Homoleptic isocyanidemetalates of 4d- and 5d-transition metals: [Nb(CNXyl)6]-, [Ta(CNXyl)6]-, and derivatives thereof

Article abstract of DOI:10.1021/ja065964q

Treatment of [M(CO)₆]^-, M = Nb, Ta, with Ag ⁺, I₂ or NO⁺ in the presence of CNXyl provided [M(CNXyl)₇]⁺, M(CNXyl)₆I, or cis-[M(CNXyl) ₄(NO)₂]⁺, which are isocyanide analogues of the unknown carbonyl complexes [M(CO)₇]⁺, M(CO)₆I, or cis-[M(CO)₄(NO)₂]⁺, respectively. Reduction of M(CNXyl)₆I by cesium graphite gave the respective Cs[M(CNXyl)₆], which have been structurally characterized and represent the first isolable homoleptic isocyanidemetalates for second or third row transition metals. Nitrosylation of [Ta(CNXyl)₆]^- affords a rare example of a mononitrosyl tantalum complex, Ta(CNXyl) ₅NO, which is an isocyanide analogue of the unknown Ta(CO) ₅NO. This study emphasizes, inter alia, the remarkable versatility of the CNXyl ligand compared to CO in stabilizing various electronic environments at heavier group 5 metal centers.

Full text of DOI:10.1021/ja065964q

Published on Web 01/11/2007
Homoleptic Isocyanidemetalates of 4d- and 5d-Transition
Metals: [Nb(CNXyl)6]-, [Ta(CNXyl)6]-, and Derivatives Thereof1
Mikhail V. Barybin,*,† William W. Brennessel, Benjamin E. Kucera,
Mikhail E. Minyaev, Victor J. Sussman, Victor G. Young, Jr., and John E. Ellis*
Contribution from the Department of Chemistry, UniVersity of Minnesota,
Minneapolis, Minnesota 55455
Received August 23, 2006; E-mail: ellis@chem.umn.edu
Abstract: Treatment of [M(CO)6]-, M ) Nb, Ta, with Ag+, I2 or NO+ in the presence of CNXyl provided
[M(CNXyl)7]+, M(CNXyl)6I, or cis-[M(CNXyl)4(NO)2]+, which are isocyanide analogues of the unknown
carbonyl complexes [M(CO)7]+, M(CO)6I, or cis-[M(CO)4(NO)2]+, respectively. Reduction of M(CNXyl)6I by
cesium graphite gave the respective Cs[M(CNXyl)6], which have been structurally characterized and
represent the first isolable homoleptic isocyanidemetalates for second or third row transition metals.
Nitrosylation of [Ta(CNXyl)6]- affords a rare example of a mononitrosyl tantalum complex, Ta(CNXyl)5NO,
which is an isocyanide analogue of the unknown Ta(CO)5NO. This study emphasizes, inter alia, the
remarkable versatility of the CNXyl ligand compared to CO in stabilizing various electronic environments
at heavier group 5 metal centers.
Ta,11 encouraged us to determine whether analogous isocyanide
complexes were possible for these metals. Recently, reports on
Introduction
Almost 50 years ago in a comprehensive review on transition
metal isocyanide complexes, Malatesta pointed out that “no
isocyanide complexes are so far known corresponding to the
carbonylmetalates [Fe(CO)4]2-, [Co(CO)4]-, and [Rh(CO)4]-,
...”2 Afterward, related homoleptic metal anions with PF3,
P(OMe)3, ethylene, and other acceptor ligands were reported.3
However, nearly 30 years elapsed before the first isocyanide
complex of this type saw the light of day. In 1989, Warnock
andCooperpublishedtheirremarkablesynthesisof[Co(CNXyl)4]-,
Xyl ) 2,6-dimethylphenyl,4 thereby providing the first evidence
that a series of homoleptic isocyanidemetalates, formally
analogous to the long known carbonylmetalates,5 might be
possible. Subsequently, Cooper and co-workers reported on
related isocyanidemetalates of Ru(II-)6 and Mn(I-)7 and pre-
sented a full account of the cobaltate synthesis, including a
structural authentication of [Co(CNXyl)4]-.8
[V(CNXyl)6]- and derivatives thereof12 and preliminary findings
on related niobium and tantalum chemistry were published.13
Now we present the first full account of the latter investigations.
