. Inorg.

Chem. 1997, 36, 5031.

(81) Wang, C.; Bley, B.; Balzer-Jo¨llenbeck, G.; Lewis, A. R.; Sui, S.

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Chem. Manuscript to be submitted.

(83) Willner, H.; Bach, C.; Wartchow, R.; Wang, C.; Rettig, S. J.; Trotter,

J.; Jonas, V.; Thiel, W.; Aubke, F. Inorg. Chem. 2000, 39, 1933.

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S. J.; Trotter, J.; Willner, H.; Aubke, F. Inorg. Chem. 2000, 39, 2172.

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H.; Rettig, S. J.; Trotter, J.; Aubke, F. Inorg. Chem. 1999, 38, 3684.

(97) Szillagy, R. K.; Frenking, G. Organometallics 1997, 16, 4807.

(98) Jonas, V.; Thiel, W. Organometallics 1998, 17, 353.

(99) Frenking, G.; Fro¨hlich, N. Chem. ReV. 2000, 100, 717.

(100) Diefenbach, A.; Bickelhaupt, F. M.; Frenking, G. J. Am. Chem.

Soc. 2000, 122, 6449.

(101) Veldkamp, A.; Frenking, G. Organometallics 1993, 12, 82.

(102) Lynn, M. A.; Bursten, B. E. Inorg. Chim. Acta 1995, 229, 437.

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Chem. Eur. J. 1999, 5, 2573.

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[Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂

J. Am. Chem. Soc., Vol. 123, No. 4, 2001 591

Table 1. Crystallographic Data for [M(CO)₄][Sb₂F₁₁]₂, M ) Pd,

A recent report on the use of [Pt(CO)₄][Sb₂F₁₁]₂as polymeri-

zation catalyst¹⁰⁵and the use of various cationic carbonyl

complexes in the carbonylation of olefins^106-109suggest begin-

ning practical applications of these materials. The existence of

Pt^a

compound

[Pd(CO)₄][Sb₂F₁₁]₂

[Pt(CO)₄][Sb₂F₁₁]₂

empirical formula

formular weight

crystal system

space group

a [Å]

b [Å]

c [Å]

C₄F₂₂O₄PdSb₄

1123.41

monoclinic

No. 14, P2₁/c

12.583(2)

9.8172(9)

17.9895(7)

103.1183(10)

2164.2(4)

4

C₄F₂₂O₄PtSb₄

1212.10

monoclinic

No. 14, P2₁/c

12.757(2)

9.726(2)

17.8576(7)

103.3576(7)

2155.7(5)

4

derivatives such as dinuclear [{Pt(CO)₃}₂]²⁺in H₂SO₄and

110

of cations such as [cyclo-C₂H₄{P(R_F)₂}₂Pt(CO)₂]²⁺, R_F

)

C₂F₅,¹¹¹which are generated in superacids, points to the

emergence of a viable new chemistry of cationic platinum

carbonyl derivatives.

â [deg]

V [Å³]

Experimental Section

Z

F

Apparatus. Volatile materials were manipulated in Monel or Pyrex

vacuum-lines of known volume. The monel lines were fitted with

Whitey valves RS4-316 and Swagelok fittings, while the Pyrex vacuum

line was fitted with Teflon stem valves. Both lines were equipped with

capacitance pressure gauges (type 280 E Setra Instruments, Acton, MA).

For reactions in SbF₅or HSO₃F Pyrex reactors of ∼50 mL volume

were used, fitted with Kontes valves and Teflon-coated magnetic stirring

bars. Reactions in anhydrous HF or in HF-SbF₅as well as recrystal-

lizations were carried out in Kel-F reaction vials of about 30 mL

volume, fitted with a Monel top, equipped with a stainless steel Whitey

valve RS4-316. A drawing of the reactor is included in the Supporting

Information as Figure S1. Solid materials were manipulated in a

Vacuum Atmosphere Corporation Dry-Lab model DI 001-SG dry box

filled with nitrogen and fitted with a HE 493 Dry Train.

_calc[g/cm³]

3.448

3.734

T [°C]

radiation

λ [Å]

-93(1)

Mo KR

0.71069

59.35

0.71069

115.80

µ [cm^-1

R (F)

]

0.025

0.047

0.025

0.048

R_w(F²)

^aR ) Σ||F_o| - |F_c||/Σ|F_o| (I g 3σ(I)), R_w) (Σw(|F_o| - |F_c²|)²/

2

2 2

Σw|F_o| )^1/2(all data)

eight scans were co-added for each spectrum, using an apodized

resolution of 2 or 4 cm^-1. The samples were crushed between CaF₂,

AgBr (Korth, Kiel, Germany), or polyethylene (Cadillac, Hannover,

Germany) disks inside the drybox. Raman spectra were recorded at

room temperature with a Bruker RFS 100/S FT Raman spectrometer

using the 1064 nm exciting line (∼500 mW) of a Nd:YAG laser (Adlas,

DPY 301, Lu¨beck, Germany). Crystalline samples were contained in

large melting point capillaries (2 mm od.) for recording spectra in the

Chemicals and Synthetic Procedures. Platinum and palladium

powder (both 99.9% pure) were obtained as a gift from the Degussa-

Hu¨ls Company. Fluorosulfuric acid tech. grade (Orange County

Chemicals) was purified by double distillation at room temperature as

described previously.⁵⁵Antimony(V) fluoride (Atochem North America,

formerly Ozark-Mahoning) was purified by atmospheric pressure

distillation, followed by a vacuum distillation. To remove moisture as

[H₃O][Sb₂F₁₁],⁶³a small amount of SbF₅was added to anhydrous HF

(Air Products), and the mixture was stored in a Kel-F vial. Bis(fluoro-

sulfuryl)peroxide, S₂O₆F₂was synthesized by catalytic (AgF₂) fluorina-

region 5000-80 cm^-1with a spectral resolution of 2 cm^-1

.

(b) NMR Spectroscopy. ¹³C NMR spectra were obtained on a

Bruker MSL-200 FT spectrometer, operating at 50.322 MHz. Solid-

state ¹³NMR spectra (MAS) were recorded using a broad band probe

MAS DLK 39-82 MHz. Samples were contained in ZrO₂crucibles (7

mm in diameter) fitted with a Kel-F insert and lid. The rotational

frequency was 5 kHz for [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt. Adamantane

(28.8 and 37.9 ppm) was used as a standard, and measured chemical

shifts were converted to the TMS scale.

112

tion of SO₃(Dupont) with elemental fluorine (Air Products) as

described.^112,113The starting materials and intermediates Pt(SO₃F)₄,⁷⁰

Pd[Pd(SO₃F)₆],^68,69and cis-M(CO)₂(SO₃F)₂,⁷¹M ) Pd, Pt, were all

obtained according to published methods. [Pt(CO)₄][Sb₂F₁₁]₂was

prepared from cis-Pt(CO)₂(SO₃F)₂by carbonylation in liquid SbF₅as

described.⁷⁵[Pd(CO)₄][Sb₂F₁₁]₂was obtained by the reductive carbon-

ylation of Pd[Pd(SO₃F)₆] in HF-SbF₅. Carbon monoxide (CP grade,

stated purity 99.5%) was obtained from Medigas Corp. and purified

by passing the gas through a trap cooled to -196 °C. ¹³CO (99%

enriched) was obtained from IC Chemicals.

(c) X-ray Diffraction. All measurements were made at 180 ( 1 K

on a Rigaku/ADSC-CCD area detector diffractometer using graphite

monochromated Mo KR radiation. X-ray crystallographic analyses of

[M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt: Crystallographic data appear in Table

1. The final unit-cell parameters were based on 11 367 (Pd) and 11 597

(Pt) reflections respectively with 2Θ ) 4-60°. The data were

processed^114,115and corrected for Lorentz and polarization effects and

absorption (semiempirical multiscan, based on a three-dimensional

analysis of symmetry-equivalent data). The structures were solved by

direct methods. All atoms were refined with anisotropic thermal

parameters. No secondary extinction corrections were necessary. Neutral

atom scattering factors and anomalous dispersion corrections were taken

from the International Tables for X-ray Crystallography.^116,117

(d) Differential Scanning Calorimetry (DSC). Thermo-analytical

measurements were made with a Netzsch DSC 204 instrument.

Temperature and sensitivity calibrations in the temperature range of

20-500 °C were carried out with naphthalene, benzoic acid, KNO₃,

Warning: Of the reagents used in this study HF, HSO₃F, F₂, CO,

SO₃, SbF₅, and S₂O₆F₂are either highly corrosive, strongly oxidizing

or toxic. Vacuum lines, used for the handling of these reagents, are

contained inside of well ventilated, fume hoods. Safety equipment

should be worn and available safety information sheets should be

consulted together with the original literature cited here, when

attempting the reactions described.

Instrumentation. (a) Vibrational Spectroscopy. Infrared spectra

were recorded at room temperature on an IFS-66v FT spectrometer

(Bruker, Karlsruhe, Germany). Two different detectors together with a

Ge/KBr or a 6 µm Mylar beam splitter operating in the region 5000-

400 or 550-80 cm^-1, respectively, were used. One hundred and twenty

AgNO₃, LiNO₃, and CsCl. The heating rate employed was 10 K min^-1

and the furnace was flushed with dry nitrogen. For the evaluation, the

software Netzsch Proteus 4.0 was employed.

The samples were contained in sealed stainless steel crucibles, which

were coated on the inside with gold or rhodium (courtesy of Degussa-

Hu¨ls), of an approximate mass of 1.3 g and a volume of 100 µL, which

,

(105) Weber, L.; Barlmeyer, M.; Quasdorff, J. M.; Sievers, H. L.;

Stammler, H.-G.; Neumann, B. Organometallics 1999, 18, 2497.

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Org. Chem. 1999, 64, 6306.

(107) Xu, Q.; Souma, Y. Top. Catal. 1998, 6, 17.

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62, 1594.

(114) teXsan: Crystal Structure Analysis Package 1.8; Molecular

Structure Corp.: The Woodlands, TX; 1996.

(115) d*TREK: Area Detector Software; Molecular Structure Corp.:

The Woodlands, TX; 1998.

(116) International Tables for X-ray Crystallography; Kynoch Press

(present distributor Kluwer Academic Publishers: Boston, MA): Birming-

ham, UK, 1974; Vol. IV, p 99.

(117) International Tables for Crystallography; Kluwer Academic

Publishers: Boston, MA, 1992; Vol. C, p 200.

(109) Xu, Q.; Nakatani, H.; Souma, Y. J. Org. Chem. 2000, 65, 1540.

(110) Xu, Q.; Heaton, B. T.; Jacob, C.; Mogi, K.; Ichihashi, Y.; Souma,

Y.; Kanamori, K.; Eguchi, T. J. Am. Chem. Soc. 2000, 122, 6862.

(111) Houlis, J. F.; Roddick, D. M. J. Am. Chem. Soc. 1998, 120, 11020.

(112) Dudley, F. B.; Cady, G. H. J. Am. Chem. Soc. 1957, 79, 513.

(113) Zhang, D.; Wang, C.; Mistry, F.; Powell, B.; Aubke, F. J. Fluorine

Chem. 1996, 76, 83.

592 J. Am. Chem. Soc., Vol. 123, No. 4, 2001

Willner et al.

stood up to a pressure of 100 bar. In an inert atmosphere about 25-40

mg of the solid sample was weighed. Samples were studied in the

temperature range of 25-300 °C. After completion of each run, the

crucible lid was punctured in vacuo, the gaseous products as well as

the solid residues were studied by IR spectroscopy.

triple-ú basis supplemented by two sets of d polarization functions.^126,127

Spherical d-functions were applied throughout. Using analytic energy

gradients, the molecular geometries were optimized within the constraint

of D_4hpoint group symmetry. Second derivatives were obtained by

numerical differentiation of the analytic energy gradients with Gauss-

ian94 or analytically with Gaussian98, which also provided Raman

intensities at the BP86/ECP2 level. The present computational approach

was the same as in previous studies.^{98-100,128-130}

Synthetic Reactions and Crystal Growth. General Comments.

The routes to [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, involved the reductive

carbonylation of Pd[Pd(SO₃F)₆]^68,69in HF-SbF₅⁹³or of Pt(SO₃F)₄⁷⁰in

71

HSO₃F, followed by the isolation of cis-Pt(CO)₂(SO₃F)₂and the

Results and Discussion

subsequent solvolytic carbonylation to [Pt(CO)₄][Sb₂F₁₁]₂.

