Phospha-fischer carbenes: synthesis, structure, bonding, and reactions of pd(o) - and pt(o)-phosphenium complexes

Article abstract of DOI:10.1021/om800973v

The analogy between cationic group 10 metal - phosphenium complexes and Fischer carbenes has been formalized through structural and reactivity studies and by energy decomposition analysis (EDA) of the M - P bond. The studied compounds were the three-coord

Full text of DOI:10.1021/om800973v

990
Organometallics 2009, 28, 990–1000
Phospha-Fischer Carbenes: Synthesis, Structure, Bonding, and
Reactions of Pd(0)- and Pt(0)-Phosphenium Complexes
Christine A. Caputo,† Michael C. Jennings,† Heikki M. Tuononen,‡ and Nathan D. Jones*,†
Department of Chemistry, The UniVersity of Western Ontario, London, Ontario, Canada N6A 5B7, and
Department of Chemistry, UniVersity of JyVa¨skyla¨, P.O. Box 35, FI-40014 JyVa¨skyla¨, Finland
ReceiVed October 9, 2008
The analogy between cationic group 10 metal-phosphenium complexes and Fischer carbenes has been
formalized through structural and reactivity studies and by energy decomposition analysis (EDA) of the
M-P bond. The studied compounds were the three-coordinate, 16-electron species [(NHPMes)M(PPh3)2]OTf
(M ) Pt (1) and Pd (2); [NHPMes]+ is the N-heterocyclic phosphenium (NHP) cation, [PN(2,4,6-Me3-
C6H2)CH2CH2N(2,4,6-Me3-C6H2)]+, OTf ) trifluoromethanesulfonate); these were made by reaction of
[NHPMes]OTf with M(PPh3)4. The metal-phosphenium bond in both compounds was dominated by metal-
to-ligand π-donation. This differed from the M-C bonds in the analogous N-heterocyclic carbene (NHC)
complexes, (NHCMes)M(PPh3)2 (M ) Pt (6), Pd (7)), which were instead predominantly σ-type. Structural
determination of 1 by X-ray crystallography revealed the shortest yet reported Pt-P bond of 2.107(3) Å,
consistent with significant double-bond character, and trigonal planar geometries at both the P-atom within
the [NHPMes]+ ligand (∑(angles) ) 359.99°) and at the Pt-atom (∑(angles) ) 360.00°), which indicated
that 1 was better described as a Pt(0)-phosphenium rather than as a Pt(II)-phosphide. Reactions of 1
and 2 with excess PMe3 cleanly gave the four-coordinate species [(NHPMes)M(PMe3)3]OTf (M ) Pt (3)
and Pd (4)), while reaction of 1 with bis(diphenylphosphino)ethane (dppe) gave [(NHPMes)Pt(dppe-
κ2P)(dppe-κP)]OTf (5). Hydrolysis of these complexes resulted in metal hydrides and oxidation of the
NHP to phosphine oxide via intial nucleophilic attack of water at the P-atom in the coordinated [NHPMes +
ligand, which was calculated to bear a significantly positive charge in 1.
]
group elements, e.g., boron (boryls, :BR2 ),9,10 silicon (silylenes,
:SiR2),11,12 and phosphorus (phospheniums, :PR2+).13-17
N-Heterocyclic phosphenium cations (NHPs) are the “carbon
copies” of the now ubiquitous Arduengo-type carbenes
(NHCs).18 Both species may have saturated or unsaturated
“backbones”, but this report deals only with the saturated variety,
specifically the mesityl-substituted compound [NHPMes]OTf
(Chart 1). The N-heterocyclic carbenes and phospheniums are
isostructural and isovalent, but electronically inverse. NHCs are
strong σ-donors and weak π-acceptors, whereas NHPs are weak
σ-donors and good π-acceptors due to the formal positive charge
and isotropic s- (as opposed to directional sp2-) character of
the “lone pair” orbital on phosphorus.19 The two families should
therefore show reciprocal reactivity in transition metal chem-
istry.20
-
Introduction
The unfurling of new molecular metal chemistry relies to a
great extent on the discovery and development of novel ancillary
and reactive ligand types with unusual electronic and steric
demands. These initially strange ligands often permit the
generation of unfamiliar metal coordination geometries and the
stabilization of unusual oxidation states. They also impart unique
reactivity on the resulting complexes, which may in turn lead
to significant applications. There are few better examples than
that of metal-carbenes, which have revolutionized organome-
tallic chemistry and catalysis. The pioneering work in this area
by Fischer,1,2 Schrock,3,4 Grubbs,5 Arduengo,6 and Bertrand7,8
has been extended to include analogous complexes of the main
(9) Aldridge, S.; Coombs, D. L. Coord. Chem. ReV. 2004, 248, 535.
(10) Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice,
C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem.
ReV. 1998, 98, 2685.
(11) Okazaki, M.; Tobita, H.; Ogino, H. Dalton Trans. 2003, 493.
(12) Waterman, R.; Hayes, P. G.; Tilley, T. D. Acc. Chem. Res. 2007,
40, 712.
* Corresponding author. Fax: 1-519-661-3022. E-mail: njones26@
uwo.ca.
† The University of Western Ontario.
‡ University of Jyva¨skyla¨.
(1) Fischer, E. O.; Maasbo¨l, A. Angew. Chem., Int. Ed. Engl. 1964, 3,
580.
(2) Do¨tz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert, U.;
Weiss, K. Transition Metal Carbene Complexes; VCH: Weinheim, 1983.
(3) Schrock, R. R. J. Am. Chem. Soc. 1974, 96, 6796.
(4) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98.
(5) Nguyen, S. T.; Johnson, L. K.; Grubbs, R. H. J. Am. Chem. Soc.
1992, 114, 3974.
(6) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361.
(7) Lavallo, V.; Canac, Y.; Pra¨sang, C.; Donnadieu, B.; Bertrand, G.
Angew. Chem., Int. Ed. 2005, 44, 5705.
(13) Gudat, D. Coord. Chem. ReV. 1997, 163, 71.
(14) Nakazawa, H. J. Organomet. Chem. 2000, 611, 349.
(15) Nakazawa, H. In AdVances in Organometallic Chemistry; West,
R. H., A. F., Ed.; Academic Press: Amsterdam, The Netherlands, 2004;
Vol. 50, p 107.
(16) Cowley, A. H.; Kemp, R. A. Chem. ReV. 1985, 85, 367.
(17) Sanchez, M.; Mazie`res, M.-R.; Lamande´, L.; Wolf, R. Phosphorus
Compounds with Coordination Number 2: Phosphenium Cations; Thieme:
Stuttgart, Germany, 1990.
(18) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon
Copy: From Organophosphorus to Phospha-organic Chemistry; John Wiley:
Chichester, England, 1998.
(8) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew.
Chem., Int. Ed. 2008, 47, 3206.
10.1021/om800973v CCC: $40.75
2009 American Chemical Society
Publication on Web 01/20/2009

Phospha-Fischer Carbenes
Organometallics, Vol. 28, No. 4, 2009 991
Chart 1. Comparison between N-Heterocyclic Carbenes
(NHCs) and Phospheniums (NHPs) and the Specific Ligand
under Study, [NHPMes]OTf (Mes ) 2,4,6-trimethylphenyl,
TfO- ) trifluoromethanesulfonate)
(VII); the same compound results from reaction between
Pt(PPh3)3 and the NHCfNHP adduct.