Significant new results include synthesis and isolation of the
previously unknown [Nb(CNXyl)6]-, and structural character-
izations of Cs[Nb(CNXyl)6], Cs[Ta(CNXyl)6], and Ta(CNXyl)6I.
Interestingly, the only prior report on homoleptic isocyanide-
metalates involving heavier, i.e., non-3d block, transition metals
wasforrutheniumcomplexesofthegeneralformula[Ru(CNR)4]2-,
for R ) t-Bu or CNXyl. However, these species were very
thermally unstable and could not be isolated. As a result, the
only evidence for their existence was a solution IR spectrum
(for R ) Xyl) and characterization of neutral derivatives, such
as trans-Ru(CNR)4(SnPh3)2.6 The present study shows for the
first time that homoleptic isocyanidemetalates are possible for
both 4d- and 5d-metals and can exist both in solution and in
the solid state.
Our development of facile syntheses of group 5 homoleptic
metalates, [ML6]-, for L ) CO, PF3 and M ) V,9 Nb,10 and
Experimental Section14
† Present address: Department of Chemistry, University of Kansas,
Lawrence, Kansas 66045.
(1) Highly Reduced Organometallics, Part 60. Part 59: Brennessel, W. W.;
Ellis, J. E. Angew. Chem., Int. Ed., in press.
[Ta(CNXyl)7][BF4], 2. A cold (-60 °C) clear yellow solution of
[Et4N][Ta(CO)6] (0.500 g, 1.04 mmol) and CNXyl (1.100 g, 8.39 mmol)
in 150 mL of THF was rapidly transferred into a cold (-60 °C) flask,
containing solid Ag[BF4] (0.410 g, 2.11 mmol). The mixture became
(2) Malatesta, L. Prog. Inorg. Chem. 1959, 1, 283.
(3) Ellis, J. E. Inorg. Chem. 2006, 45, 3167.
(4) Warnock, G. F.; Cooper, N. J. Organometallics 1989, 8, 1826.
(5) Ellis, J. E. Organometallics 2003, 22, 3322.
(6) Corella, J. A.; Thompson, R. L.; Cooper, N. J. Angew. Chem., Int. Ed.
1992, 31, 83.
(11) Ellis, J. E.; Warnock, G. F.; Barybin, M. V.; Pomije, M. K. Chem.sEur.
J. 1995, 1, 521.
(12) (a) Barybin, M. V.; Young, V. G., Jr.; Ellis, J. E. J. Am. Chem. Soc. 1998,
120, 429. (b) Barybin, M. V.; Young, V. G., Jr.; Ellis, J. E. J. Am. Chem.
Soc. 2000, 122, 4678.
(13) (a) Barybin, M. V.; Young, V. G., Jr.; Ellis, J. E. J. Am. Chem. Soc. 1999,
121, 9237. (b) Barybin, M. V.; Young, V. G., Jr.; Ellis, J. E. Organome-
tallics 1999, 18, 2744.
(14) General experimental procedures and details of the syntheses and charac-
terizations of 1, 3, 5, 7-11 are provided in the Supporting Information.
(7) Utz, T. L.; Leach, P. A.; Geib, S. J.; Cooper, N. J. Chem. Commun. 1997,
847.
(8) Leach, P. A.; Geib, S. J.; Corella, J. A.; Warnock, G. F.; Cooper, N. J.
J. Am. Chem. Soc. 1994, 116, 8566.
(9) (a) Barybin, M. V.; Pomije, M. K.; Ellis, J. E. Inorg. Chim. Acta 1998,
269, 58. (b) Liu, X.; Ellis, J. E. Inorg. Synth. 2004, 34, 96.