(a) [Pt(CO)₄][Sb₂F₁₁]₂. For crystal growth approximately 0.5 g of

[Pt(CO)₄][Sb₂F₁₁]₂were dissolved in a mixture of ∼10% of SbF₅and

∼90% (by volume) of anhydrous HF. CO was added to give a total

pressure of ∼1.5 atm. A small amount of the substance remained

undissolved. The suspension was heated briefly (about 30 min) to 60

°C while stirring and allowed to cool to room temperature without

stirring. The solvent was then removed slowly in vacuo over a period

of 24 h. The mixture had darkened due to the formation of traces of

platinum. From the dry, gray powder small crystals of [Pt(CO)₄]-

[Sb₂F₁₁]₂were picked inside a drybox under a polarizing microscope

and fitted into Lindemann glass capillaries.

(b) [Pd(CO)₄][Sb₂F₁₁]₂. The salt was obtained by the reductive

carbonylation of ∼0.2 g of Pd[Pd(SO₃F)₆] in 10 mL of HF-SbF₅(1:1

by volume), adopted from a published procedure.⁹³The mixture was

heated at 60 °C and ∼1.5 atm of CO for 24 h and was then allowed to

cool to room temperature. The solid product mixture contained a number

of small crystals which were picked under a polarizing microscope and

fitted into Lindemann glass capillaries inside a drybox.

Synthetic Aspects. Both [Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO₄]-

[Sb₂F₁₁]₂together with [Pt(CO)₄][Pt(SO₃F)₆] are the first of a

growing list of superelectrophilic⁴⁹metal carbonyl cations, and

the only ones of these with a square planar coordination

geometry. The superacid anion [Pt(SO₃F)₆]^2-ranks just below

[Sb₂F₁₁]^-and [SbF₆]^-as a very weak nucleophile in an order

based on ¹¹⁹Sn Mo¨ssbauer spectroscopy of various dimethyl

tin(IV) salts.¹³¹

The previously reported syntheses of [M(CO)₄][Sb₂F₁₁]₂, M

) Pd, Pt,⁷⁵involve the solvolytic carbonylation of the cis-

M(CO)₂(SO₃F)₂, M ) Pd, Pt,⁷¹which in turn are obtained by

the reductive carbonylation of Pd[Pd(SO₃F)₆]^68,69or Pt(SO₃F)₄

70

in HSO₃F. The two precursors^68-70are obtained by the oxidation

112,113

of Pd or Pt metal powder, respectively, with S₂O₆F₂

in

HSO₃F^52-56as described.^68-70There are a number of simpler

alternative routes to [Pt(CO)₄][Sb₂F₁₁]₂and to a lesser extent

to [Pd(CO)₄][Sb₂F₁₁]₂, which are reviewed recently.⁷³

The ¹³C-isotopomers [M(¹³CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, were obtained

by using ¹³CO throughout the reductive carbonylations of Pt(SO₃F)₄

or Pd[Pd(SO₃F)₆] in SbF₅.

In liquid SbF₅, the reductive carbonylations of both Pd[Pd-

(SO₃F)₆]^68,69and Pt(SO₃F)₄⁷⁰lead directly to [M(CO)₄][Sb₂F₁₁]₂,

M ) Pd, Pt.⁹³We have shown, in the course of this study, that

in HF-SbF₅crystalline [Pd(CO)₄][Sb₂F₁₁]₂is directly obtained

by this route from the mixed valency palladium fluorosulfate.

The most elegant synthesis of [Pt(CO)₄][Sb₂F₁₁]₂is the

Theoretical Methods

Gradient-corrected density functional calculations were carried out

by using the Gaussian94¹¹⁸and Gaussian98¹¹⁹program systems.

Gradient corrections for exchange and for correlation were taken from

the work of Becke¹²⁰and Perdew,¹²¹respectively (usually abbreviated

as BP or BP86). Two basis sets were employed, labeled ECP1 and

ECP2. Both used a quasi relativistic effective core potential at the

transition metal together with the corresponding (8s,7p,5d)/[6s,5p,3d]

valence basis set.^122,123For carbon and oxygen, ECP1 employed the

6-31G(d) basis,^124,125whereas ECP2 used a Dunning (10s,6p)/[5s,3p]

86

reductive carbonylation of PtF₆in liquid SbF₅according to:

25-50 °C, 2 h

PtF₆+ 6 CO + 4 SbF₅

_{, 1 atm CO}8

SbF₅₍₁₎

[Pt(CO)₄][Sb₂F₁₁]₂+ 2 COF₂(1)

This approach, which produces only a single, volatile

byproduct, is efficient and fast. However the synthesis of PtF₆

is not trivial.^86,132-134Interestingly, the reductive carbonylation

of PtF₆in pure anhydrous HF produces [Pt(CO)₄][PtF₆], which

is the first example of a Pt(II) carbonyl fluoride after recent

unsuccessful attempts to synthesize cis-Pt(CO)₂F₂.⁷³As is

generally observed, late transition metal carbonyl fluorides are

rather uncommon.^135,136Attempts to convert [M(CO)₄][Sb₂F₁₁]₂,

M ) Pd, Pt, into the [SbF₆]^-salts by repeated washing with

anhydrous HF in order to produce [M(CO)₄][SbF₆]₂, M ) Pd,

Pt, are unsuccessful.

(118) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;

Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.

A.; Montgomery, J. A., Jr.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,

V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;

Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;

Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;

Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-

Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision D.4;

Gaussian, Inc.: Pittsburgh, PA, 1995.

(119) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;

Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,

Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,

A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;

Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;

Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,

D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;

Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,

I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;

Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.

W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.;

Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.5;

Gaussian, Inc.: Pittsburgh, PA, 1998.

All synthetic routes to [M(CO)₄]²⁺, M ) Pd, Pt, salts involve

Pd[Pd(SO₃F)₆],^68,69Pt(SO₃F)₄or PtF₆

as starting

70

86,132,133

materials, which are not commercially available. Their synthesis

(126) Dunning, T. H. J. Chem. Phys. 1971, 55, 716.

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[Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂

J. Am. Chem. Soc., Vol. 123, No. 4, 2001 593

compounds, is followed by endothermic events with onsets at

180 °C and 230 °C for the [Pd(CO)₄]²⁺and [Pt(CO)₄]²⁺

compounds, respectively. The peak area of ∼60 kJ mol^-1is

similar for both compounds. The thermal events are not

reversible, which argues against a simple melting process. The

gaseous products of the decomposition are COF₂and CO₂

(probably formed by hydrolysis). CO is not observed in either

run. The black solid residue with IR bands at 2238(s), 2206(s)

and 2167(w) for the Pt complex and 2249(m) ([Pd(CO)₄]²⁺),

2216(m) and 2186 (m) with an additional weak band at 1993

cm^-1for the Pd complex are tentatively identified as cis-

M(CO)₂(FSbF₅)₂, M ) Pd, Pt, by analogy to cis-M(CO)₂-

(SO₃F)₂, M ) Pd, Pt,⁷¹and to observations by C. Wang.¹³⁷

(c) Summary. While the observation of metal (Pd, Pt), liquid

SbF₅, and COF₂suggest two or even three different simultaneous

decomposition modes, four conclusions can be drawn from the

observations: (i) [Pt(CO)₄][Sb₂F₁₁]₂has greater thermal stability

than [Pd(CO)₄][Sb₂F₁₁]₂by about 60 °C, with the latter stable

at least to 140 °C and the former to ∼200 °C. (ii) CO is either

not observed (DSC measurements) or is formed only in trace

amounts. Instead, COF₂and CO₂are formed. (iii) The endo-

thermic thermal events appear to be quite similar for both

compounds. (iv) In addition to a complete breakdown of

[M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, resulting in metal formation,

the decomposition appears to proceed via intermediates such

as cis-M(CO)₂X₂, M ) Pd, Pt; X ) Sb₂F₁₁, SbF₆.

A very similar decomposition is found for [Au(CO)₂][Sb₂F₁₁]

in a glass capillary. Here Au and SbF₅are formed on heating

the sample to about 200 °C, while in the gas phase COF₂and

CO₂are observed by IR spectroscopy. A distinctly different

mode of thermal decomposition has been observed recently for

the complex [Fe(CO)₆][Sb₂F₁₁]₂.⁸⁰Initially, SbF₅is released to

give [Fe(CO)₆][SbF₆]₂, which undergoes at T ) 140-150 °C

an irreversible loss of CO to give previously reported Fe-

[SbF₆]₂.¹³⁸It is very unlikely that a controlled thermal decom-

position of [Pt(CO)₄][Sb₂F₁₁]₂will produce such elusive com-

pounds as Pt[SbF₆]₂or PtF₂. The thermal behavior of

[Au(CO)₂][Sb₂F₁₁], [Fe(CO)₆][SbF₆]₂, and [M(CO)₄][Sb₂F₁₁]₂,

M ) Pd, Pt, is in sharp contrast to that of the silver(I) tetra-

(teflato)borate CO-adducts Ag(CO)_nB(OTeF₅)₄, n ) 0, 1,

3,^139,140which undergo facile CO addition and dissociation and

are thermally unstable at ambient conditions.

Structural Aspects of [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt. (a)

Crystal Structures of [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt. The

crystallographic data for [Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄]-

[Sb₂F₁₁]₂are summarized in Table 1. The data are obtained at

identical experimental conditions at 180 ( 1 K. The structures

are refined in the same manner and nearly identical R (0.025)

and R_w(0.048) values are obtained for both salts. As can be

seen from the data in Table 1, both [Pd(CO)₄][Sb₂F₁₁]₂and

[Pt(CO)₄][Sb₂F₁₁]₂are isostructural. They crystallize in the

monoclinic spacegroup P2₁/c (Nï. 14). Packing of the four

formula units in the unit cell, shown in Supporting Information

in Figure S2, is identical for both [M(CO)₄][Sb₂F₁₁]₂, M ) Pd,

Pt, salts, and the unit cell dimensions are very similar for both,

as seen in Table 1. The unit cell volume of [Pt(CO)₄][Sb₂F₁₁]₂

is slightly smaller by 0.4% than that of the Pd(II) complex.

Interestingly, for the isostructural pair cis-M(CO)₂(SO₃F)₂, M

) Pd,⁷²Pt,⁷³where weaker inter- and intramolecular interactions

Figure 1. DSC curves in the temperature range of 120-300 °C for

[MCO)₄][Sb₂F₁₁]₂, M ) Pd, Pt.

requires highly reactive and corrosive reagents (S₂O₆F₂, F₂).

The carbonylation of the commercially available metal dichlo-

93

rides PdCl₂or PtCl₂in liquid SbF₅seems to provide an

alternative. However, the reactions are very slow, proceed in a

heterogeneous phase, are suitable for small amounts only, and

yield impure products.

Common to all methods discussed here and the synthetic

routes to other superelectrophilic⁴⁹carbonyl cation salts is the

use of superacids as reaction media, most prominently among

them antimony(V) fluoride⁴⁸and more recently HF-SbF₅.^60,61

There are at this time no alternative routes to superelectrophilic

metal carbonyl cations and their derivatives.

The compositions of [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, are

established by the reported microanalyses and the reaction

balances.⁷⁵Further extensive characterizations by differential

scanning calorimetry, vibrational spectroscopy, ¹³C MAS NMR

spectroscopy, single-crystal X-ray diffraction, and DFT calcula-

tions are the subject of this study.

Thermal Behavior of [M(CO)₄][Sb₂F₁₁]₂. (a) Observations

During the Decomposition Process. Both [Pd(CO)₄][Sb₂F₁₁]₂

and [Pt(CO)₄][Sb₂F₁₁]₂are obtained as colorless, very moisture-

sensitive solids which show surprisingly high thermal stabilities

up to 140 and 200 °C, respectively.⁷⁵When [Pt(CO)₄][Sb₂F₁₁]₂

is heated in a glass capillary beyond 200 °C, thermal decom-

position occurs, and a black solid (Pt) and a liquid (SbF₅) form.