On the basis of their strong π-acidity and reasonably
straightforward modular synthesis, NHPs have been described
as tunable CO equivalents.32 However, it may be better to think
of them as tunable NO+ surrogates instead, not only because
they are cationic but also because, in contrast to terminal CO
ligands that bind only linearly, they may in principle adopt either
planar or pyramidal (at P) coordination modes.37 These are
analogous to the linear and “bent” modes, respectively, of metal
nitrosyls and have identical implications for oxidation state
formalisms (Chart 3). The planar ligand is properly considered
a coordinated phosphenium, while the pyramidal one is a metal
phosphide. Both of these modes have been observed structurally
by Paine in CpMo(CO)2(NHPMe) (planar)38 and Cp*Fe-
(CO)2(NHPMe) (pyramidal).39 The analogy between NHP and
NO+ (and Fischer carbenes, Vide infra) was first drawn in
passing in Paine’s pioneering work and also later in a review
by Gudat,13 but to the best of our knowledge the relationship
has not been formalized via definitive structure, bonding, and
reactivity relationships.
Just as there is inconsistency in the ways that metal-NHC
bonds are drawn in the literature, representation of the
metal-NHP interaction has proven problematic (see Chart 2
for the ways that group 9 and 10 metal complexes have been
drawn, and Chart 4 for various generic representations).
Although a range of descriptions is warranted on the basis of
differences in structure and reactivity, and therefore a universally
applicable drawing is impossible, historical representation of
the metal-NHP interaction appears to be complicated by the
ligand’s carbene-like nature, formal positive charge, dual
bonding modes, and capacity for strong π-back-bonding. The
representations in Chart 4 may be considered to be on the same
“resonance continuum”, sometimes by deconvolution of the
dative bond, DfA, to its charge-separated, valence bond
representation, D+-A-,40 but these diagrams have different
chemical meanings and should have significant implications in
interpreting and predicting the structures and reactivity of the
NHP complexes they represent. For example, it should be
possible to substitute the phosphenium ligand in A, and B should
contain a trigonal pyramidal phosphorus center with a lone pair
(as in Chart 3), but it has been used in the literature to represent
coordinated planar phospheniums.
Often considered “phosphine replacement” ligands,21-23
NHCs have triggered an explosion of research in the field of
homogeneous catalysis. By comparison, NHPs have garnered
much less attention as ancillary ligands in catalysis. However,
reinvigorated investigation of their metal chemistry over the last
several years has begun to uncover their remarkable propert-
ies.13,24
The use of diaminophospheniums as ligands dates to Parry’s
1978 synthesis of [(CO)4Fe(P{NR2}2)]PF6.25 Since then, the
synthesis, structure, bonding, and reactivity of early and mid
transition metal complexes (e.g., of Cr,26,27 Mo,28,29 W,26,27 Fe,30
and Ru31) have been explored, principally by Nakazawa.14 To
date, however, there are exceedingly few reported examples of
late (groups 9 and 10) transition metal-NHP complexes. The
groups of Baker32 and Richeson20 have both made NHP-
containing analogues of Wilkinson’s catalyst (I-III; Chart 2),
and Gudat has disclosed a Co complex (IV).33 To the best of
our knowledge, there are currently only four reported NHP
complexes of the group 10 metals. The first of these was
[(NHPMes)Ni(CO)3][BHsBu3] (V) by Parry.34 Niecke has de-
scribed a Ni complex with a zwitterionic phosphenium-con-
taining ligand (VI),35 while Baker has shown that [NHPMes]OTf
reacts directly with Pt(PPh3)3 to give [(NHPMes)Pt(PPh3)2]OTf
(1).36 One of the PPh3 ligands in this complex can be replaced
by an Enders-type NHC to give [(NHPMes)Pt(PPh3)(NHC)]OTf
(19) Tuononen, H. M.; Roesler, R.; Dutton, J. L.; Ragogna, P. J. Inorg.
Chem. 2007, 46, 10693.
(20) Spinney, H. A.; Yap, G. P. A.; Korobkov, I.; DiLabio, G.; Richeson,
D. S. Organometallics 2006, 25, 3541.
(21) Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913.
(22) Bourissou, D.; Guerret, O.; Gabba¨ı, F. P.; Bertrand, G. Chem. ReV.
2000, 100, 39.
(23) Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. Engl. 1997,
36, 2162.
We recently reported an improved synthesis of a family of
free saturated NHP ligands.41 In this article, we present a series
of cationic Pt(0)- and Pd(0)-NHP complexes (Chart 5,
including 1 via a new synthesis) followed by their reactions
with both monodentate and chelating tertiary phosphines, and
also with H2O.
The solid state structure of 1 presented herein, together with
the results of electronic structure calculations and reactivity
patterns, affords insight into the nature of the bonding in late
metal-NHP complexes and allows the analogy with carbenes
and linear nitrosyls to be put on a solid footing. The Pt and Pd
compounds are best described as phosphorus analogues of
(24) Gudat, D.; Haghverdi, A.; Hupfer, H.; Nieger, M. Chem.-Eur. J.
2000, 6, 3414.
(25) Montemayor, R. G.; Sauer, D. T.; Fleming, S.; Bennett, D. W.;
Thomas, M. G.; Parry, R. W. J. Am. Chem. Soc. 1978, 100, 2231.
(26) Nakazawa, H.; Yamaguchi, Y.; Miyoshi, K. J. Organomet. Chem.
1994, 465, 193.
(27) Yamaguchi, Y.; Nakazawa, H.; Itoh, T.; Miyoshi, K. Bull. Chem.
Soc. Jpn. 1996, 69, 983.
(28) Gudat, D.; Haghverdi, A.; Nieger, M. J. Organomet. Chem. 2001,
383, 617.
(29) Nakazawa, H.; Miyoshi, Y.; Katayama, T.; Mizuta, T.; Miyoshi,
K.; Tsuchida, N.; Ono, A.; Takano, K. Organometallics 2006, 25, 5913.
(30) Nakazawa, H.; Kishishita, M.; Nakamoto, T.; Nakamura, N.;
Ishiyama, T.; Miyoshi, K. Chem. Lett. 2000, 230.
(31) Kawamura, K.; Nakazawa, H.; Miyoshi, K. Organometallics 1999,
18, 4785.
(32) Abrams, M. B.; Scott, B. L.; Baker, R. T. Organometallics 2000,
19, 4944.
(33) Burck, S.; Daniels, J.; Gans-Eichler, T.; Gudat, D.; Na¨ttinen, K.;
Nieger, M. Z. Anorg. Allg. Chem. 2005, 631, 1403.
(34) Snow, S. S.; Jiang, D. X.; Parry, R. W. Inorg. Chem. 1987, 26,
1629.
(35) Oberdo¨rfer, R.; Nieger, M.; Niecke, E. Chem. Ber. 1994, 127, 2397.
(36) Hardman, N. J.; Abrams, M. B.; Pribisko, M. A.; Gilbert, T. M.;
Martin, R. L.; Kubas, G. J.; Baker, R. T. Angew. Chem., Int. Ed. Engl.
2004, 43, 1955.
(37) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987.
(38) Hutchins, L. D.; Paine, R. T.; Campana, C. F. J. Am. Chem. Soc.
1980, 102, 4521.
(39) Hutchins, L. D.; Duesler, E. N.; Paine, R. T. Organometallics 1982,
1, 1254.
(40) Parkin, G. Organometallics 2006, 25, 4744.
(41) Caputo, C. A.; Price, J. T.; Jennings, M. C.; McDonald, R.; Jones,
N. D. Dalton Trans. 2008, 3461.

992 Organometallics, Vol. 28, No. 4, 2009
Chart 2. Summary of Reported Group 9 and 10 Metal-NHP Complexesa
Caputo et al.
a
With the exception of 1 and VII, for which graphics were not given in the original reports, the cations are drawn exactly as they have been
represented in the literature. In all Pt and Rh compounds, the anion is TfOs; for Ni, it is [HB(sBu)3]s.
Chart 3. Structural Analogy between Metal-Nitrosyls and
-Phospheniums
Experimental Section
General Considerations. Reagents were obtained from com-
mercial sources and used as supplied, unless otherwise indicated.