(10) Barybin, M. V.; Ellis, J. E.; Pomije, M. K.; Tinkham, M. L.; Warnock,
G. F. Inorg. Chem. 1998, 37, 6518.
9
10.1021/ja065964q CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 1141-1150
1141

A R T I C L E S
Barybin et al.
turbid and was stirred at -60 °C for 15 min. Then the reaction vessel
was removed from the cold bath, and the mixture was warmed to room
temperature. At about -15 °C, extensive gas evolution began, and the
reaction mixture turned deep red. Precipitation of white [Et4N][BF4]
and deposition of silver mirror were observed. After the evolution of
CO ceased (∼1 h), the mixture was filtered to give a grayish white
filter cake and a deep orange-red filtrate. The latter was concentrated
to 70 mL. Heptane (120 mL) was added to this concentrated solution
to cause precipitation of dark violet microcrystals. These were filtered
off, washed with pentane (4 × 15 mL), and dried under a vacuum.
Recrystallization from THF/heptane followed by drying in vacuo for
2 h afforded 1.005 g of dark purple microcrystalline, 2, 81% yield.
Mp: 198-199 °C (dec). Anal. Calcd for C63H63N7BF4Ta: C, 63.80;
H, 5.35; N, 8.27. Found: C, 63.64; H, 5.24; N, 8.45. IR (THF): νCN
2141 vw, 2029 vs, 1993 s cm-1; (Nujol mull): νCN 2144 vw, 2045 m
filter cake was washed with additional cold THF. The solution in the
receiving flask turned dark purple-brown. About 20 min after filtration,
iridescent green microcrystals began to form. The reaction mixture was
stirred at -78 °C for 12 h and then was warmed to room temperature
over a period of 4 h. The green microcrystals were filtered off, washed
thoroughly with THF (1 × 10 mL) and Et2O (4 × 20 mL). Additional
washing with THF (1 × 15 mL) and Et2O (4 × 20 mL) followed by
drying under vacuum for 2 h afforded 0.410 g of very finely divided,
iridescent green microcrystalline 6, 41% yield. Anal. Calcd for
C72H90N8O6KTa: C, 62.50; H, 6.56; N, 8.10. Found: C, 61.80; H, 6.37;
N, 7.87. IR (HMPA): νCN 2022 vw br, 1992 vw br, 1871 s sh, 1812
vs br cm-1; (Nujol mull): νCN 2024 vw, 1999 vw, 1871 s sh, 1762 vs
br cm-1. Compound 6 is practically insoluble in THF, DME, and CH3-
CN. Attempts to grow single crystals of 6 from HMPA/Et2O were
unsuccessful since the compound decomposes appreciably in HMPA
solution within several days.
1
sh, 2011 vs, 1978 s, νBF 1061 m br cm-1. H NMR (300 MHz, THF-
-C10H8)3]
Treatment of [Na(cryptand 2.2.2)][Ta(η4 15 with 6 equiv of
d8, 22 °C): δ 2.36 (s, 6H, o-CH3), 7.14 (s, 3H, m- and p-H) ppm.
13C{1H} NMR (75.5 MHz, THF-d8, 22 °C): δ 19.06 (o-CH3), 129.20
(p-C), 129.22 (m-C), 134.17 (o-C), 187.52 (W1/2 ) 6 Hz, CN) ppm.
Compound 2 (0.250 g, 0.211 mmol) was dissolved in 30 mL of THF.
This solution was filtered through a medium porosity frit. Aliquots of
the filtrate were transferred into two Schlenk tubes immersed in an ice
water bath. Cold (0 °C) pentane was layered onto the deep orange-red
solutions of 2. The tubes were kept at 4 °C for 4 days. After this period,
large dark violet X-ray quality crystals formed in both tubes.
CNXyl in THF at -78 °C, followed by warming over a period of
several hours to 0 °C, provided the analogous green microcrystalline
[Na(cryptand 2.2.2)][Ta(CNXyl)6]. This substance has identical IR
spectral features to those observed for the corresponding potassium
salt, vide infra. Unfortunately, the isolated yield was only 4%, based
on the [Ta(C10H8)3]- salt, so no attempts to optimize the conditions
for this synthesis were carried out.