The gas-phase IR spectrum shows only traces of CO. Instead,

COF₂and some CO₂are observed. The solid residue, after

keeping the sample at 220 °C for 3 min, still contains CO bound

to Pt. The observed IR bands at 2220 (m) and 2178 (s) cm^-1

are lower than the E_umode of [Pt(CO)₄]²⁺at 2244 cm^-1and

are tentatively attributed to cis-Pt(CO)₂(FSbF₅)₂.¹³⁷For cis-

Pt(CO)₂(SO₃F)₂two IR bands of similar intensities are found

at 2219 and 2185 cm^-1respectively.⁷¹

(b) Differential Scanning Calorimetry. The results of the

DSC measurements for the temperature range of 120-300 °C

are shown in Figure 1. As can be seen, the thermal behaviors

of both [Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂are similar.

An initial small shoulder centered at ∼50 °C, found for both

(138) Gantar, D.; Leban, I.; Frlec, B.; Holloway, J. H. J. Chem. Soc.,

Dalton Trans. 1987, 2379.

(139) van Seggen, D. M.; Hurlburt, P. K.; Noirot, M. D.; Anderson, O.

P.; Strauss, S. H. Inorg. Chem. 1992, 31, 1423.

(140) Hurlburt, P. K.; Rack, J. J.; Luck, J. S.; Dec, S. F.; Webb, J. D.;

Anderson, O. P.; Strauss, S. H. J. Am. Chem. Soc. 1994, 116, 10003.

(137) Wang, C. Ph.D. Thesis, University of British Columbia, 1996.

594 J. Am. Chem. Soc., Vol. 123, No. 4, 2001

Willner et al.

(CO)₄]²⁺and [Pt(CO)₄]²⁺are identical within error limits, as

are the average C-O distances of 1.106(6) and 1.110(9) Å,

respectively. The average M-C, M ) Pd, Pt, and C-O

distances for both cations together with the average CO

stretching wavenumbers ν˜(CO)_avand the CO stretching force

constants f_COare listed in Table 3, where they are compared to

the corresponding data for other structurally and spectroscopi-

cally characterized Pd(II)- and Pt(II) carbonyl derivatives such

as cis-M(CO)₂(SO₃F)₂, M ) Pd, Pt,^71-73cis-Pt(CO)₂Cl₂,⁵

dinuclear trans-Pt₂(CO)₂I₂(µ-I)₂¹⁴¹and three carbonyl trihalide

anions of the type [MCl₃(CO)]^-, M ) Pd,¹⁴¹Pt,¹⁴²and

[PdBr₃(CO)]^-,¹⁴¹all with [NBut₄]⁺as countercation. All anionic

complexes have been previously characterized by vibrational

spectroscopy.^22,143

As can be seen in Table 3, the average M-C distances for

the cation [Pt(CO)₄]²⁺are the longest Pt-C internuclear

distances so far reported. Equally unprecedented for Pt(II)

carbonyls are the high ν˜(CO)_avand f_COvalues. On substitution

of two or more CO groups by anionic ligands (SO₃F, Cl, Br, I)

the Pt-C distance and the CO bond strength both decrease.

The vibrational data (ν˜(CO)_av, f_CO) provide a more precise

measure of the strength of the CO bond, than do the CO

distances, which for all compounds listed in Table 3 are identical

within esd values, while the ν˜(CO)_avwavenumbers are spread

Figure 2. Ortep plot (30% probability ellipsoids) of a formula unit of

[Pt(CO)₄][Sb₂F₁₁]₂.

Table 2: Selected Bond Parameters for [M(CO)₄][Sb₂F₁₁]₂, M )

Pd, Pt^a

M ) Pd

M ) Pt

over 166 cm^-1

.

(a) Bond Length [Å]

2.000(3)

For [Pd(CO)₄]²⁺not only the M-C distances but also ν˜(CO)_av

and f_COare nearly identical to those of [Pt(CO)₄]²⁺as discussed

above. However, for the isostructural pairs cis-M(CO)₂-

(SO₃F)₂^72,73and [MCl₃(CO)]^-,^141,143M ) Pd, Pt, longer M-C

bonds and shorter C-O bonds are found for the palladium

compounds, in agreement with previous observations.^22,144These

differences, previously noted in isostructural Pd- and Pt carbonyl

derivatives,^22,143,144must be, as recently discussed,⁷³due to

effects caused by the anionic ligands.

Sb(1)-F(1)

Sb(2)-F(1)

Sb(3)-F(12)

Sb(4)-F(12)

M(1)-C(1)

M(1)-C(2)

M(1)-C(3)

M(1)-C(4)

O(1)-C(1)

O(2)-C(2)

O(3)-C(3)

O(4)-C(4)

2.011(3)

2.034(3)

2.011(3)

2.034(3)

1.984(8)

1.980(8)

1.987(8)

1.979(9)

1.086(9)

1.106(9)

1.110(9)

1.118(9)

2.049(3)

2.015(3)

2.032(3)

1.991(6)

2.006(6)

1.987(6)

1.984(6)

1.110(6)

1.100(6)

1.105(6)

1.111(7)

A comparison to tabulated data from the Cambridge data

compilation¹⁴⁵is only possible for the Pt(II) carbonyl species.

Compared to the listed mean Pt-C distance of 1.854 Å and

the upper quartile q_uvalue of 1.878 Å, based on 29 samples,

the observed average distances for [Pt(CO)₄][Sb₂F₁₁]₂(1.992(6)

(b) Bond Angles [deg]

93.0(2)

C(1)-M(1)-C(2)

C(1)-M(1)-C(4)

C(2)-M(1)-C(3)

C(3)-M(1)-C(4)

Sb(1)-F(1)-Sb(2)

Sb(3)-F(12)-Sb(4)

M(1)-C(1)-O(1)

M(1)-C(2)-O(2)

M(1)-C(3)-O(3)

M(1)-C(4)-O(4)

92.2(3)

90.7(3)

89.8(3)

89.6(2)

88.8(2)

88.6(2)

151.6(2)

158.8(2)

176.3(5)

177.0(5)

176.6(5)

176.1(5)

5

73

Å), cis-Pt(CO)₂Cl₂(1.897(5) Å), and cis-Pt(CO)₂(SO₃F)₂

87.4(3)

153.8(2)

161.1(2)

176.4(7)

177.1(7)

178.1(7)

177.0(7)

(1.882(3) Å) are all longer. The CO distances for all compounds

listed in Table 3 are at the shorter end of the tabulated data¹⁴⁵

and fall frequently below the lower quartile q_lof 1.132 Å.¹⁴⁵

A useful structural comparison is possible with the square

planar{M(CO_eq)₄} moiety in the structures of [M(CO)₅Cl]-

[Sb₂F₁₁]₂, M ) Rh, Ir,^78,83and data for the recently reported

^aAtom labeling differs between two structures, corresponding

parameters are presented side-by-side.

cation [Rh(CO)₄]⁺,^94,95which is isoelectronic to [Pd(CO)₄]²⁺

.

For both [Rh(CO)₄][1-Et-CB₁₁F₁₁]⁹⁴and [Rh(CO)₄][Al₂Cl₇]⁹⁵

nearly identical average Rh-C distances of 1.951(6)⁹⁴and

1.95(1) Å,⁹⁵respectively, are observed. In addition to similar

C-O distances, the ν˜(CO) values in the Raman spectra are

within 5-10 wavenumbers of each other. It appears, that

structural and spectroscopic data for [Rh(CO)₄]⁺in both salts^94,95

show a remarkable independence from the counteranion, which

in our opinion is due to the absence of significant interionic

interactions (vide infra).

are observed, the unit cell volume of cis-Pt(CO)₂(SO₃F)₂is

larger by 4.1% than that of cis-Pd(CO)₂(SO₃F)₂.⁷³

(b) Internal Bond Parameters for the [M(CO)₄]²⁺, M )

Pd, Pt, Cations. A formula unit of [Pt(CO)₄][Sb₂F₁₁]₂is shown

in Figure 2 and of [Pd(CO)₄][Sb₂F₁₁]₂in Figure S3. Selected

bond lengths and bond angles are collected in Table 2. A close-

up look of the [M(CO)₄]²⁺, M ) Pd, Pt, cations is shown in

the Supporting Information as Figure S4. As can be seen, there

are small departures from planarity. There are also similar

angular distortions. The C-M-C angles, M ) Pd, Pt, for CO

ligands cis to each other range from 88.6(2) to 93.0(2)° for the

Pd(II) cation and from 87.4(3) to 92.3(3)° for [Pt(CO)₄]²⁺. As

is commonly found for cationic metal carbonyl complexes, the

M-C-O groups, M ) Pd, Pt, show a very slight departure

from linearity.

(141) Andreini, B. P.; Belli Dell’Amico, D.; Calderazzo, F.; Venturi,

M. G.; Pelizi, G.; Segre, A. J. Organomet. Chem. 1988, 354, 357.

(142) Russell, D. R.; Tucker, P. A.; Wilson, S. J. Organomet. Chem.

1976, 104, 387.

(143) Browning, J.; Goggin, P. L.; Goodfellow, R. J.; Norton, M. G.;

Rattray, A. J.; Taylor, B. F.; Mink, J. J. Chem. Soc., Dalton Trans. 1977,

2061.

(144) Belli Dell’Amico, D.; Calderazzo, F.; Zandona, N. Inorg. Chem.

1984, 23, 137.

(145) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,

D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.

Both M-C and C-O distances vary slightly for both cations.

The d(M-C)_avvalues of 1.992(6) and 1.982(9) Å for [Pd-

[Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂

J. Am. Chem. Soc., Vol. 123, No. 4, 2001 595

Table 3: Selected Structural and Vibrational Data for Pd(II) and Pt(II) Carbonyl Complexes

compound

d(M-C)_av[Å]

d(C-O)_av[Å]

ν˜(CO)_av[cm^-1

]

f_CO^a[10²N m^-1

]

ref^b

c, c

[Pd(CO)₄][Sb₂F₁₁]₂

[Pt(CO)₄][Sb₂F₁₁]₂

cis-Pd(CO)₂(SO₃F)₂

cis-Pt(CO)₂(SO₃F)₂

cis-Pt(CO)₂Cl₂

1.992(6)

1.982(9)

1.932(5)

1.882(3)

1.897(5)

1.88(3)

1.82(1)

1.87(1)

1.87(3)

1.106(6)

1.110(9)

1.108(6)

1.116(6)

1.115(6)

1.06(4)

1.12(2)

1.11(1)

1.10(3)

2259

2261

2218

2202

2171

2106^d

2095

2132

2120

20.63

20.64

19.87

19.56

18.67

17.93

17.74

18.37

18.16

c, c

71, 72

71, 73

5, 61

trans-Pt₂(CO)₂I₂(µ-I)₂

141

[PtCl₃(CO)]^{- e}

142, 143

141, 142

[PdCl₃(CO)]^{- e}

[PdBr₃(CO)]^{- e}

^aApproximation according to Cotton and Kraihanzel.^{182 b}Where two references are listed, the first refers to structural data, the second reference

refers to vibrational data. ^cThis work. ^dSolution in n-heptane. ^e[NBu₄]⁺as cation.

Table 4: Calculated and Experimental Bond Lengths [Å] and Partial Atomic Charges from Natural Population Analysis^afor the Isoelectronic

Square Planar (D_4h) Cations [M(CO)₄]ⁿ⁺, M ) Co, Rh, Ir, Ni, Pd, Pt, Au, Hg; n ) 1, 2, 3, 4, and for Gaseous CO

experimental

d(MC) d(CO)

BP86/ECP1

BP86/ECP2

d(MC)

d(CO)

d(MC)

d(CO)

q_M

q_C

q_O

q_C+q_O

q_C-q_O

[Co(CO)₄]⁺

[Rh(CO)₄]⁺

[Ir(CO)₄]⁺

[Ni(CO)₄]²⁺

[Pd(CO)₄]²⁺

[Pt(CO)₄]²⁺

[Au(CO)₄]³⁺

[Hg(CO)₄]⁴⁺

CO_(g)

-

1.826

1.965

1.971

1.886

2.015

2.009

2.081

2.278

-

1.147

1.146

1.147

1.136

1.132

1.136

1.150

1.829

1.967

1.972

1.884

2.016

2.009

2.080

2.260

1.137

1.135

1.136

1.125

1.121

1.124

1.138

-0.08

-0.15

-0.12

0.50

0.43

0.42

0.55

0.56

0.55

0.54

0.56

0.55

0.54

0.59

0.46

-0.28

-0.27

-0.16

-0.17

-0.15

-0.05

0.04

0.27

0.29

0.28

0.38

0.39

0.50

0.63

0.00

0.83

0.84

0.81

0.70

0.72

0.70

0.59

0.55

0.92

1.951(6)^b)

1.117(10)^c)

-

1.992(6)

1.982(9)

-

1.106(6)

1.110(9)

-

1.00

1.48

1.12819^d)

-0.46

^aBP86/ECP2 values in units of e. ^bReference 94. ^cReference 95. ^dReference 151.