These were of American Chemical Society (ACS) grade or finer.
Solvents were dried and deoxygenated either by N2 purge followed
by passage through alumina columns (Innovative Technology or
MBraun solvent purification systems) or by distillation under N2
from the appropriate drying agent. All reactions were carried out
under N2 atmosphere using standard Schlenk techniques unless
stated otherwise. Pt(PPh3)4 and Pd(PPh3)4 were made according to
literature procedures.45,46
1H, 13C{1H}, 19F{1H}, and 31P{1H} NMR data were recorded
on a 400 MHz Varian Mercury (400.085 MHz for for 1H, 100.602
MHz for 13C, 376.458 MHz for 19F) or 400 MHz Varian Inova
spectrometer (399.762 MHz for 1H, 100.520 MHz for 13C, 161.825
MHz for 31P). Unless otherwise indicated, spectra were recorded
at rt in CDCl3 solution using residual solvent proton (relative to
external SiMe4, δ 0.00) or solvent carbon (relative to external SiMe4,
δ 0.00) as an internal reference. All 31P{1H} NMR spectra were
recorded relative to an external standard (85% H3PO4, δ 0.00).
Downfield shifts were taken as positive, and all coupling constants
are given in hertz.
Chart 4. Selection of Generic Representations of Metal-
NHP Complexes Found in the Literature
High-resolution mass spectrometry data were recorded using a
Finnigan MAT 8200 instrument. Elemental analyses (C, H, N) were
performed by Guelph Chemical Laboratories Inc., Guelph, ON,
Canada.
Chart 5. NHP-Containing Compounds Made in This Study
(all anions are TfO-)
Synthesis of Pt and Pd Metal Complexes. [(NHPMes)Pt-
(PPh3)2]OTf (1). The synthesis of this compound (previously
reported by Baker and co-workers)36 was modified to use
Pt(PPh3)4 instead of Pt(PPh3)3. In a small vial, Pt(PPh3)4 (0.53
g, 0.42 mmol) was dissolved in C6H6 (10 mL). A solution of
[NHPMes]OTf (0.20 g, 0.42 mmol) in C6H6 (5 mL) was added
(42) Bernasconi, C. F.; Leyes, A. E. J. Chem. Soc., Perkin Trans. 2
1997, 1641.
(43) Zoloff Michoff, M. E.; de Rossi, R. H.; Granados, A. M. J. Org.
Chem. 2006, 71, 2395.
(44) Bernasconi, C. F.; Flores, F. X.; Kittredge, K. W. J. Am. Chem.
Soc. 1997, 119, 2103.
(45) Ugo, R.; Cariati, F.; La Monica, G. In Reagents for Transition Metal
Complex and Organometallic Syntheses; Angelici, R. J., Ed.; John Wiley
& Sons Inc.: New York, 1990; Vol. 28, p 123.
Fischer carbenes with PdM double bond character and localiza-
tion of positive charge at P (as in D, Chart 4). To demonstrate
this analogy with Fischer carbenes, we show that the coordinated
phosphenium ligand is inert to substitution by monodentate and
chelating phosphines. We show also that the complexes undergo
attack by H2O at the NHP P-atom to generate metal-hydrides
in a mechanism akin to that proposed for the hydrolysis of
Fischer carbenes.42-44
(46) Coulson, D. R. In Reagents for Transition Metal Complex and
Organometallic Syntheses; Angelici, R. J., Ed.; John Wiley & Sons Inc.:
New York, 1990; Vol. 28, p 107.

Phospha-Fischer Carbenes
Organometallics, Vol. 28, No. 4, 2009 993
dropwise to give a dark yellow solution. The solvent was frozen
and removed in Vacuo to give a yellow solid, which was washed
with Et2O (2 × 10 mL) to remove free PPh3, then dried in Vacuo
to give the title compound as a yellow solid. Yield: 0.26 g (49%).
X-ray quality crystals of 1 · 1/2C6H6 were grown from concen-
trated C6H6 solution by slow evaporation. The 1H NMR
spectroscopic data were identical to those reported by Baker.36
However, chemical shift differences due to solvent were observed
in the 31P{1H} NMR spectrum. 31P{1H} NMR (CDCl3): δ 44.3
Computational Details. Molecular geometries of all studied
compounds were optimized with DFT using the GGA PBEPBE
exchange-correlation functional47-49 with the def-TZVP basis
sets;50,51 ECP basis sets of similar valence quality were used for
the transition metal nuclei.52 Although the use of the GGA
functional leads to a slight overestimation of all bond lengths, it
was preferred over the hybrid counterpart PBE1PBE for reasons
of computational efficiency, i.e., the ability to use multipole
accelerated RI approximation and to run calculations in parallel.
All optimizations were carried out with the Turbomole 5.10 program
package.53,54 Energy decomposition analyses for optimized struc-
tures were performed with the ADF 2007.01 program.55 The
analysis followed the Morokuma-Rauk-Ziegler partition sch-
eme56-58 and utilized the PBEPBE GGA functional47-49 in
combination with STO-type all-electron basis sets of TZP quality.59,60
Scalar relativistic effects were taken into account in all EDA
calculations by employing the ZORA Hamiltonian.61-63 Repre-
sentations of the molecular orbitals of 2 and 7 given in Figure 2
were constructed using the gOpenMol program.64,65
1
2
1
2
(d, JPPt ) 4243, JPP ) 228), 290.0 (t, JPPt ) 6446, JP-P
)
228). In addition, the following additional characterization data
were collected. 19F{1H} NMR (CDCl3): δ -78.1 (s). ESI-MS:
C56H56N2P3Pt+ calcd (found) 1044.3299 (1044.3367, M+). Anal.
Calcd for C57H58F3N2O3P3PtS: C, 57.2; H, 4.9; N, 2.3. Found:
C, 57.2; H, 5.0; N, 2.2.
[(NHPMes)Pd(PPh3)2]OTf (2). This compound was made in the
same way as 1. Thus, reaction of Pd(PPh3)4 (0.59 g, 0.52 mmol)
and [NHPMes]OTf (0.27 g, 0.57 mmol) gave 0.30 g (52%) of a
yellow solid. 1H NMR (C6D6): δ 2.05 (s, 6H, p-CH3), 2.08 (s, 12H,
3
o-CH3), 4.10 (d, 4H, CH2, JHP ) 4.0), 6.54 (s, 4H, Ar), 7.01 (pt,
18H, Ar), 7.13 (pq, 12H, Ar). 13C{1H} NMR (CDCl3, 263 K): δ
Results and Discussion
17.8, 21.3, 52.5, 128.8, 130.0, 130.3, 131.8, 133.3, 135.0, 136.0,
Synthesis and Structure. The phosphenium triflate
[NHPMes]OTf reacts quickly and cleanly with equimolar
M(PPh3)4 precursors at rt to generate [(NHPMes)M(PPh3)2]OTf
(1, M ) Pt; 2, M ) Pd) with elimination of 2 equiv of PPh3
(Scheme 1). As mentioned in the Introduction, 1 has
previously been made (although not structurally character-
ized) by reaction of [NHPMes]OTf with Pt(PPh3)3; 2 represents
the first well-defined Pd-NHP complex, although related
species have been assumed to form in situ in catalytic
processes.66,67
The geometries of mixed phosphine-phosphenium complexes
of the type [(NHPMes)M(PR3)n]OTf are easily elucidated from
31P{1H} NMR data. The spectrum of 1 in CDCl3 exhibits an
upfield doublet (with Pt satellites) for the PPh3 ligands (δ 44.3,
2
138.8. 31P{1H} NMR (CDCl3): δ 25.1 (d, JPP ) 149), 260.0 (t,
2JPP
) δ -78.2 (s). ESI-MS:
149). 19F NMR (CDCl3):
C56H56N2P3Pd+ calcd (found) 955.3 (955.3, M+). Anal. Calcd for
C57H56F3N2O3P3PdS: C, 61.9; H, 5.1; N, 2.5. Found: C, 62.0; H,
5.1; N, 2.5.