Results and Discussion
Ta(CNXyl)6I, 4. A cold (-78 °C) brown solution of I2 (0.563 g,
2.22 mmol) in 20 mL of THF was added dropwise to a cold (-78 °C)
yellow solution of [Et4N][Ta(CO)6] (1.030 g, 2.15 mmol) in 20 mL of
THF over a period of 15 min. A turbid mango-colored mixture formed,
which was stirred at -78 °C for 30 min. Then a cold (-78 °C) solution
of CNXyl (2.350 g, 17.9 mmol) in 70 mL of THF was transferred into
the reaction flask via cannula. The mixture became darker and was
stirred at ca. -78 °C for 2 h. The reaction vessel was removed from
the cold bath, and the mixture was warmed to room temperature. At
ca. -10 °C, an extensive gas evolution began as the reaction mixture
turned deep red. Precipitation of white [Et4N]I was observed. After
stirring at room temperature for about 60 h, the reaction mixture was
filtered to provide an off-white filter cake and a deep red-maroon filtrate.
The filter cake was washed with an additional 10 mL of THF. All
solvent was removed from the filtrate under a vacuum to give a
somewhat oily deep red-maroon solid. The solid was triturated in 200
mL of pentane resulting in dark maroon (almost black) microcrystals.
These were filtered off and washed thoroughly with pentane (4 × 20
mL). The product was redissolved in toluene (200 mL), and the resulting
red-maroon solution was filtered. All toluene was removed under a
vacuum, and the residue was triturated in 150 mL of pentane. The solid
obtained was filtered off, washed with pentane (3 × 20 mL), and dried
under a vacuum for 2 h to afford 1.880 g of microcrystalline, free-
flowing, dark maroon 4, 80% yield. Mp: 119-122 °C (dec). Anal.
Calcd for C54H54N6ITa: C, 59.24; H, 4.97; N, 7.68. Found: C, 58.98;
H, 4.96; N, 7.74. IR (THF): νCN 2169 vw, 2032 vs, 1996 s, 1969 m
sh, 1850 vw sh cm-1; (Nujol mull): νCN 2140 vw, 2097 w, 2057 m sh,
2015 vs, 1983 vs, 1844 m sh cm-1. 1H NMR (300 MHz, C6D5CD3, 22
°C): δ 2.40 (s, 6H, o-CH3), 6.70 (s br, 3H, m- and p-H) ppm. 13C
{1H} NMR (75.5 MHz, C6D5CD3, 22 °C): δ 19.54 (o-CH3), 126.54
(p-C), 128.33 (m-C), 130.34 (i-C), 133.61 (o-C), 199 (br, W1/2 ≈ 150
Hz, CN) ppm. Compounds 3 and 4 form dichroic crystals, which appear
to be dark maroon in microcrystalline form or dark green as relatively
large crystals. X-ray quality single crystals of 4 were grown from THF-
pentane over a period of 6 days at room temperature.
Shortly after a conventional procedure for the preparation and
isolation of bis(naphthalene)vanadium(0) was developed,16 it
was shown to readily shed both naphthalene ligands in the
presence of 6 equiv of CNXyl, to afford high yields of
V(CNXyl)6, the first isolable 17 electron homoleptic zerovalent
metal isocyanide complex. One electron oxidation and reduction
of this species gave the corresponding 16 and 18 electron
complexes [V(CNXyl)6]+ and [V(CNXyl)6]-, respectively.12
Their robust character suggested that related niobium and
tantalum species should exist. Early attempts to access the
homoleptic metalates, [M(CNXyl)6]-, M ) Nb, Ta, via direct
reactions of CNXyl with tris(anthracene)niobate(1-)17 or impure
solutions of tris(naphthalene)tantalate(1-)15 failed, due to facile
polymerization of the isocyanide ligand.
We then considered the use of the hexacarbonylmetalates-
(1-) of Nb and Ta5 as precursors to homoleptic isocyanides.
Although [M(CO)6]- are normally inert toward ligand substitu-
tion reactions,18 upon mild oxidation they readily lose one or
more CO groups to afford substituted derivatives of the unknown
[M(CO)7]+.19-22 When relatively good donor groups are present
during the oxidations, such as halides, organophosphanes, and
similar ligands, invariably heteroleptic M(I) complexes form,
e.g., [Nb2(CO)8Cl3]-19 and Ta(CO)3(PMe3)3Br,21 which are also
derivatives of the unknown M(CO)6X, X ) halide. Indeed,
Rehder and co-workers had previously shown that halogen
oxidations of mixtures of [Nb(CO)6]- or [Ta(CO)6]- and several
(15) Brennessel, W. W.; Ellis, J. E.; Pomije, M. K.; Sussman, V. J.; Urnezius,
E.; Young, V. G., Jr. J. Am. Chem. Soc. 2002, 124, 10258.