Slightly longer average M-C distances are found for the four

equatorial CO groups in [Rh(CO)₅Cl]²⁺(2.014(12) Å)⁸³and

[Ir(CO)₅Cl]²⁺(2.017(10) Å)^78,83both with [Sb₂F₁₁]^-as anion.

Very slightly shorter Ir-C distances of 2.006(6) and 1.999(6)

Å are reported for the trans-Ir-(CO)₂-segment in mer-Ir(CO)₃-

(SO₃F)₃.¹⁴⁶The ν˜(CO)_avvalues for [Rh(CO)₅Cl]²⁺(2247

and experimental CO distances have been noted for [Fe-

(CO)₆]^{2+ 80}

As the calculated bond parameters are closer to

experimental data at the BP86/ECP2 rather than at the BP86/

ECP1 level of theory, we shall therefore in the remainder of

this paper only discuss the BP86/ECP2 results.

.

The data for the hypothetical D_4hcations [Au(CO)₄]³⁺and

[Hg(CO)₄]⁴⁺indicate, that M-C distances increase with

increasing ion charge, while CO distances remain in about the

same region. It appears unlikely that either of these two cations

can be obtained, using presently known synthetic methods. On

the other hand [Co(CO)₄]⁺and [Ir(CO)₄]⁺may have a chance

to be prepared. Vibrational evidence for the existence of

cm^-1),⁸³[Ir(CO)₅Cl]²⁺(2246 cm^{-1 78,83}

and mer-Ir(CO)₃(SO₃F)₃

)

(2213 cm^-1),¹⁴⁶are all lower than ν˜(CO)_avof 2268 cm^-1for

3+ 78,86

6

[Ir(CO) ]

with f_CO20.78 × 10²N m^-1

.

It is remarkable how in isostructural pairs such as [M(CO)₄]²⁺

,

M ) Pd, Pt, [M(CO)₅Cl]²⁺, M ) Rh, Ir,^78,83or [M(CO)₆]²⁺, M

) Ru, Os,^81,82the effective radii for the 4d and 5d metal ions

appear to be identical, most likely as a result of relativistic

effects.^147,148As a consequence, very similar structural {d(M-

C), d(C-O)} and spectroscopic {ν˜(CO)_av, f_CO} features are

observed within each pair. For the square planar cations

[Rh(CO)₄]⁺,^94,95[Pd(CO)₄]²⁺, and [Pt(CO)₄]²⁺the experimental

average metal-carbon and carbon-oxygen bond lengths are

compared to data from DFT calculations in Table 4. The

calculations also include other possibly square planar, homo-

leptic carbonyl cations such as [Co(CO)₄]⁺, [Ir(CO)₄]⁺,

[Ni(CO)₄]²⁺, [Au(CO)₄]³⁺, and [Hg(CO)₄]⁴⁺. Of these, only for

[Co(CO)₄]⁺and [Ir(CO)₄]⁺limited vibrational information is

available.^149,150

[Ir(CO)₄]⁺in molten AlCl₃will be discussed below, and

149

Co(CO)₄Y, where Y is an anionic ligand, is recently detected

in superacid solution.¹⁵⁰According to calculations for [Co-

(CO)₄]^{+ 152}the triplet state appears to be lower than the singlet

state. Collision-induced dissociation studies of [Co(CO)_n]⁺, n

) 1-5,¹⁵³point to the possible existence of [Co(CO)₅]⁺which

would be isoelectronic to Fe(CO)₅.

The structural characterizations of [M(CO)₄][Sb₂F₁₁]₂, M )

Pd, Pt, reported here, complete a series of data for dipositive

metal carbonyl cations with known structures, all obtained as

[Sb₂F₁₁]^-or [SbF₆]^-salts. The metals range from group 8 to

group 12; all are synthesized in superacids. Their generation

involves the four major synthetic methods,⁴⁸(a) solvolytic

carbonylation ([Hg(CO)₂]²⁺),^76,77(b) reductive carbonylation

([M(CO)₄]²⁺, M ) Pd, Pt, [M(CO)₆]²⁺, M ) Ru, Os),^81,82(c)

oxidative carbonylation ([Fe(CO)₆]²⁺), and (d) oxidation by SbF₅

([M(CO)₅Cl]²⁺, M ) Rh, Ir)^78,83complemented by the recently

discovered conversion of [M(CO)₆][Sb₂F₁₁]₂into [M(CO)₆]-

[SbF₆]₂, M ) Fe, Ru, Os,^80,82by elution with anhydrous HF.

All salts are thermally stable well above 100 °C.^75-83Most

As can be seen in Table 4, agreement between experimental

and calculated data at two levels of theory is fair. The computed

M-C distances are slightly longer, generally by about 0.02 Å.

The C-O distances are overestimated by about 0.03 Å at the

BP86/ECP1 level and by 0.02 Å at the BP86/ECP2 level, which

reflects the corresponding errors for the free CO molecule (see

Table 4).^149,151Similar discrepancies between calculated^96,98-100

(146) Wang, C.; Lewis, A. R.; Batchelor, R. J.; Einstein, F. W. B.;

Willner, H.; Aubke, F. Inorg. Chem. 1996, 35, 1279.

(147) Pyykko¨, P.; Desclaux, J. P. Acc. Chem. Res. 1979, 12, 276.

(148) Pyykko¨, P. Chem. ReV. 1988, 88, 563.

(149) Bach, C. Ph.D. Thesis, Universita¨t Hannover, 1998.

(150) Xu, Q.; Inoue, S.; Souma, Y.; Nakatani, H. J. Organomet. Chem.

2000, 606, 147.

(151) Herzberg, G. Spectra of Diatomic Molecules, 2nd ed.; Van

Nostrand: Toronto, Canada, 1966; p 521.

(152) Jonas, V., unpublished results.

(153) Goebel, S.; Haynes, C. L.; Khan, F. A.; Armentrout, P. B. J. Am.

Chem. Soc. 1995, 117, 6994 and references therein.

596 J. Am. Chem. Soc., Vol. 123, No. 4, 2001

Willner et al.

Table 5: Crystallographic, Structural, and Spectroscopic Properties of Selected Superelectrophilic Metal Carbonyl Cations in Their [Sb₂F₁₁]^-

or [SbF₆]^-Salts

unit cell

space

group

point

group

volume

Z

value

d(M-C)_av

d(C-O)_av

ν˜(CO)_av

f_CO‚10²

group

cation

[Å³]

[cm^-1

]

[N m^-1

]

ref

12

10

[Hg(CO)₂]²⁺

[Pd(CO)₄]²⁺

[Pt(CO)₄]²⁺

P2₁/n

P2₁/c

P2₁

D_4h

C_4V

O_h

967.1(2)

2164.2(4)

2155.7(5)

1237.7(2)

1226.5(4)

1198.4(2)

850.5(2)

1215.1(2)

853.1(2)

1210.8(2)

850.3(2)

2

4

2

2.083(10)

1.992(6)

1.982(9)

2.011(10)

2.026(10)

1.911(5)

1.908(7)

2.039(5)

2.024(5)

2.027(5)

2.022(7)

1.104(12)

1.106(6)

1.110(9)

1.102(10)

1.086(20)

1.104(6)

1.108(9)

1.094(10)

1.101(7)

1.102(7)

1.104(7)

2280

2259

2261

2247

2246

21.0

77

a

20.63

20.64

20.41

20.39

9

8

[Rh(CO)₅Cl]²⁺

[Ir(CO)₅Cl]²⁺

[Fe(CO)₆]²⁺

[Fe(CO)₆]^{2+ b}

[Ru(CO)₆]²⁺

[Ru(CO)₆]^{2+ b}

[Os(CO)₆]²⁺

[Os(CO)₆]^{2+ b}

83

80

82

P2₁

P2₁/n

P4/mnc

P2₁/n

P4/mnc

P2₁/n

P4/mnc

2215

2214

2209

19.82

19.80

19.71

}

O_h

^aThis work. ^b[SbF₆]^--salts, all others [Sb₂F₁₁]^-as anion.

cations are completely characterized by analytical, vibrational,^79-83

NMR, and computational methods^{96-98,101-104}in addition to the

molecular structure determination by single-X-ray diffrac-

tion.^77,78,82,83

and the recently reported group 6 carbonyl cations in the salts

[W(CO)₆(FSbF₅)][Sb₂F₁₁]⁸⁴and in polymeric [{Mo(CO)₄}(cis-

µ-F₂SbF₄)₃]_x[Sb₂F₁₁]_x⁸⁵establishes the superelectrophilic metal

carbonyl cations and their derivatives as a substantial and

important group. Even though square planar M(CO)₄species

are unknown for neutral carbonyls^5,14-16and the highly reduced

carbonylates,²⁵the cations [M(CO)₄]²⁺, M ) Pd, Pt, and

[Rh(CO)₄]^{+ 94,95}are now structurally and spectroscopically well

characterized and are the subject of theoretical calculations in

this study. The vast majority of the superelectrophilic cations

have the fluoroantimonate(V) species [SbF₆]^-and [Sb₂F₁₁]^-as

counteranions with [Pt(CO)₄][Pt(SO₃F)₆]⁷⁴and [Pt(CO)₄][PtF₆]⁸⁶

notable exceptions. The role of [Sb₂F₁₁]^-in providing thermally

stable salts of the [M(CO)₄]²⁺cations, M ) Pd, Pt, is now

examined.

(c) The [Sb₂F₁₁]^-ions in [M(CO)₄][Sb₂F₁₁]₂. A complete

listing of all Sb-F interatomic distances and F-Sb-F bond

angles is found in the Supporting Information section. While

the Sb-F interatomic distances are unremarkable, the bond

angles listed show two abnormalities: (i) F_ax-Sb-F_eqangles

are slightly wider than 90 °C, while F_eq-Sb-F_bangles are

consequently more acute, by about 5°. It appears that the

equatorial fluorine “lean” slightly toward the weakly bonded

Sb-F_b-Sb group, in agreement with the VSEPR theory of

molecular structure.^155-158(ii) As will be discussed below, the

Sb-F_b-Sb bridge angle R departs significantly from 180°.