[(NHPMes)Pt(PMe3)3] (3). Neat PMe3 (0.25 mL, 2.4 mmol) was
added dropwise to a yellow CH2Cl2 (2 mL) solution containing 1
(0.33 g, 0.28 mmol) in a small vial. The reaction mixture was stirred
for 10 min at rt, during which it turned red. The solvent was
removed in Vacuo, and the orange residue was washed with Et2O
(2 × 10 mL) to remove free PPh3, then dried under vacuum. Yield:
1
0.22 g (88%). H NMR (CDCl3): δ 1.17 (m (br m, 27H, CH3),
3
2.25 (s, 6H, p-CH3), 2.34 (s, 12H, o-CH3), 3.79 (d, 4H, CH2, JHP
) 2.0), 7.28 (br s, 4H, m-CH). 13C{1H} NMR (CDCl3, 273 K): δ
18.7, 21.0, 24.6, 50.7, 129.9, 134.3, 136.6, 138.6. 31P{1H} NMR
1
2
1
(CDCl3): δ -47.1 (d, JPPt ) 3283, JPP ) 125), 207.6 (q, JPPt
)
(47) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865.
(48) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1997, 78,
1396.
(49) Perdew, J. P.; Ernzerhof, M.; Burke, K. J. Chem. Phys. 1996, 105,
9982.
(50) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100,
5829.
(51) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem.
Acc. 1997, 97, 119.
(52) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chem. Acc. 1990, 77, 123.
(53) TURBOMOLE, Program Package for ab initio Electronic Structure
Caluclations; Version 5.10 ed.; Theoretical Chemistry Group, University
of Karlsruhe: Karlsruhe, Germany, 2008.
(54) Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel, C. Chem. Phys.
Lett. 1989, 162, 165.
(55) ADF2007.01 Theoretical Chemistry Program; SCM, V. U.: Am-
sterdam, The Netherlands, 2007.
(56) Morokuma, K. J. Chem. Phys. 1971, 55, 1236.
(57) Kitaura, K.; Morokuma, K. Int. J. Quantum Chem. 1976, 10, 325.
(58) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1.
(59) van Lenthe, E.; Baerends, E. J. J. Comput. Chem. 2003, 24, 1142.
(60) Chong, D. P.; van Lenthe, E.; van Gisbergen, S.; Baerends, E. J.
J. Comput. Chem. 2004, 25, 1030.
(61) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993,
99, 4597.
(62) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994,
101, 9783.
6162, JPP ) 125). 19F NMR (CDCl3): δ -78.1 (s). ESI-MS:
C29H53N2P4Pt+ calcd (found) 748.2807 (748.2743, M+). Anal. Calcd
for C30H53F3N2O3P4PtS: C, 40.1 H, 5.95; N, 3.1. Found: C, 39.65;
H, 6.1; N, 2.6.
2
[(NHPMes)Pd(PMe3)3]OTf (4). This compound was made in the
same way as 3. Thus reaction of 2 (0.30 g, 0.27 mmol) and PMe3
1
(0.112 mL, 1.09 mmol) gave 0.13 g (58%) of an orange solid. H
NMR (CDCl3): δ 1.03 (br s, 27H, CH3), 2.24 (s, 6H, p-CH3), 2.34
3
(s, 12H, o-CH3), 3.86 (d, 4H, CH2, JHP ) 4.4), 6.95 (s, 4H, Ar).
2
13C{1H} NMR (CDCl3): δ 18.7, 21.2, 21.4, 52.1 (d, JPC ) 6.1),
128.6 (d, 2JPC ) 7.6), 130.3, 133.8 (d, 2JPC ) 17.7), 134.3 (d, 2JCP
2
) 9.2), 136.3 (d, JCP ) 3.1), 139.0. 31P{1H} NMR (CDCl3, 253
K): δ -29.6 (d, 2JPP ) 73), 242.8 (d, 2JPP ) 73). 19F NMR (CDCl3):
δ -74.2 (s). ESI-MS: C29H53N2P4Pd+ calcd (found) 585.2 (583.1,
M+ - PMe3), (507.1, M+ - 2 PMe3). Anal. Calcd for
C30H53F3N2O3P4PdS: C, 44.53 H, 6.60; N, 3.46. Found: C, 44.25;
H, 6.52; N, 3.16.
[(NHPMes)Pt(dppe-K2P)(dppe-KP)]OTf (5). This compound was
prepared in the same way as 3. Thus, reaction of 1 (102 mg, 8.50
mmol) and dppe (68.0 mg, 17.0 mmol) gave 94.7 mg (76%) of a
dark orange solid. 1H NMR (CDCl3): δ 1.26 (br s, 4H, PCH2), 1.58
(br s, 4H, PCH2), 2.10 (s, 6H, p-CH3), 2.26 (s, 12H, o-CH3), 3.94
(br s, 4H, NCH2), 6.59 (br s, 4H, Ar), 6.93-7.25 (br m, 40H, Ar).
(63) van Lenthe, E.; Ehlers, A. E.; Baerends, E. J. J. Chem. Phys. 1999,
110, 8943.
(64) Laaksonen, L. J. Mol. Graph. 1992, 10, 33.
(65) Bergman, D. L.; Laaksonen, L.; Laaksonen, A. J. Mol. Graph.
Model. 1997, 15, 301.
(66) Ackermann, L.; Born, R. Angew. Chem., Int. Ed. 2005, 44, 2444.
(67) Sakakibara, K.; Yamashita, M.; Nozaki, K. Tetrahedron Lett. 2005,
46, 959.
3
31P{1H} NMR (CDCl3, 263 K): δ -14.2 (d, JPP ) 28.5), 9.7 (m,
1
2
1
1JPPt ) 3502), 26.3 (m, JPPt ) 2994, JPP ) 105), 217.5 (m, JPPt
) 6914). 19F NMR (CDCl3): δ -78.0 (s). ESI-MS: C72H77N2P5Pt+
calcd (found) 1319.4 (1320.4, M+ + 1), (921.9, M+ - dppe). The
inevitable appearance of [PtH(dppe)2]OTf hydrolysis product in all
samples precluded satisfactory elemental analysis.

994 Organometallics, Vol. 28, No. 4, 2009
Caputo et al.
Figure 1. Stacked 31P{1H} NMR spectra for [(NHPMes)Pd(PPh3)2]OTf (2, bottom) and [(NHPMes)Pd(PMe3)3]OTf (4).
on coordination (∆δ) of [NHPMes]+ to Pt is 86.9 ppm downfield,
which implies a dramatic deshielding of the phosphorus center.
The Pt-P coupling constant is very much larger than those
obtained by Glueck and co-workers68 for a range of Pt(II)-phos-
phide complexes (ca. 900-1000 Hz), which implies that the
bound NHP in 1 is probably not pyramidal in solution. Similar
2
NMR data are obtained for 2: an upfield doublet (δ 25.1, JPP
) 149 Hz) for the PPh3 ligands and a downfield triplet (δ 260.0,
2JPP ) 149 Hz; ∆δ ) 56.9) for the coordinated NHP in a 2:1
ratio show that 1 and 2 are isostructural (Figure 1).
Crystals of 1 · 1/2C6H6 suitable for X-ray crystallographic
analysis were grown at rt by slow evaporation of C6H6 solution;
the solid state structure is given in Figure 2. As expected from
the NMR data, the structure is trigonal planar at Pt (∑(angles)
) 360.00°). The geometry at the phosphenium P-atom is also
trigonal planar (∑(angles) ) 359.99°). The N-P-N plane of
Figure 2. ORTEP representation of the molecular structure of the
cation of 1. Thermal ellipsoids are drawn at 30% probability.