With a linear Sb-F_b-Sb moiety, together with eclipsed SbF₄

groups, the di-octahedral anion would be centro symmetrical

(point group D_4h). The mutual exclusion rule for IR- and Raman

active vibrations permits easy detection. The D_4hconformer is

however quite rare and found in [H₃F₂][Sb₂F₁₁]¹⁵⁹and in the

metal carbonyl salts [Au(CO)₂][Sb₂F₁₁],^65,66,134[Rh(CO)₄]-

[Sb₂F₁₁],⁹⁵and curiously in [Re(CO)₆][Re₂F₁₁]¹⁶⁰where Re has

replaced Sb in the anion. In all examples the cations are

unipositive. In the structurally characterized carbonyl com-

plexes^134,160there are no significant interionic interactions in

their [M₂F₁₁]^-salts, M ) Sb, Re. A bent [Sb₂F₁₁]^-ion is

reported recently in a salt of the composition [Au(CO)₂]₂[SbF₆]-

[Sb₂F₁₁]¹⁶¹and in [H₂F][Sb₂F₁₁].¹⁵⁹

Selected crystallographic, structural, and vibrational data for

the fluoroantimonate(V) salts with dipositive metal carbonyl

cations are collected in Table 5. They allow some general

observations and conclusions: (i) All pairs of salts in groups 9

and 10 are isostructural, as are the members of the two triads

in group 8 with either [Sb₂F₁₁]^-or [SbF₆]^-as anions and M )

Fe, Ru, Os. As a consequence, crystallographic, structural, and

vibrational data within each pair or triad are extremely sim-

ilar. (ii) The unit cell volume of isostructural compounds

decreases from the 4d to the corresponding 5d complexes on

average by 0.3 to 0.9% as a consequence of relativistic

effects^147,148and possibly significant interionic, secondary¹⁵⁴

contacts of the C- -F type.⁸³(iii) There are in all salts with centro

symmetrical cations (point groups D_∞h, O_h) very slight depar-

tures from ideal geometry, which do not affect the mutual

exclusion rule for IR and Raman active fundamentals.^77,80,82(iv)

For the homoleptic carbonyl cations [M(CO)_n]²⁺in groups 12,

10, and 8 with n ) 2 (linear), 4 (square planar), or 6 (octahedral),

respectively, the strength of the C-O bond, measured as d(C-

O)_av, ν˜(CO)_av, and f_CO, decreases with increasing n, as the

s-orbital contributions to the metal hybrid orbitals of σ-symmetry

gradually decrease. The ν˜(CO)_avand f_COvalues for [Hg-

(CO)₂]^{2+ 76,77}and [M(CO)₄]²⁺, M ) Pd, Pt, together with those

for [Ir(CO)₆]^{3+ 78,86}are the three highest sets of values reported

to date for metal carbonyl cations.⁴⁸However, the force

constants f_COfor the three cations, found between 20.63⁷⁵and

76,77

21.0 × 10²N m^-1

,

are slightly lower than f_COof 21.3 ×

10²N m^-1calculated for HCO⁺,^48,64,76,77which suggests some

residual electron density in π* molecular orbitals of CO, caused

either by π-back-bonding or significant interionic C- -F interac-

tions. (v) Since [Ir(CO)₆]^{3+ 78,86}is not yet structurally character-

ized and [Rh(CO)₆]³⁺is still unknown, their monochloropenta-

carbonyl derivatives [M(CO)₅Cl]²⁺, M ) Rh, Ir,^78,83are included

in Table 5. While substitution of a single CO in [Ir(CO)₆]³⁺by

Cl^-results in a slight reduction of ν˜(CO)_avfrom 2268 to 2247

cm^-1and f_COfrom 20.78 to 20.40 × 10²N m^{-1 78,83,86}

,

the CO

(155) Gillespie, R. J. Molecular Geometry; van Nostrand: London, 1972.

(156) Gillespie, R. J.; Hargittai, I. The VSEPR Model of Molecular

Geometry; Allyn and Bacon: Boston, 1991.

(157) Gillespie, R. J. J. Chem. Soc. ReV. 1992, 22, 59.

(158) Gillespie, R. J.; Robinson, E. A. Angew. Chem., Int. Ed. Engl.

1996, 35, 495.

(159) Mootz, D.; Bartmann, K. Angew. Chem., Int. Ed. Engl. 1988, 27,

391.

(160) Bruce, D. M.; Holloway, J. H.; Russell, D. R. J. Chem. Soc., Dalton

Trans. 1978, 1627.

bonds in the cations [M(CO)₅Cl]²⁺, M ) Rh, Ir, are still stronger

than in the octahedral cations [M(CO)₆]²⁺, M ) Ru, Os, as a

consequence of the higher oxidation state of the metal ions in

the first group of cations.⁸²

In summary, the survey presented in Table 5, complemented

by similar data for structurally characterized cis-Pt(CO)₂Cl₂,⁵

cis-M(CO)₂(SO₃F)₂, M ) Pd,^71,72Pt,^71,73mer-Ir(CO)₃(SO₃F)₃,¹⁴⁶

(154) Alcock, N. W. AdV. Inorg. Radiochem. 1972, 15, 1.

(161) Ku¨ster, R.; Seppelt, K. Z. Anorg. Allg. Chem. 2000, 626, 236.

[Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂

J. Am. Chem. Soc., Vol. 123, No. 4, 2001 597

Table 6: Characteristic Structural Features of the [Sb₂F₁₁]^-Anion in Salts with Metal Carbonyl Cations

point

group anion

bridge angle

dihedral

angle ψ [°]

bond lengths in

bridge [Å]

cation

R [°]

ref

134

161

77

b

83

79, 80, c

160

85

[Au(CO)₂]⁺

D_4h

C₁

180.0

0

n.a.

36.5

9

38

0.5

42

31

41

34

49

38

0

1.979(3); 1.979(3)

2.025(2)

[Au(CO)₂]^{+ a}

[Hg(CO)₂]²⁺

[Pd(CO)₄]²⁺

152.5(3)

147.6(3)

158.8(2)

151.64(15)

161.1(2)

153.8(2)

154.7(6)

156.3(5)

153.2(6)

156.5(6)

148.5(2)

180

2.008(4); 2.035(4)

2.015(3); 2.032(3)

2.000(3); 2.049(3)

2.013(3); 2.034(3)

2.011(3); 2.038(3)

2.023(9); 1.977(10)

2.038(9); 1.982(9)

1.946(9); 2.089(10)

1.980(10); 2.007(9)

2.053(3); 1.998(3)

2.009(2); 2.009(2)

2.051(6); 2.014(6)

2.034(5); 2.022(5)

∼C_2V

C₁

[Pt(CO)₄]²⁺

∼C_2V

C₁

D_4h

C₁

[Rh(CO)₅Cl]²⁺

[Ir(CO)₅Cl]²⁺

[Fe(CO)₆]²⁺

[Re(CO)₆]^{+ d}

[{Mo(CO)₄}₂(F₂SbF₄)₃]⁺

[W(CO)₆(FSbF₅)]⁺

150.0(3)

149.4(3)

19

30

84

^a[Sb₂F₁₁]^-‚[SbF₆]^-as anion. ^bThis work. ^cIdentical R and ψ angles as well as Sb-F_bdistances are found for [Ru(CO)₆][Sb₂F₁₁]₂and

[Os(CO)₆][Sb₂F₁₁]₂. ^d[Re₂F₁₁]^-as anion.

In most metal carbonyl [Sb₂F₁₁]^-salts, two simultaneous

distortions from D_4hsymmetry are noted:¹⁶⁰(a) bending about

F_bwhich is expressed in terms of a bridge angle R and (b)

rotation of the two SbF_{4 eq}groups into a staggered conformation

expressed in terms of a dihedral angle ψ.¹⁶⁰Where both

rotational and bending processes are present, the [Sb₂F₁₁]^-anion

has no symmetry (point group C₁). The C₁conformation is most

commonly encountered. A summary of older examples is found

in ref 160. More recent results, obtained mainly by us, are listed

in Table 6. In addition to bending and rotational distortion, two

minor deformations are noted: (a) a slight asymmetry within

the bridge where both Sb-F_bdistances are no longer equal and

(b) a slight lengthening of the Sb-F bonds where F is involved

in secondary bonding.

The crystallographic results for the anions in [M(CO)₄]-

[Sb₂F₁₁]₂, M ) Pd, Pt, are surprising on three accounts: (i)

Both [Sb₂F₁₁]^-anions in both salts are no longer symmetry-

related and have different bridge and dihedral angles. This is

usually not the case for salts with other centrosymmetrical

cations such as [Hg(CO)₂]^{2+ 77}(D_4h) or [M(CO)₆]²⁺, M ) Fe,⁸⁰

Ru,⁸²Os,⁸²(O_h), where the [Sb₂F₁₁]^-anions have in each case

the same bond parameters including the angles R and ψ. (ii)

While the internal bond parameters for the two anions in both

salts are very similar and often identical within quoted esd

values, the bridge angles for both sets of data differ by about

2-3°. (iii) One of the two conformers in each salt has a dihedral

angle of 9° (Pd) and 0.5° (Pt), respectively. This implies that

departure from D_4hsymmetry involves primarily bending about

the bridging F-atom, but no significant rotation of the two SbF₄

groups. The approximate symmetry of this unique conformation

is C_2ν. Both conformers are shown in Figure S6, taken from

the [Pd(CO)₄][Sb₂F₁₁]₂structure. We have argued in the

past,^48,63,83that interionic interactions are the cause of the

observed distortions of the [Sb₂F₁₁]^-anions, to strengthen and

maximize such interionic contacts. The secondary contacts for

[M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, will be discussed in the next

section.

coordination. The F- -C contacts in [Hg(CO)₂][Sb₂F₁₁]₂are

equally significant and more numerous (10 per cation). The

bifurcated interactions, shown in Figure S5, are seen as

strengthening the Hg-C bonds, which are already strong on

account of relativistic effects.^147,148They are also important in

extending the molecular structure into a three-dimensional

framework.⁷⁷An illustration of the coordination environments

77

of carbon and mercury in [Hg(CO)₂][Sb₂F₁₁]₂may be found

in Figure 4 of ref 48.

It seems the “open” square planar structures of the [M(CO)₄]²⁺

cations (see Figure S4) with point group D_4hin the salts

[M(CO)₄][Sb₂F₁₁]₂become interesting test cases, because the

formation of secondary M- -F, M ) Pd, Pt, and C- -F contacts

are both possible for stereochemical reasons. To assist in the

discussion, we have listed in Table 4 the partial charges

calculated from natural population analysis for square planar

[M(CO)₄]ⁿ⁺, M ) Rh, Ir, Ni, Pd, Pt, Au, Hg, all with d⁸

configurations, and n ) 1 to 4. As can be seen, for [M(CO)₄]²⁺

,

M ) Pd, Pt, both M and C are positively charged. The calculated

partial charges on the four C-atoms (0.56 or 0.55) are slightly

higher than those for Pd (0.43) and Pt (0.44), which would favor

C- -F over M- -F contacts.

To judge the strength of the interionic contacts, we have

chosen to use the van der Waals radii of Bondi.¹⁶²It is

convenient to arbitrarily classify those contacts, which are

equal or shorter than the sum of the van der Waals radii¹⁶²by

10% or more as “significant” and those between this “signifi-

cance” limit and the sum of the van der Waals radii as

“marginal”. For C- -F interactions, the corresponding contact

distances are 2.85 and 3.17 Å, respectively.¹⁶²

The relevant sums of van der Waals radii are summarized in

Table S1, where the observed M- -F and C- -F interionic contacts

are listed. In addition a number of relatively long F- -F and

O- -F contacts are observed, which are seemingly due to repul-

sive interactions. Occasionally marginal O- -F contacts are ob-

served in bifurcated interactions together with stronger C- -F

contacts for the same carbonyl group. The M- -F and C- -F

contacts for Pd, Pt, and each of the four carbon atoms (C(1) to

C(4)) are listed in order of decreasing strength. Also listed for

each carbon atom are d(C- -F)_avvalues for those contacts, that

are shorter than the sum of the van der Waals radii (3.17 Å).

Since a different numbering system is used for both [M(CO)₄]²⁺

cations, M ) Pd, Pt, individual data are not directly comparable.

The secondary¹⁵⁴interionic contacts for a formula unit of

(d) Interionic interactions in [M(CO)₄][Sb₂F₁₁]₂, M ) Pd,

Pt. Significant interionic contacts in superelectrophilic salts are

usually of the F- -C-type.^79-85This is not surprising, since in

octahedral^80,82,83or seven-coordinate^84,85metal carbonyl cations,

the central metal is not accessible for F atoms of the anion. For

linear [Hg(CO)₂]²⁺, both significant Hg- -F and C- -F contacts

are observed, but Hg- -F contacts are found only in bifurcated

fluorine bridges (see Figure S5). The Hg- -F contacts produce

a distorted octahedral environment for the central metal by 2:4

(162) Bondi, A. J. Phys. Chem. 1964, 68, 441.

598 J. Am. Chem. Soc., Vol. 123, No. 4, 2001

Willner et al.

interionic contacts, it is difficult to state clearly, why [Pt(CO)₄]-

[Sb₂F₁₁]₂has a smaller unit cell volume than [Pd(CO)₄][Sb₂F₁₁]₂

(see Table 1 or 5). The only discernible differences in the two

salts are the slightly wider Sb-F_b-Sb bridge angles R in both

conformers (about 2.0-2.5°) and smaller dihedral angle ψ of

the C_2νform of the [Sb₂F₁₁]^-anion in the Pt(II) compound,

which allow for very slightly tighter packing. In any event, the

very small contraction in unit cell volume by 0.4% appears to

be due to very subtle structural differences between both

[M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, salts. (vi) As can be seen in

Figure 3 all eight significant C- -F interactions involve only

equatorial F-atoms of the anions. The unprecedented C_2ν

conformation of the [Sb₂F₁₁]^-anion is ideally suited to form

four significant C- -F contacts to match the four C atoms of

the square planar [M(CO)₄]²⁺cation, M ) Pd, Pt. In the

staggered C₁conformer, F(21) forms two very strong bifurcated

contacts (2.771(10) and 2.697(10) Å) to C(2) and C(3), while

the adjacent F(18) atom is involved in slightly longer contacts

(2.848(9) and 2.889(9) Å) to C′(1) and C′(4) atoms of a neighbor

cation (see Figure 3). (vii) The involvement of all four C-atoms

of the [M(CO)₄]²⁺cations, M ) Pd, Pt, in forming four to five

secondary interionic contacts, explains the relatively high

thermal stabilities of the salts.

It is tempting to attribute the observed formation of COF₂

during the thermal decomposition of [M(CO)₄][Sb₂F₁₁]₂(vide

supra) to the existence of C- -F contacts in the solid-state

structures of the salts, but at the decomposition temperatures

of 150-200 °C fluorination of CO by SbF₅is a possibility¹⁶⁶

as well. In contrast to these observations, for several polycar-

bonyl adducts of Ag(I) and Cu(I),^{42,43,140,167}the reversible

dissociation and re-addition of CO has been observed at ambient

conditions.

Figure 3. Significant and marginal secondary contacts within a formula

unit of [Pt(CO)₄][Sb₂F₁₁]₂.

[Pt(CO)₄][Sb₂F₁₁]₂are shown in Figure 3. An identical arrange-

ment due to secondary contacts is found for [Pd(CO)₄][Sb₂F₁₁]₂.

The following conclusions are reached: (i) The interionic

interactions observed for [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, are

almost exclusively of the C- -F type. They contribute, as can

be seen in Figures 3 and S2, to tight packing of anions and

cations in the unit cell, as well as to the formation of extended

structures. (ii) There are for each of the four CO-groups four to

five C- -F interactions. Of these approximate 20 contacts, eights

two for each carbonsare termed “significant” (between

2.592(10)-2.791(10) Å). They are involved in direct C- -F

contacts within a formula unit as seen in Figure 3. Additional

borderline or marginal contacts reflect interionic interactions

to adjacent [Pt(CO)₄]²⁺cations (see Figure S2 for the packing

arrangement within the unit cell) and aid in the formation of

extended structures. (iii) There is in each salt only a single,

very marginal M- -F contact, M ) Pd, Pt, which is formed as

a bifurcated M- -F- -C interaction together with a significant

C- -F contact. There is in the [M(CO)₄][Sb₂F₁₁]₂salts, M )

Pd, Pt, no evidence for a coordination expansion of the metal

from 4 to 6 (termed 4:2 coordination), which is frequently found,

for example, in related square planar gold(III) complexes such

Extended structures are also found for the square planar

isostructural molecular complexes cis-M(CO)₂(SO₃F)₂, M )

5

Pd,⁷²Pt,⁷³and cis-Pt(CO)₂Cl₂which are all structurally

characterized. For example, for cis-Pd(CO)₂(SO₃F)₂,⁷²four inter-

or intramolecular C- -O contacts in the range of 2.839(6)-

3.172(6) Å are found for each CO group with only a single

marginal Pd- -O interlayer contact of 3.007(4) Å.⁷²In cis-

5

Pt(CO)₂Cl₂the square planar molecules are stacked into

columns by intermolecular C- -Cl contacts. In all instances,^5,72,73

the observed secondary contacts are weaker and less numerous

than they are in the [M(CO)₄][Sb₂F₁₁]₂salts, M ) Pd, Pt.

For [Rh(CO)₄][Al₂Cl₇],⁹⁵the shortest long-range C- -Cl

contact is with 3.366 Å just barely below the sum of the van

der Waals radii¹⁶²of 3.45 Å, while the shortest Rh- -Cl contact

of 3.746 Å is longer than 3.40 Å, estimated as the sum of the

van der Waals radii.¹⁶²Significant interactions are, as stated

above, commonly absent in metal carbonyl salts with complex

charges of +1 or less, as in linear [Au(CO)₂][Sb₂F₁₁]^67,134and

octahedral [Re(CO)₆][Re₂F₁₁].¹⁶⁰For the recently synthesized

compound [Rh(CO)₄][Sb₂F₁₁]⁹⁵the vibrational analysis suggests

D_4hsymmetry for both the cation and the anion. This is

suggestive of the absence of significant interionic interactions.

For [Rh(CO)₄][1-Et-CB₁₁F₁₁]⁹⁴the only interionic interactions

listed are a single Rh- -H contact of 3.21 Å and 5 Rh- -F contacts

for the formula unit in the range of 3.220(9)-3.588(9) Å which

are claimed to affect ν˜(CO)_avrelative to matrix isolated

[Rh(CO)₄]⁺.^168-170However, all secondary interionic contacts

to rhodium listed⁹⁴are longer than the sum of the van der Waals

165

as AuF₃,¹⁶³Au(SO₃F)₃,¹⁶⁴or Au[AuF₄]₂in their solid-state

structures. (iv) The strength of the interionic C- -F contacts in

both [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, salts is very similar. The

shortest contact of 2.591(10) Å is found for the Pt(II) salt,

closely followed by a C- -F contact of 2.613(7) Å for the Pd(II)

complex; however the average C- -F contacts are with 2.865

(Pt) and 2.866 (Pd) Å identical for both salts. (v) With very

similar internal bond parameters for both cations (see Table 2)

and the four anions in both salts, as well as nearly identical

(163) Einstein, F. W. B.; Rao, P. R.; Trotter, J.; Bartlett, N. J. Chem.

Soc. A 1967, 478.

(164) Willner, H.; Rettig, S. J.; Trotter, J.; Aubke, F. Can. J. Chem.

1991, 69, 391.

(166) Emele´us, H. J.; Wood, J. F. J. Chem. Soc. 1948, 2183.

(167) Strauss, S. H. J. Chem. Soc., Dalton Trans. 2000, 1.

(168) Zhou, M. F.; Andrews, L. J. Phys. Chem. A 1999, 103, 7773.

(169) Zhou, M. F.; Andrews, L. J. Am. Chem. Soc. 1999, 121, 9141.

(170) Zhou, M. F.; Andrews, L. J. Chem. Phys. 1999, 111, 4548.

(165) Schmidt, R.; Mu¨ller, B. G. Z. Anorg. Allg. Chem. 1999, 625, 605.

[Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂

J. Am. Chem. Soc., Vol. 123, No. 4, 2001 599

radii¹⁶²and are seemingly not due to attractive interactions.⁹⁴

In [Rh(CO)₄][1-Et-CB₁₁F₁₁] there are also 14 marginal C- -F

contacts, three or four for each CO, between 2.926 and 3.155

Å, which are all shorter than the sum of the van der Waals

radii of 3.17 Å.¹⁶²These contacts have been omitted by the

authors,⁹⁴but are more likely to affect ν˜(CO)_avthan the long

Rh- -F contacts.

vibrational fundamentals. Of the 13 observable fundamentals,

only the three CO-stretching vibrations for [M(CO)₄]²⁺, M )

Pd,⁷⁵Pt,^74,75,86and [M(CO)₄]⁺, M ) Rh,^94,149,151Ir,¹⁴⁹have been

previously reported and have been discussed in the preceding

section. In this study, we have made use of the salts [Pd(CO)₄]-

[Sb₂F₁₁]₂,⁷⁵[Pt(CO)₄][Sb₂F₁₁]₂,⁷⁵[Pt(CO)₄][Pt(SO₃F)₆],⁷⁴[Pt(CO)₄]-

[PtF₆],⁸⁶[Rh(CO)₄][Sb₂F₁₁],⁹⁵and [Rh(CO)₄][Al₂Cl₇]⁹⁵as well

as their ¹³C-isotopomers, to arrive at a reasonably complete set

of vibrational data for the [M(CO)₄]ⁿ⁺cations, M ) Pd, Pt (n

)2), Rh (n ) 1).

The vibrational wavenumbers for the salts [M(CO)₄][Sb₂F₁₁]₂,

M ) Pd, Pt, are listed in Table 7. The IR and Raman spectra

for [Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂are depicted in

Figures 4 and 5. As can be seen, the spectra in the range of

720-500 and 320-200 cm^-1are dominated by bands due to

the [Sb₂F₁₁]^-anion. This is not unexpected, because the two

anions in both salts are of low symmetry (C₁or ∼C_2ν) and are

not symmetry-related. In contrast the [Sb₂F₁₁]^-ions in [Au-

(CO)₂][Sb₂F₁₁]⁶⁷and in [Rh(CO)₄][Sb₂F₁₁]⁹⁵have D_4hsymmetry

and only three Raman and three IR bands are observed for the

anion with no coincidences.

Due to the unique nature of square planar M(CO)₄moieties

in metal carbonyl chemistry^14-17,25there are no isoelectronic

and isosteric metal carbonyl complexes, which could serve as

precedents for [M(CO)₄]²⁺, M ) Pd, Pt, in a manner similar to

M(CO)₆, M ) Cr, Mo, W, or [Re(CO)₆]⁺, which have all been

completely characterized by various vibrational methods.^178-180

A comparison with these species is very useful in the vibrational

analyses of the cations [M(CO)₆]²⁺, M ) Fe,⁸⁰Ru, Os,⁸²where

all vibrational modes are experimentally detected. It is hence

not surprising, that only 10 of the 13 observable fundamentals

for [M(CO)₄]²⁺, M ) Pd, Pt, are identified and assigned. This

is similar to the situation for the isoelectronic cyano anions

[M(CN)₄]^2-, M ) Pd, Pt,^178,181where, due to solvent interference

and cation dependency of vibrational modes, assignments are

incomplete and occasionally ambiguous.¹⁷⁸

For the isostructural pair [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt,

three interesting observations are made: (i) The intensities of

the cation bands in the IR spectra relative to those due to the

anions are higher for the platinum compound than for its

palladium analogue (see Figures 4 and 5), in accordance also

with calculated band intensities (Table S3). In addition, the

separations between the three CO stretching modes ν₁, ν₆, and

In summary the interionic C- -F contacts observed for

[M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, here are stronger and more

numerous than in other [Sb₂F₁₁]^-salts of superelectrophilic

metal carbonyl cations such as [Hg(CO)₂]^{2+ 77}

,

[M(CO)₅Cl]²⁺

,

M ) Rh, Ir,^78,83and [M(CO)₆]²⁺, M ) Fe, Ru, Os.^80,81,82With

the exception of the four Hg- -F contacts already discussed, there

is no evidence for other than occasional, very marginal single

M- -F contacts in any of the above-mentioned superelectrophilic

metal carbonyl salts as well as in the structures of [Au(CO)₂]-

[Sb₂F₁₁],^67,134[Au(CO)₂]₂[Sb₂F₁₁‚SbF₆],¹⁶¹[Re(CO)₆][Re₂F₁₁],¹⁶⁰

[W(CO)₆(FSbF₅)][Sb₂F₁₁],⁸⁴polymeric[{Mo(CO)₄}₂(cis-µ-

F₂SbF₄)₃]_x[Sb₂F₁₁]_x,⁸⁵[M(CO)₆][SbF₆]₂, M ) Fe, Ru, Os,^80,81,82

and, using accepted criteria, in [Rh(CO)₄][1-Et-CB₁₁F₁₁].⁹⁴

Similarly there are no significant M- -O contacts in the

fluorosulfates cis-M(CO)₂(SO₃F)₂, M ) Pd,⁷²Pt,⁷³and mer-

146

Ir(CO)₃(SO₃F)₃nor are there significant M- -Cl contacts in

the metal carbonyl chlorides cis-Pt(CO)₂Cl₂,⁵Au(CO)Cl¹⁷¹and

the salt [Rh(CO)₄][Al₂Cl₇].⁹⁵The frequently observed significant

C- -X, X ) F, O, Cl, contacts are found in predominantly

σ-bonded metal carbonyl cations, in agreement with views on

the strongly polarized C-O group in these cations.¹⁷²

While we know of no occasion where in thermally stable

cationic metal carbonyl salts M- -X, X ) F, Cl, O, interactions

play a significant or dominant role, we also know of no instance

where in metal carbonyls of limited thermal stability, which

readily lose CO, like the polycarbonyls of silver(I) or cop-

per(I),^42,167,173or the recently reported adducts of the type

[HB(3.5‚(CF₃)₂pz)₃]M(CO), pz ) pyrazol; M ) Cu, Ag,

Au,^174-176any significant secondary contacts to carbon of the

CO ligand are reported.