H-atoms and all but the ipso-C-atoms of the PPh3 ligands have
been omitted for clarity. Selected experimental [and calculated]
bond distances (Å) and angles (deg): Pt-P1, 2.1073(9) [2.185];
P1-N2, 1.640(3) [1.695]; P1-N5, 1.628(3) [1.695]; Pt-P2,
2.3178(9) [2.383]; Pt-P3, 2.3113(8) [2.383]; N5-P1-N2, 93.68(16)
[92.0]; N5-P1-Pt, 134.15(12) [134.0]; N2-P1-Pt, 132.16(12)
[134.0]; P1-Pt-P3, 121.41(3) [122.0]; P1-Pt-P2, 121.70(3)
[122.0]; P2-Pt-P3, 116.89(3) [115.9]; P2-Pt-P1-N5, -61.66(19)
[-69.1]; P3-Pt-P1-N5, 119.02(19) [110.9].
the [NHPMes +
] ligand makes a torsion angle of approximately
62° with the coordination plane of the metal. The Pt-NHP bond
(Pt-P1: 2.107(3) Å) is significantly shorter than the Pt-PPh3
bonds (Pt-P2: 2.3178(9) Å and Pt-P3: 2.3113(8) Å) by ca.
0.21 Å, consistent with the large Pt-P coupling constant, and
indicates a strong Pt-P bond and greater back-bonding to the
phosphenium than to the PPh3 ligands. To the best of our
knowledge, this Pt-P bond is the shortest reported to date; the
previous record of 2.116(3) Å was held by Baker’s complex,
VII (Chart 2);36 formal PtdP double bonds in phosphinidenes
lie in the range 2.2-2.3 Å,69 which contains the average
Pt-PPh3 single bond of 2.28 Å.70 These data imply that by
metrical considerations alone the Pt-NHP interaction in 1 may
be thought of as a double bond. No short contacts between either
the NHP P-atom or the Pt-atom and the TfO- anion are observed
in the solid state; the closest distances are 3.282 and 3.370 Å
between a “backbone” C-atom and the OTf- O-atoms.
Scheme 1. Synthesis of Pd(0)- and Pt(0)-NHP Metal
Complexes (anion is TfO-)
1JPtP ) 4143 Hz, 2JPP ) 228 Hz) and a dramatically downfield
triplet (with Pt satellites) for the coordinated [NHPMes]+ ligand
(68) Scriban, C.; Glueck, D. S.; DiPasquale, A. G.; Rheingold, A. L.
Organometallics 2006, 25, 5435.
(69) Kourkine, I. V.; Chapman, M. B.; Glueck, D. S.; Eichele, K.;
Wasylishen, R. E.; Yap, G. P. A.; Liable-Sands, L. M.; Rheingold, A. L.
Inorg. Chem. 1996, 35, 1478.
(70) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.
1
2
(δ 290.0, JPtP ) 6446 Hz, JPP ) 228 Hz) with relative 2:1
peak intensity. These data are similar to those reported by Baker
for 1 in CD2Cl2 solution.36 They unequivocally confirm the
presence of two equivalent phosphine and one phosphenium
ligand in a trigonal-planar coordination environment. The shift

Phospha-Fischer Carbenes
Organometallics, Vol. 28, No. 4, 2009 995
Scheme 2. Phosphine Substitution Reactions of 1 and 2 with PMe3 (all anions are TfO-)
Scheme 3. Substitution Reactions of 1 with dppe (anion is
TfO-)
The question arises whether to consider 1 as a Pt(0)-NHP
complex or a Pt(II)-phosphide (Chart 3). In 1, and all other
structurally characterized group 9 and 10 metal-NHP com-
plexes,20,32,35,36 the geometry at the NHP P-atom is trigonal
planar, which clearly rules out the Pt(II)-phosphide description
(analogous to “bent” metal-nitrosyls) and thereby discounts
formal two-electron oxidation of the metal by the phosphenium.
Hence, structural characteristics alone imply that 1 and 2 are
best described as 16-electron metal(0)-NHP complexes.
Although 1 should be considered as a compound of Pt(0), it
reacts differently from homoleptic Pt(0)-phosphine complexes:
it does not dissociate phosphine in solution (within the limits
of 31P NMR detection); it is inert to oxidative addition by MeI
and MeOTf; and it does not bind the electron-poor alkene
dimethyl maleate. These findings indicate that 1 is significantly
less electron rich than other group 10 metal(0) phosphine
compounds, probably due to the formal positive charge borne
by the NHP ligand.
Chart 4 shows a selection of generic representations of
metal-NHP complexes found in the literature. Clearly, B is
not a good description of either 1 or 2 because it implies a
trigonal-pyramidal geometry at the NHP P-atom. Representation
of the M-NHP interaction in 1 and 2 as D (equivalent to E)
should mean (i) that the NHP is not easily substituted (the
opposite of what would be predicted by A) and (ii) that the
ligand should suffer attack by good nucleophiles; representation
instead as C would predict attack by nucleophiles at the metal
rather than the phosphorus center. In fact, both predictions (i)
and (ii) are borne out by experiment.
To test the first hypothesis, we surveyed the reaction of 1
and 2 with a variety of monodentate and chelating phosphines,
and in no case was the NHP ligand displaced from the inner
coordination sphere of the metal. This finding is consistent with
Baker’s observation that reaction of 1 with an NHC leads to
the mixed NHP/NHC compound VII, in which the phosphenium
ligand is retained.36 In our study, reaction of 1 and 2 with an
excess of the small, basic phosphine PMe3 quickly and cleanly
gives [(NHPMes)M(PMe3)3]OTf (3, M ) Pt; 4, M ) Pd). The
substitution of PPh3 by PMe3 is cooperative, so that when only
a single equivalent of PMe3 is added, 3 and 4 are generated,
but two-thirds of the starting material is present in the reaction
mixture at completion (Scheme 2). In no case is a mixed PMe3/
PPh3 complex detected, and the NHP ligand is always retained.
The formation of four- rather than the three-coordinate
products could be explained by the smaller steric demand of
PMe3 compared to PPh3 (Tolman cone angles: 118° and 143°,
respectively)71 and also to the higher Lewis basicity of the
incoming ligand. The 31P{1H} NMR spectroscopic data of 3
and 4 are similar to those of the parent compounds, except that
in these cases the downfield peaks due to the coordinated NHP
are quartets, rather than triplets, due to coupling of the NHP to
three equivalent phosphines in the four-coordinate products
(Figure 1). These complexes are best described as 18-electron
metal(0)-NHP complexes. Putative metal(II) compounds bear-
ing three phosphine ligands should be square planar and
therefore have two chemically distinct PR3 environments, one
trans to the NHP and the other cis. However, in 3 and 4 only
a single PMe3 environment is evident by NMR spectroscopy,
which indicates the presence of the three equivalent phosphine
ligands in a tetrahedral complex and thereby reinforces the idea
that these compounds (together with 1 and 2) are in fact metal(0)
species.
Reaction of 1 with the chelating phosphine, bis(diphenylphos-
phino)ethane (dppe), gives 5 with substitution of both PPh3
ligands (Scheme 3). This compound has a distinctive 31P{1H}
NMR spectrum that unequivocally describes the structure. The
spectrum of 5 displays a low-field multiplet (with Pt “satellites”)
at 215.6, an upfield multiplet (26.4, showing Pt “satellites”),
yet another multiplet in this region (9.73, showing Pt “satel-
lites”), and a doublet even further upfield (-14.2, no coupling
to Pt). The chemical shifts, multiplicities, and relative integra-
tions of these four chemically inequivalent 31P environments
are consistent with the presence of coordinated [NHPMes]+ and
one chelating and one monodentate dppe ligand, i.e., the
compound [(NHPMes)Pt(dppe-κ2P)(dppe-κP)]OTf (Scheme 3).
This once again is clear evidence of the substitutionally inert
nature of the coordinated NHP and reinforces the notion that 1
is properly considered as a phospha-Fischer carbene.
Bonding. The qualitative bonding scheme describing cyclic
aminophosphenium ions as σ-donors and π-acceptors emerged
concurrently with the publication of the first X-ray crystal
structures of NHP-metal complexes.38 By contrast, quantitative
treatments of the M-P bond in these systems are rare and vastly
outnumbered by analyses of the M-C interaction in metal-NHC
complexes, which have been intensely scrutinized using a
(71) Tolman, C. A. Chem. ReV. 1977, 77, 313.

996 Organometallics, Vol. 28, No. 4, 2009
Caputo et al.
myriad of computational approaches.72-78 The most authorita-
tive investigations into M-NHP bonding have been published
by Gudat and co-workers.24,33,79 Together with others in the
field,80 they have analyzed the M-P bond primarily with orbital-
based approaches, such as Mulliken and natural population
analyses. To the best of our knowledge, there are no reports of
an energy decomposition analysis (EDA) of M-NHP bonding
within the framework of a fragment molecular orbital (FMO)
approach.56-58 This combination of methods is aptly suited for
the analysis of donor-acceptor interactions because it allows a
simple quantification of the relative importance of σ- and
π-contributions to bonding.
Chart 6. Theoretical Pt and Pd NHC Complexes, with MdC
Bonds Drawn Analogously to the MdP Bonds of 1-4
Table 1. Results of Energy Decomposition Analyses (kJ mol-1) of
1-4 and 6-9
∆Eorbint
The EDA approach has been extensively described in the
literature,81 and only a brief overview is presented herein. The
energy associated with bond formation between two (or more)
fragmentsstypically the metal and the ligand(s)sboth possess-
ing the geometry they take in the optimized complex, is referred
to as the total bonding energy ∆Etot.82 According to EDA, ∆Etot
∆Etot
∆E0
∆Eelstat ∆EPauli
a
b
1
2
3
4
6
7
8
9
-318.5 195.5 -641.9 837.3 -191.0 (37%) -323.0 (63%)
-265.1 138.8 -496.5 635.2 -133.2 (33%) -270.7 (67%)
-359.0 175.9 -644.3 820.2 -534.9
-293.2 136.7 -506.6 643.2 -429.9
-226.3 121.5 -803.8 925.3 -230.2 (66%) -117.5 (34%)
can be expressed as a sum of steric, ∆E0, and orbital, ∆Eorbint
,
-157.4
67.2 -555.7 622.8 -140.4 (63%)
-84.2 (37%)
interactions, i.e., ∆Etot ) ∆E0 + ∆Eorbint. The steric interaction
consists of a purely electrostatic term, ∆Eelstat, and a Pauli
repulsion term, ∆EPauli, between occupied orbitals on both
fragments. The former stabilizes the bond, while the latter
weakens it. It is the relative importance of the two terms that
determines the effect of ∆E0 on the total bonding energy. The
orbital interaction energy has a stabilizing contribution and is
calculated as the energy gain resulting from the relaxation of
the FMOs to their optimal form in the complex. This term has
an instructive interpretation as a sum of contributions from the
various irreducible representations of the molecular point group.
Therefore, by assessing the symmetry of the system, the σ- and
π-characteristics of the interaction can be determined in a
straightforward manner.
-191.1 114.9 -718.0 832.8 -305.9
-126.9 68.2 -495.1 563.3 -195.1
The geometries of all systems were fully optimized prior to
EDA calculations. Selected calculated metrical parameters for
1 are given in the legend for Figure 2, and complete structural
data are deposited as Supporting Information (SI). The calculated
geometries are in good overall agreement with the experimental
data and display all the expected trends; therefore an in-depth
analysis of geometrical parameters is not warranted. However,
special attention should be drawn to three points: (i) The M-P
bond lengths in 1-4 are not significantly affected by the identity
of the metal fragment and indicate multiple bonding in all
compounds (the calculated bond lengths lie in the range 2.179-
2.199 Å). In contrast, the calculated M-C bond lengths in the
analogous NHC complexes 6-9 vary more widely (from 2.050
to 2.145 Å) and are slightly longer than typical measured
NHC-Pt(0) and NHC-Pd(0) single bonds in two coordinate
complexes (2.02-2.0985 and 1.99-2.11 Å, respectively).86-90
(ii) The geometry at the P- and C-atoms of 1-4 and 6-9,
respectively, is invariably trigonal planar, with the sum of bond
angles being very close to 360°. (iii) The optimized geometries
of 3 and 4 are tetrahedral at the metal center, in agreement with
the NMR data.
We have examined the bonding in complexes 1-4 using the
EDA method. Because the M-P bond is the interaction of
interest, the complexes were divided into two parts: the
[NHPMes +
] ligand and the formally 14- and 16-electron frag-
ments M(PPh3)2 and M(PMe3)3, respectively (M ) Pd, Pt).
Analogous valence isoelectronic complexes involving the neutral
NHCMes ligand (6-9, Chart 6) were included in the analysis
for comparative purposes; the results can also be contrasted with
those published earlier for a variety of Fischer carbenes.75,83,84
Results of the EDA calculations are summarized in Table 1.
They show that changing the metal from Pd to Pt strengthens
the M-P bond by ca. 60 kJ mol-1; a change in the composition
of the metal fragment from M(PPh3)2 to M(PMe3)3 further
strengthens the bond by 40 kJ mol-1. The overall effect of steric
interaction in 1-4 is destabilizing, unlike the case of Fischer
carbenes,75,83,84 because of the increased Pauli repulsion.
However, the total orbital interaction, which is the more
informative of the two contributors to ∆Etot, outweighs the steric
(72) Hu, X.; Tang, Y.; Gantzel, P.; Meyer, K. Organometallics 2003,
22, 612.
(73) Hu, X.; Castro-Rodrigues, I.; Olsen, K.; Meyer, K. Organometallics
2004, 23, 755.
(74) Nemcsok, D.; Wichmann, K.; Frenking, G. Organometallics 2004,
23, 3640.
(75) Frenking, G.; Sola`, M.; Vyboishchikov, S. F. J. Organomet. Chem.
2005, 690, 6178.
(76) Jacobsen, H.; Correa, A.; Costabile, C.; Cavallo, L. J. Organomet.
Chem. 2006, 691, 4350.
(77) Boehme, C.; Frenking, G. Organometallics 1998, 17, 5801.
(78) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L.
Coord. Chem. ReV. 2008, doi:10.1016/j.ccr.2008.06.006.
(79) Gudat, D. Eur. J. Inorg. Chem. 1998, 1087.
(80) Takano, K.; Tsumura, H.; Nakazawa, H.; Kurakata, M.; Hirano,
T. Organometallics 2000, 19, 3323.
(81) Bickelhaupt, F. M.; Baerends, E. J. ReViews in Computational
Chemistry; Wiley-VCH: New York, 2000; Vol. 15.
(82) The total bonding energy calculated in this manner is, however,
not the negative of bond dissociation enthalpy, as the former quantity does
not take into account the fragment relaxation and reorganization processes
inherent in real bonding schemes.
(85) Gelloz-Berthon, G.; Buisine, O.; Brie`re, J.-F.; Michaud, G.; Ste´rin,
S.; Mignani, G.; Tinant, B.; Declercq, J.-P.; Chapon, D.; Marko´, I. E. J.
Organomet. Chem. 2005, 690, 6156.
(86) Arnold, P. L.; Cloke, F. G. N.; Geldbach, T.; Hitchcock, P. B.
Organometallics 1999, 18, 3228.