The presence of electrophilic carbon centers and the genera-

tion in superacid media establish a strong link between the

superelectrophilic⁴⁹metal carbonyl cations such as [M(CO)₄]²⁺

,

M ) Pd, Pt, discussed here and the large family of carbo-

cations.⁵⁰There is a recent example¹⁷⁷where fluoro-substituted

carbocations are stabilized by [As₂F₁₁]^-to form stable com-

pounds with the anion in a ∼C₁conformation similar to the

conformation of the [Sb₂F₁₁]^-anion commonly encountered in

metal carbonyl cations^77,80,83-85(see Table 6).

ν

₁₃are slightly wider for the Pt complex (22 and 23 cm^-1) than

for the Pd compound (15 and 14 cm^-1). These observations

suggest a stronger ionic contribution to the Pd-C bond relative

to those for the Pt-C bond. (ii) While ν˜(CO)_avand f_COvalues

for both cations are very similar, there are large variations in

the skeletal vibrations. The M-C stretching modes ν₂, ν₇, and

Spectroscopic Characterization of [M(CO)₄]ⁿ⁺, M ) Rh,

Ir, Pd, Pt; n ) 1, 2. (a) Vibrational Spectroscopy. For square

planar cations of the type [M(CO)₄]ⁿ⁺, M ) Rh, Ir, Pd, Pt; n )

1 or 2, the irreducible representations of fundamental vibrations

and their activities are:

ν₁₅and two of the M-C-O deformation modes are all higher

for [Pt(CO)₄]²⁺than for [Pd(CO)₄]²⁺. (iii) The totally symmetric

A_1gCO-stretching mode is also observed in the IR spectra of

both cations as a weak band. The break-down of the mutual

exclusion rule appears to be due to the observed small departure

from 90° C-M-C angles and from planarity (Figure S4), due

Γ_vib) 2 A_1g(Ra, p) + A_2g(-) + 2 A_2u(IR) +

2 B_1g(Ra, dp) + 2 B_2g(Ra, dp) + 2 B_2u(-) +

E_g(Ra, dp) + 4 E_u(IR)

(177) Christe, K. O.; Zhang, X.; Bau, R.; Hegge, J.; Olah, G. A.; Prakash,

G. K. S.; Sheehy, J. A. J. Am. Chem. Soc. 2000, 122, 481 and references

therein.

(178) Jones, L. H. Inorganic Vibrational Spectroscopy; Marcell

Decker: New York, 1971; Vol. 1, p 129.

(179) Jones, L. H.; McDowell, R. S.; Goldblatt, M. Inorg. Chem. 1969,

8, 2349.

with seven Raman active, six IR active and three inactive

(171) Jones, P. G. Z. Naturforsch., B: Chem. Sci. 1982, 37, 823.

(172) Goldman, A. S.; Krogh-Jesperson, K. J. Am. Chem. Soc. 1996,

118, 12159.

(173) Rack, J. J.; Webb, J. D.; Strauss, S. H. Inorg. Chem. 1996, 35,

277.

(174) Dias, H. V. R.; Lu, H.-L. Inorg. Chem. 1995, 35, 5380.

(175) Dias, H. V. R.; Jin, W. J. Am. Chem. Soc. 1995, 117, 11381.

(176) Dias, H. V. R.; Jin, W. Inorg. Chem. 1996, 35, 3694.

(180) Abel, E. W.; McLean, R. A. N.; Tyfield, S. P.; Braterman, P. S.;

Walker, A. P.; Hendra, P. J. J. Mol. Spectrosc. 1969, 30, 29.

(181) Sweeny, D. M.; Nakagawa, I.; Mizushima, S.; Quagliano, J. V. J.

Am. Chem. Soc. 1956, 78, 889.

(182) Cotton, F. A.; Kraihanzel, C. S. J. Am. Chem. Soc. 1962, 84, 4432.

600 J. Am. Chem. Soc., Vol. 123, No. 4, 2001

Willner et al.

Table 7: Observed Vibrational Wavenumbers (cm^-1) for [M(CO)₄][Sb₂F₁₁]₂, M ) Pd,Pt and Estimated Band Intensities, Assignment, and

Description of Vibrational Modes

[Pd(CO)₄][Sb₂F₁₁]₂

[Pt(CO)₄][Sb₂F₁₁]₂

assignments

[M(CO)₄]⁺

description

[Sb₂F₁₁]^-

IR

int

Ra

int

IR

int

Ra

int

D_4h

2278

(vw)^a

2278

2263

(vs)

(s)

2288

(vw)

2289

2267

(vs)

(s)

ν₁

A_1g

B_1g

E_u

ν_s(CO)^b

ν_as(CO)

ν_as(¹³CO)

ν₆

2249

2207

718

(vs)

(w)

(sh)

(vs)

2244

2204

712

(vs)

(w)

(vs)

ν₁₃

ν₁₃′

E_u

714

709

697

(w)

(sh)

(w)

714

704

695

(w)

708

ν (SbF_ax)

}

690

(vs)

689

(vs)

686

(m)

686

(m)

675

662

(s)

674

664

(s)

668

656

649

598

585

(s)

(w)

668

657

649

594

587

(m)

(s)

(m)

(w)

ν (SbF_4eq

)

648

604

596

503

484

(sh)

(vw)

(w)

(m)

648

604

596

502

518

(sh)

(w)

(s)

}

ν (SbFSb)

ν₁₄

ν₈

E_u

δ (MCO)

ν_s(MC)

ν_as(MC)

δ (MCO)

437

(vw)

n.o.

B_2g

A_2u

A_1g

B_1g

E_u

427

(m)

473

(s)

ν₄

383

363

(w)

(vw)

436

408

(w)

(vw)

ν₂

ν₇

ν₁₅

ν₁₂

336

(sh)

360

(w)

n.o.

341

(vw)

E_g

315

305

(s)

313

305

(s)

(m)

}

δ(SbF_ax)

δ(SbF_eq)

δ(FSbF)

305

275

(m)

(w)

304

277

(m)

(w)

267

250

228

196

136

(vs)

(sh)

(vw)

268

250

228

197

134

(vs)

(sh)

(vw)

230

201

135

95

(m)

(vw)

(m)

231

201

139

99

(m)

(vw)

(m)

ν₉

B_2g

δ (CMC)

^aw ) weak, m ) medium, s ) strong, v ) very, sh ) shoulder. ^bs ) symmetric, as ) asymmetric, ν ) stretching mode, δ ) deformation

mode.

Figure 4. The IR and Raman spectra for [Pd(CO)₄][Sb₂F₁₁]₂.

Figure 5. The IR and Raman spectra for [Pt(CO)₄][Sb₂F₁₁]₂.

to secondary contacts in both salts and the presence of two

different [Sb₂F₁₁]^-anions in both compounds. In contrast for

[M(CO)₆][Sb₂F₁₁]₂and [M(CO)₆][SbF₆]₂, M ) Fe,⁸⁰Ru, Os,⁸²

secondary interionic contacts are weaker and less numerous than

here. The anions are symmetry-related, and no departure from

O_hsymmetry is noted in the vibrational spectra.^79-82

The experimental and calculated vibrational wavenumbers for

the square planar cations [M(CO)₄]ⁿ⁺, M ) Rh, Pd, Pt; n ) 1,

2, are listed in Table 8. The assignments are supported by

experimental and calculated ¹³C isotope shifts (Table S2) and

intensities of IR and Raman bands (Table S3). The inclusion

of data for [Rh(CO)₄]⁺is warranted, as a result of the vibrational

analysis of [Rh(CO)₄][Sb₂F₁₁]⁹⁵and [Rh(CO)₄][Al₂Cl₇],⁹⁵where

9 of the observable 13 vibrational fundamentals are detected.

For [Ir(CO)₄]⁺; however, the vibrational information is at present

limited to two Raman lines in the CO stretching range, which

are observed on melts in AlCl₃(Table S4).¹⁴⁹

As can be seen in Table S3 the “missing” fundamentals are

of rather low calculated intensities (e.g., ν₅(A_2u), ν₈(B_2g), or

ν

₁₆(E_u)) or are like ν₁₂(E_g) for [Pd(CO)₄]²⁺, seemingly obscured

by anion bands. Since [Pd(CO)₄]²⁺is only stabilized by

[Sb₂F₁₁]^-, unlike [Pt(CO)₄]^{2+ 74,75,86}or [Rh(CO)₄]⁺,^94,95avoid-

ance of anion interference is not possible. In addition to their

predicted low intensities, ν₅, ν₉, and ν₁₆are expected to fall

below 100 cm^-1, which makes their observation difficult.

As can be seen in Table 8, agreement between experimental

and calculated band positions is reasonable, considering that

[Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂

J. Am. Chem. Soc., Vol. 123, No. 4, 2001 601

Table 8: Experimental and Calculated Vibrational Wavenumbers (cm^-1) for the Square Planar (D_4h) Cations [M(CO)₄]ⁿ+ (M ) Rh, Pd, Pt)

cation:

[Rh(CO)₄]⁺

calcd

[Pd(CO)₄]⁺

calcd

[Pt(CO)₄]⁺

calcd

vibrational modes

expt^a

∆ν

expt^a

∆ν

expt^a

∆ν

v₁

v₂

v₃

v₄

v₅

v₆

v₇

v₈

A_1g

A_2g

A_2u

B_1g

B_2g

B_2u

E_g

v(CO)

v(MC)

δ(MCO)

δ(CMC)

v(CO)

2214

406^c

(i.)^d

465

n.o.^d

2174

406^c

n.o.

(i.)

2198

423

320

437

54

2150

404

481

91

430

30

316

2119

545

353

86

-16

+17

2278

383

(i.)

427

n.o.

2263

363

437

95

(i.)

n.o.

2249

484

336

n.o.

2258

374

305

424

85

2236

349

432

90

407

56

298

2221

482

330

93

-20

-9

2289

436

(i.)

473

n.o.

2267

408

n.o.

99

(i.)

341

2244

518

360

n.o.

2265

424

325

469

83

2237

399

462

96

444

56

319

2216

513

340

95

-24

-12

-28

-3

-4

-24

-2

-27

-14

-5

-30

-9

v(MC)

δ(MCO)

δ(CMC)

δ(MCO)

δ(CMC)

δ(MCO)

ν(CO)

δ(MCO)

ν(MC)

δ(CMC)

v₉

-5

-3

v₁₀

v₁₁

v₁₂

v₁₃

v₁₄

v₁₅

v₁₆

(i.)

306

2137

543

331

n.o.

+10

-18

+2

-22

-28

-5

E_u

-28

-2

-6

+22

-20

b

^a[Sb₂F₁₁]^-salt, ref 95. AlCl₃melt, ref 149. ^cν₂and ν₇are accidentally degenerated. ^dAbbreviations: n.o. ) not observed; i. ) inactive mode.

substantially lower than those for [Ir(CO)₄]⁺and [Pt(CO)₄]²⁺

,

Table 9: Calculated and Experimental C-O and M-C Force

Constants f[10²N m^-1] for Square Planar [M(CO)₄]ⁿ⁺Cations

respectively. The apparent weakness in the Pd-CO bonds is

reflected in the observed ν(M-C)- and δ(MCO)-stretching and

deformation wavenumbers in Tables 8 and S4, when compared

to the corresponding fundamentals for [Pt(CO)₄]²⁺. The differ-

ence in bond strength between [Pd(CO)₄]²⁺and [Pt(CO)₄]²⁺is

consistent with differences in thermal stability for [M(CO)₄]-

[Sb₂F₁₁]₂, M ) Pd, Pt, and the results of the DSC measurements

discussed above. The rather weak Pd-CO bonds in [Pd(CO)₄]²⁺

provide a good explanation why there are so few Pd(II)

carbonyls known with two or more terminal CO ligands^18-23

in comparison to the more numerous Pt(II) carbonyls of this

type.

f_CO

f_MC

cation

experimental^a

BP86/ECP2^b

[Rh(CO)₄]⁺

[Ir(CO)₄]⁺

19.00

18.50

18.39

19.75

19.86

19.79

20.29

19.62

2.18

2.69

1.73

2.20

1.76

0.80

[Ni(CO)₄]²⁺

[Pd(CO)₄]²⁺

[Pt(CO)₄]²⁺

[Au(CO)₄]³⁺

[Hg(CO)₄]⁴⁺

20.63

20.64

^aCalculated according to the Cotton-Kraihanzel approximation.¹⁸²

^bHarmonic force constants from ref 130.