(87) Caddick, S.; Cloke, F. G. N.; Hitchcock, P. B.; Leonard, J.; Lewis,
A. K. d. K.; McKerrecher, D.; Titcomb, L. R. Organometallics 2002, 21,
4318.
(88) Jackstell, R.; Harkal, S.; Jiao, H.; Spannenberg, A.; Borgmann, C.;
Ro¨ttger, D.; Nierlich, F.; Elliot, M.; Niven, S.; Cavell, K. J.; Navarro, O.;
Viciu, M. S.; Nolan, S. P.; Beller, M. Chem.-Eur. J. 2004, 10, 3891.
(89) Clement, N. D.; Cavell, K. J.; Ooi, L. Organometallics 2006, 25,
4155.
(83) Lein, M.; Szabo´, A.; Kova´cs, A.; Frenking, G. Faraday Discuss.
2003, 124, 365.
(84) Cases, M.; Frenking, G.; Duran, M.; Sola`, M. Organometallics 2002,
21, 4182.
(90) Fantasia, S.; Nolan, S. P. Chem.-Eur. J. 2008, 14, 6987.

Phospha-Fischer Carbenes
Organometallics, Vol. 28, No. 4, 2009 997
term for all systems. Furthermore, the C2-symmetry of 1 and 2
allows us to describe the M-P bond by clean deconvolution of
∆Eorbint in these complexes into σ- and π-MOs, which belong
to the irreducible representations a and b of the C2 point group,
respectively91 (Table 1). This treatment reveals that the bond
has approximately 2/3 π- and 1/3 σ-character. Although the
same analysis cannot be conducted for the tetrahedral com-
pounds 3 and 4, we note that their similarity to 1 and 2 in terms
of other bonding characteristics (Table 1), together with the
FMO analysis (Vide infra), imply predominant π-bonding also
in these complexes.
These findings are in stark contrast to those of NHC
complexes 6-9. Although carbenes have long been considered
pure σ-donors, recent computational treatments have demon-
strated a small to moderate (10-30%) π-bonding character, the
significance of which increased with the increasing formal
d-electron count.75,76,92 Energy decomposition analysis for 6-9
yielded similar π-bonding contributions of 34% and 37% for 6
and 7, respectively. For Fischer carbenes, the percentage of
∆Eorbint attributable to π-bonding has been reported to lie
between 25% and 50%.75,83,84 Therefore, the bonding in 1-4
is clearly of Fischer type also at the quantitative level, and as
anticipated,79 the M-NHP bond is dominated by π-back-
bonding.
The bonding in Fischer carbenes and M-NHC complexes can
be explained qualitatively by dative and retrodative components,
whose relative contributions lead to formal double and single M-C
bonds in the former and latter species, respectively.93,94That is, in
both cases there generally exists one σ-type MO that is a linear
combination of occupied ligand-based orbitals (typically the
highest occupied FMO) and unoccupied metal-based d-orbitals,
and one π-type MO that is a linear combination of unoccupied
ligand-based orbitals (typically the lowest unoccupied FMO)
and occupied metal-based d-orbitals. The MOs of 1 and 2 differ
significantly from the above classical description, however, and
reflect the dominant nature of the π-interaction. Figure 3
illustrates this difference by comparing the important bonding
MOs of 2 and 7. In the NHP complexes 1 and 2, the unoccupied
metal-based d-orbitals make only a negligible to small contribu-
tion to the HOMOs of the resultant complexes (6.8% and 0.0%),
whereas the σ-bonding MOs of the NHC complexes 6 and 7
contain 12.8% and 6.0% contributions, respectively, from the
unoccupied metal-based d-orbitals. By contrast, there is a much
greater mixing between the occupied metal-based d-orbitals and
Figure 3. Important bonding MOs in [(NHPMes)Pd(PPh3)2]+ (2) and
(NHCMes)Pd(PPh3)2 (7). In all cases the coordination plane of the
metal is oriented parallel to the page.
π-interaction; the percentage contributions of LUMO+1 to the
bonding MOs of the metal complex are 6.0% and 5.1% in 1
and 2, respectively.
The qualitative bonding description of 3 and 4 (as well that
of 8 and 9) closely follows the above discussion. The most
notable difference is the increased importance of the σ-type
dative interaction: the unoccupied metal-based d-orbitals have
8.0% and 4.4% contributions to the occupied MOs of
the resulting complexes 3 and 4, respectively. Even so, the
retrodative π-interaction dominates the bonding in both com-
pounds. It can therefore be concluded that the net flow of
electrons in 1-4 takes place from metal to ligand, but as
evidenced by the calculated data, the transfer of electrons
involves significantly less than an electron pair. Thus, the NHP
complexes can be considered as containing the metal center in
a formal zero oxidation state.
Hydrolysis. Serendipitous discovery of the reaction between
1 and H2O confirmed the second of the two hypotheses given
above, i.e., that these phospha-Fischer carbenes should be
susceptible to attack by nucleophiles. Indeed, 1 is highly
sensitive to moisture and needs to be handled under a rigorously
inert atmosphere. Despite repeated drying of solvents, the
presence of adventitious water eventually leads to the decom-
position of these complexes. Overnight incubation of reaction
mixtures of [NHPMes]OTf and Pt(PPh3)4 in wet CH2Cl2 or CDCl3
solution invariably gives a mixture of the well-known Pt(II)-hydr-
ide [PtH(PPh3)3]OTf (10)95-98 and the phosphine oxide 11,
the lowest unoccupied FMO of the [NHPMes +
] fragment: this
ligand-based FMO contributes 18.8% and 15.3% to the HOMOs
of 1 and 2, respectively. The analogous contributions to the
π-bonding MOs of complexes 6 and 7 are only 8.3% and 5.2%,
respectively. There is also a third bonding contribution in 1 and
2 that is absent from the analogous NHC complexes: due to
the cationic charge borne by the [NHPMes +
fragment, the
]
π-symmetric LUMO+1 of the ligand is also sufficiently low in
energy to accept electron density from the occupied metal-based
d-orbitals. This results in a second back-bonding contribution
that is, however, of lesser importance than the primary
(91) We note that this is only a rough division, as proper definition of
symmetry implies the presence of a mirror plane and not a C2 axis. Orbitals
that should be classified antisymmetric and symmetric transfer as the a and
b representations of the C2 point group, respectively. Consequently, the
reported contributions to bonding are slightly under- and overestimated.
However, the error is expected to be only minor, as the metal-phosphorus
bond lies on the C2axis.
(95) Thomas, K.; Dumler, J. T.; Renoe, B. W.; Nyman, C. J.; Roundhill,
D. M. Inorg. Chem. 1972, 11, 1795.
(96) Chinakov, V. D.; Il’inich, G. N.; Zudin, V. N.; Likholobov, V. A.;
Nekipelov, V. M. J. Organomet. Chem. 1989, 366, 421.
(97) Goel, R. G.; Srivastava, R. C. J. Organomet. Chem. 1983, 244,
303.
(92) Xile, H.; Yongjun, T.; Gantzel, P.; Meyer, K. Organometallics 2003,
22, 612.
(93) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C79.
(94) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939.
(98) Caputo, R. E.; Mak, D. K.; Willett, R. D.; Roundhill, S. G. N.;
Roundhill, D. M. Acta Crystallogr. 1977, B33, 215.

998 Organometallics, Vol. 28, No. 4, 2009
Scheme 4. Complete in Situ Hydrolysis of 1 in Wet CH2Cl2 Solution (anion is TfO-)
Caputo et al.
Scheme 5. Proposed Mechanisms for the Hydrolysis of 1 (anion is TfO-)
which has been independently characterized by Ackermann and
Born (Scheme 4).66
kJ mol-1 more stable than the alternative platinum hydroxide.
We therefore conclude that route (a) is the more reasonable
pathway. This mechanism is further supported by the calculated
LUMO of 1, which is localized predominantly on the NHP
P-atom, with smaller contributions from the metal and N-atoms
(Figure 4), and by the analogous reactivity of group 6
metal-NHP complexes, which undergo nucleophilic attack by
Me- and EtO- at the NHP, although dissociation of the resultant
phosphine is not observed in these cases.101
In both routes (a) and (b), an additional molar equivalent of
PPh3 is necessary for formation of the observed products. In
crude reaction mixtures toward the preparation of 1 (Scheme
4), we never observe deposition of Pt black, presumably owing
Hydrolysis of 1 may reasonably occur via one of two routes,
both of which formally involve hydration of the PtdP double
bond, but are differentiated by the regiochemistry of addition:
(Scheme 5a) initial attack of H2O at the NHP, followed by
protonation of the Pt center and subsequent release and
tautomerization of the modified NHP ligand to give 11;
coordination of excess PPh3 in solution either prior to, or after
this release, would generate the hydride 10; (Scheme 5b)
nucleophilic attack of H2O at the Pt center and subsequent
deprotonation of coordinated water by NHP-turned-phosphide.
The resultant Pt(II)-hydroxide may then decompose by either
or both of two reductive elimination pathways (both analogous
to that determined by Alper99 for the oxidation of phosphines
by Pd/OH-) either to generate OdPPh3 and an analogue of 10
that incorporates a secondary phosphine derived from the NHP
and/or to generate 10 and 11. Although the formation of
OdPPh3 is favored statistically by a 2:1 margin in route (b),
we never observe it, nor do we see by NMR spectroscopy any
metallic species other than 10 that incorporate phosphine and
hydride.
The calculated natural atomic orbital charge distribution100
in 1 (+1.09 for P, +0.03 for Pt) also implicates the more cationic
NHP phosphorus atom as the likely site for nucleophilic attack
by water. In addition, the total energies calculated for the P-OH/
Pt-H and Pt-OH/P-H bonded intermediates in Scheme 5
indicate strong energetic preference for pathway (a): the initial
attack of H2O at P leads to a structure that is approximately 90
Figure 4. Calculated representation of the LUMO of 1 showing
localization predominantly at the NHP P-atom. The metal coordina-
tion plane is approximately parallel to the page, and that of the
NHP ligand is perpendicular to it.
(99) Grushin, V. V.; Alper, H. Organometallics 1993, 12, 1890.
(100) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985,
83, 735.

Phospha-Fischer Carbenes
Organometallics, Vol. 28, No. 4, 2009 999
Scheme 6. Hydrolysis of Isolated 1
Scheme 7. Overall Hydrolysis Reactions of 1-5 in CH2Cl2 or CDCl3 Solution at rt (all anions not shown are TfO-)
2
1
to the presence of excess PPh3 from the Pt(PPh3)4 starting
material in these systems. However, deliberate addition of H2O
to concentrated CH2Cl2 solutions containing pure, isolated
samples of 1 gives the expected hydrolysis products 10 and 11,
and Pt black because of the stoichiometric deficiency of PPh3
in these reactions (Scheme 6). No intermediates en route to the
bulk metal are detected.
(δ 22.2, JHP ) 22 Hz, JPPt ) 2344 Hz)105 is very similar to
106
that of the known compound [PtH(dppe-κ2P)2]PF6
and
analogous to that of the Pd complex, which has been made
107
previously from [Pd(dppe-κ2P)2]2+
.
Conclusions
Taken together, metrical parameters, computational results,
and spectroscopic and reactivity data collectively support the
interpretation that the most appropriate representation of 1
(and 2) is that which shows an MdP double bond and a
formal positive charge residing on the NHP phosphorus, i.e.,
the phospha-Fischer carbene D, which is formally equivalent
to E (Chart 4), albeit with diminished and augmented σ- and
π-interactions, respectively, when compared to traditional
carbon-based systems. This representation of the group 10
metal(0) compounds is warranted also on the grounds of
direct analogy to the characteristics generally attributed to
Fischer carbenes108 and highlights the “diagonal relation-
ship”18 between C and P: Fischer carbenes are derived from
singlet :CR2, contain an electrophilic carbene center, have
π-donor groups adjacent to the carbene center, are typically
associated with late metals in low oxidation states, and are
found in conjunction with π-accepting ancillary ligands. We
note also that Fischer carbenes are recognized as two-electron
The phosphine-containing hydrolysis product depends on the
M-NHP starting material. Thus, the expected hydride
[HPt(PMe3)3]OTf102 does not form as the final product when 3
is hydrolyzed; instead, we observe the known homoleptic
103,104
complex [Pt(PMe3)4]OTf2
(Scheme 7). Presumably, the
predicted cation [HPt(PMe3)3]+ is formed initially, but due to
the greater Lewis basicity of PMe3 relative to PPh3, it is much
more hydridic than 10 and very easily protonated by adventitious
H2O; the resultant coordinatively unsaturated [Pt(PMe3)3]2+ is
then trapped by PMe3 to give the final product.
Hydrolysis of 5 gives the five-coordinate platinum hydride
[PtH(dppe-κ2P)2]OTf. A characteristic pentet (with Pt satellites)
1
is observed in the high-field region of the H NMR spectrum
of this species (δ -10.6, 1JHPt ) 645 Hz, 2JHP ) 32 Hz), which
unambiguously indicates the presence of a hydride coupled to
four equivalent phosphines and platinum. The 31P{1H} spectrum
(105) The 1H decoupler was on for this 31P experiment; however it was
unable to fully decouple the hydride proton. As a result, a doublet was still
visible, but the 1JHP value was slightly smaller than that observed in the 1H
NMR spectrum.
(106) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc.
1999, 121, 1432.
(107) Aresta, M.; Quaranta, E. J. Organomet. Chem. 2002, 662, 112.
(108) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; John Wiley & Sons, Inc.: New Haven, CT, 2001.
(101) Nakazawa, H.; Ohta, M.; Miyoshi, K.; Yoneda, H. Organome-
tallics 1989, 8, 638.
(102) Packett, D. L.; Syed, A.; Trogler, W. C. Organometallics 1988,
7, 159.
(103) Hahn, F. E.; Langenhahn, V.; Lu¨gger, T.; Pape, T.; Le Van, D.
Angew. Chem., Int. Ed. 2005, 44, 3759.
(104) Kozelka, J.; Lu¨thi, H.-P.; Dubler, E.; Kunz, R. W. Inorg. Chim.
Acta 1984, 86, 155.

1000 Organometallics, Vol. 28, No. 4, 2009
Caputo et al.
donors in both the covalent and ionic counting schemes and
do not change the formal oxidation state of the metal. We
attribute these same characteristics to NHP complexes of
Pt(0) and Pd(0).
of Canada (NSERC), a Petro-Canada Young Innovator’s
Award (to N.D.J.), the Academy of Finland (to H.M.T.), and
an Ontario Graduate Scholarship (to C.A.C.).
Despite advances in synthesis, the onward reactivity of
late metal-NHP complexes is all but unknown, and our initial
investigations into substitution and hydrolysis reactions
described herein will be followed by catalytic studies. The
unique sterically encumbered and π-acidic character of NHP
ligands holds great potential for rich and exciting new
chemistry, as does the “operational unsaturation”109,110 that
lies in the possibility for trigonal-planar to pyramidal
isomerism.
Supporting Information Available: Calculated coordinates for
1-4 and 6-9 and X-ray crystallographic data for 1 (CIF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM800973V
(109) Johnson, T. J.; Huffman, J. C.; Caulton, K. G. J. Am. Chem. Soc.
1992, 114, 2725.
Acknowledgment. We acknowledge financial support
from the Natural Sciences and Engineering Research Council
(110) Johnson, T. J.; Coan, P. S.; Caulton, K. G. Inorg. Chem. 1993,
32, 4594.