The observed weakness of the Pd-CO bond, reflected in the

the calculations refer to isolated cations and do not account for

the role interionic contacts play. Some of the deviations are

systematic and reflect well-documented trends in BP86/ECP2

results:^{98-100,128-130}for example, CO-stretching frequencies in

transition metal carbonyls are on average underestimated by 28

cm^-1at this level,^98-100which reflects a similar error for the

free CO molecule.¹²⁸For the salts [M(CO)₄][Sb₂F₁₁]₂, M ) Pd,

Pt, the difference between calculated and experimental wave-

numbers, expressed as ∆ν, falls between -2 and -30 cm^-1. In

other [Sb₂F₁₁]^-salts with cations such as [M(CO)₅Cl]²⁺, M )

Rh, Ir,⁸³or [M(CO)₆]²⁺, M ) Fe,^79,80Ru, Os,^81,82ν(CO)-

stretching vibrations are also observed at wavenumbers higher

than the calculated values.^83,98-100

trend for the M-C force constants f_M-C([Pt(CO)₄]²⁺

>

[Rh(CO)₄]⁺> [Pd(CO)₄]²⁺), is not apparent from the structural

data for the isostructural pair [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt,

but is revealed by the vibrational analysis presented above for

the tetracarbonyl cations. A detailed bonding discussion for

isostructural cis-M(CO)₂(SO₃F)₂, M ) Pd, Pt, has appeared very

recently.⁷³

Additional results of DFT calculations for the so far unknown

square planar d⁸cations [M(CO)₄]ⁿ⁺, Mⁿ⁺) Co⁺, Ni²⁺, Au³⁺

and Hg⁴⁺are found in the Supporting Information (Tables S4-

S6). In the 5d series of isoelectronic D_4h-cations [M(CO)₄]ⁿ⁺

ν˜(CO) and f_COgradually increase in the order of Ir⁺< Pt²⁺

<

Au³⁺but then decrease for Hg⁴⁺. A similar trend is found for

For the three [Pt(CO)₄]²⁺salts an interesting anion depend-

octahedral cations of the type [M(CO)₆]^{n+ 83,98-100}

where the

,

ency is noted in the CO-stretching region: [Pt(CO)₄][Sb₂F₁₁]₂,⁷⁵

strength of the CO bond increases in the order of Re⁺< Os²⁺

< Ir³⁺< Pt⁴⁺but declines for Au⁵⁺. It is noteworthy, that the

species with a calculated maximum of predicted CO bond

Ra, 2289, 2267 cm^-1IR, 2244 cm^-1, ν˜(CO)_av2261 cm^-1

;

[Pt(CO)₄][Pt(SO₃F)₆],⁷⁴Ra 2281, 2257 cm^-1, IR, 2235 cm^-1

ν˜(CO)_av, 2252 cm^-1; [Pt(CO)₄][PtF₆]⁸⁶Ra 2276, 2248 cm^-1

,

strength, square planar [Au(CO)₄]³⁺and octahedral [Pt(CO)₆]⁴⁺

,

IR 2223 cm^-1with ν˜(CO)_av2242.5 cm^-1. With increasing

nucleophilicity of the anion,¹³¹all three CO stretches and

ν˜(CO)_avdecrease by about 10 cm^-1. Of interest in this context

are recent matrix isolation studies,^168-170where ν˜(CO) in the

IR spectrum of matrix isolated (Ne) [Rh(CO)₄]⁺is found at

have so far remained undetected. The latter cation is not formed

in the reductive carbonylation of PtF₆⁸⁶as hoped, but formation

of [Pt(CO)₄]²⁺is observed instead.

(b) ¹³C NMR Spectroscopy. The spectroscopic characteriza-

2162 cm^{-1 170}compared to 2137 cm^-1in [Rh(CO)₄][Sb₂F₁₁].⁹⁵

,

tion of the square planar cations [Rh(CO)₄]⁺and [M(CO)₄]²⁺

,

M ) Pd, Pt, in this study is completed by ¹³C NMR measure-

ments on ¹³C-enriched samples. For [Rh(CO)₄]⁺in HSO₃F at

-80 °C, a ¹³C chemical shift of 173 ppm is noted. Since

[M(CO)₄]²⁺, M ) Pd, Pt, are converted in HSO₃F to cis-

M(CO)₂(SO₃F)₂, ¹³C MAS NMR spectra are recorded on solid

[M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt. The chemical shifts are 144

and 137 ppm, respectively. The observation that ν˜(CO)_av

Therefore the ν˜(CO) values in BP86/ECP2 calculations are

possibly even more underestimated than indicated by the data

in Table 8.

To summarize some of the observed trends, experimental and

calculated force constants are listed in Table 9. While f_COvalues

for the two pairs of 4d and 5d cations remain reasonably

constant, the f_MCvalues for [Rh(CO)₄]⁺and [Pd(CO)₄]²⁺are

602 J. Am. Chem. Soc., Vol. 123, No. 4, 2001

Willner et al.

increases with increasing nuclear charge, while δ¹³C decreases

within the 4d or 5d series is well established, on the basis of

experimental data for [M(CO)₆]-species.^48,78

for the three principal coordination geometries, linear, square

planar, and octahedral. In all superelectrophilic [M(CO)_n]-

[Sb₂F₁₁]_msalts, C- -F interionic contacts are observed and play

an important role, while M- -F interactions are either irrelevant

or not observed.

A similar decrease in ¹³C chemical shifts vertically down a

group is well documented for octahedral metal carbonyl species

such as [M(CO)₆]²⁺, M ) Fe, Ru, Os,^48,79-82in group 8, while

ν˜(CO)_avand f_COremain almost constant. The decrease in ¹³C

chemical shifts in a given group appears to be determined by

paramagnetic contributions to the chemical shift⁹⁶for both

octahedral and square planar carbonyl cations.

Acknowledgment. Financial support by the Natural Sciences

and Engineering Research Council of Canada NSERC (operating

funds to F.A. and J.T.); Deutsche Forschungsgemeinschaft DFG

(operating funds to H.W., PdF scholarship to M.B.); Fonds der

Chemischen Industrie (operating funds to H.W.); Schweiz-

erischer Nationalfonds (grant to W.T.); Alexander v. Humboldt

Foundation (F. Lynen Fellowship to R.B., Research Prize and

re-invitation of F.A.); North Atlantic Treaty Organization NATO

(collaborative research grant H.W. and F.A.) is gratefully

acknowledged. Dr. G. Balzer (Universita¨t Hannover) is thanked

for his help in obtaining ¹³C NMR spectra. We are indebted to

Dr. Brian Patrick (UBC - X-ray crystallography) and Ms.

Elizabeth Varty (UBC - Illustrations), who made useful

contributions. Dr. Q. Xu and Professor Y. Souma are thanked

for preprints of their recent work. Degussa Hu¨ls is thanked for

a gift of Pd and Pt metal powder.

1

Spin coupling constants J(M-¹³C) are only obtainable for

[Rh(CO)₄]⁺and [Pt(CO)₄]²⁺. For [Rh(CO)₄]⁺, where ¹⁰³Rh

1

(100% abundance) with s ) /₂, J(¹⁰³Rh-¹³C) is found to be

61.1 Hz. For [Pt(CO)₄]²⁺with ¹⁹⁵Pt (33.8% abundance) of s )

1

¹/₂, J(¹⁹⁵Pt-¹³C) is found to be 1550 ( 10 Hz in a ¹³C MAS

spectrum and 1576 ( 2 Hz for a solution of [Pt(CO)₄][Sb₂F₁₁]₂

in HSO₃F.

Summary and Conclusions

The extensive synthetic, structural, spectroscopic, and com-

putational study of [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, described

here, establishes the square planar [M(CO)₄]²⁺cations as an

essential part of a series of thermally stable, superelectrophilic

metal carbonyl cations. The complete series extends in the 5d-

block from groups 12 to 6 and includes linear [Hg(CO)₂]^{2+ 76,77}

and [Au(CO)₂]⁺(D_∞h),^64,67square planar [Pt(CO)₄]²⁺(D_4h),

octahedral [Ir(CO)₅Cl]²⁺(C_4ν),^78,83[Os(CO)₆]²⁺(O_h),^81,82and

[Re(CO)₆]⁺(O_h),¹⁶⁰and seven-coordinate [W(CO)₆(FSbF₅)]⁺

(C_2ν).⁸⁴All can be generated only in superacidic media, ^46,47,51-62

and all cations are almost exclusively stabilized by the superacid

anion [Sb₂F₁₁]^-.⁶³

All members in this group are structurally characterized by

single-crystal X-ray diffraction. Important members such as

[Au(CO)₂]⁺,⁶⁷[Pt(CO)₄]²⁺, [Ir(CO)₅Cl]²⁺,⁸³and [Os(CO)₆]^{2+ 81,82}

have also been extensively investigated by a complete vibra-

tional analysis and DFT calculations. With the recently com-

pleted molecular structure determinations of cis-M(CO)₂(SO₃F)₂,

M ) Pd, Pt,^72,73cis-Pt(CO)₂Cl₂,⁵and of the cation [Rh(CO)₄]⁺

with two different anions, [1-Et-CB₁₁F₁₁]^{- 94}and [Al₂Cl₇]^{- 95}

the square planar coordination geometry is now very well

represented for metal carbonyl compounds.

Supporting Information Available: Dimensions and design

of a Monel-Kel-F reactor used for solution in HF-SbF₅; a

stereoview of the unit cells of [M(CO)₄][Sb₂F₁₁]₂; the molecular

structure of [Pd(CO)₄][Sb₂F₁₁]₂showing one formula unit; the

[M(CO)₄]²⁺cations, M ) Pd, Pt; selected bifurcated interionic

interactions of [Hg(CO)₂][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂; the

[Sb₂F₁₁]^-anions in [Pd(CO)₄][Sb₂F₁₁]₂; interionic contacts [Å]

in [M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt; calculated and experimental

¹³C isotope shifts for the cations [M(CO)₄]ⁿ⁺, M ) Rh, Pd, Pt,

n ) 1, 2; calculated and experimental vibrational band intensities

for [M(CO)₄]ⁿ⁺, M ) Rh, Ir, Pd, Pt; calculated vibrational

wavenumbers (cm^-1) and ¹³CO shifts (cm^-1) for square planar

(D_4h) cations [M(CO)₄]ⁿ⁺, M ) Co, Ir, Ni, Au, Hg; calculated

band intensities for [Co(CO)₄]⁺, [Ni(CO)₄]²⁺, [Au(CO)₄]³⁺and

[Hg(CO)₄]⁴⁺; calculated vibrational frequencies (cm^-1) at BP86/

ECP1 for square planar cations [M(CO)₄]ⁿ⁺, M ) Co, Rh, Ir (n

) 1); Ni, Pd, Pt (n ) 2); Au (n ) 3) and Hg (n ) 4) (PDF).

X-ray crystallographic files in CIF format for the complex salts

[Pd(CO)₄][Sb₂F₁₁]₂and [Pt(CO)₄][Sb₂F₁₁]₂. This material is

available free of charge via the Internet at http://pubs.acs.org.

The detailed structural and spectroscopic characterization of

[M(CO)₄][Sb₂F₁₁]₂, M ) Pd, Pt, completes the extensive

characterization of the superelectrophilic metal carbonyl cations

JA002360S

Article Doi

Superelectrophilic tetrakis(carbonyl)palladium(II)- and -platinum(II) undecafluorodiantimonate(V), [Pd(CO)4] [Sb2F11]2 and [Pt(CO)4][Sb2F11]2: Syntheses, physical and spectroscopic properties, their crystal, molecular, and extended structures, and density functional calculations

DOI: 10.1021/ja002360s

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Article abstract of DOI:10.1021/ja002360s

Full text of DOI:10.1021/ja002360s

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