(16) Pomije, M. K.; Kurth, C. J.; Ellis, J. E.; Barybin, M. V. Organometallics
1997, 16, 3582.
(17) Brennessel, W. W.; Ellis, J. E.; Roush, S. N.; Strandberg, B. R.;
Woisetschla¨ger, O. E.; Young, V. G. Chem. Commun. 2002, 2356.
(18) Davison, A.; Ellis, J. E. J. Organometal. Chem. 1971, 31, 239.
(19) Calderazzo, F.; Pampaloni, G.; Zanazzi, P. F. J. Chem. Soc., Chem.
Commun. 1982, 1304.
[K(cryptand 2.2.2)][Ta(CNXyl)6], 6. A deep red-maroon solution
of 4 (0.800 g, 0.73 mmol) in 70 mL of THF was cooled to -78 °C
and then was transferred to a cold (-78 °C) suspension of KC8 (0.396
g, 2.93 mmol) in 15 mL of THF to give a dark brown solution/slurry.
After stirring for 2.5 h, the cold mixture was filtered at -78 °C into a
flask, containing solid cryptand 2.2.2 (0.300 g, 0.80 mmol). The black
(20) Calderazzo, F.; Castellani, M.; Pampaloni, G.; Zanazzi, P. F. J. Chem. Soc.,
Dalton Trans. 1985, 1989.
(21) Leutkins, M. L.; Santure, D. J.; Huffman, J. C.; Sattelberger, A. P. J. Chem.
Soc., Chem. Commun. 1985, 552.
(22) Calderazzo, F.; Pampaloni, G.; Zanazzi, P. F. Chem. Ber. 1986, 119, 2796.
9
1142 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Homoleptic Isocyanidemetalates of 4d/5d Transition Metals
A R T I C L E S
isocyanides gave only heteroleptic isocyanide complexes, such
as Nb(CNcyclohexyl)4Br3, Nb(CO)2(CNXyl)4I, and Ta(CO)3-
(CNtBu)3I.23 For these reasons, we decided to investigate the
oxidations of [M(CO)6]- solely in the presence of isocyanides.
Also noteworthy is a 1992 report by Lippard and co-workers
on the first synthesis of monovalent isocyanides of Nb and Ta,
M(CO)(CNR)(dmpe)2Cl, (R ) alkyl, dmpe ) 1,2-bis(dimeth-
ylphosphano)ethane), and related compounds, which were
obtained by alternative routes.24
Initially CNXyl was employed in our studies because it is a
better acceptor and weaker donor than alkyl isocyanides2 and
on this basis appeared better suited than the latter to afford the
desired homoleptic complexes of Nb(I) and Ta(I). Also, in
contrast to other aryl isocyanides, which are often thermally
unstable and highly malodorous liquids at room temperature,2
xylyl isocyanide is an easily handled solid of fair thermal
stability and relatively low volatility under normal conditions.
Whether homoleptic alkylisocyanidemetalates(1-) of the group
5 metals are accessible remains unknown.
Synthesis and Properties of [M(CNXyl)7][BF4], M ) Nb
(1), Ta (2). Addition of THF solutions of [Et4N][M(CO)6], M
) Nb, Ta, and about 8 equiv of CNXyl to 2 equiv of solid
AgBF4 at -60 °C gave no immediate reaction. However, when
the reaction mixtures were warmed to about +15 °C (Nb) or
-15 °C (Ta), rapid evolution of gas occurred and the color of
the solutions abruptly changed from golden yellow to deep red.
Following removal of insoluble [Et4N][BF4] and silver metal
by filtration, unexceptional workups provided dark purple
microcrystals of 1 and 2 in about 80% yields (eq 1).
Figure 1. Molecular structure of the cation in 2: 50% thermal ellipsoids;
hydrogens omitted for clarity. See text and Table 1 for relevant distance
and angle data.
observed at 187.5 ppm and occurs about 22 ppm downfield of
the corresponding absorption for free CNXyl (165.4 ppm). This
shift, along with the aforementioned IR data, provides convinc-
ing evidence for significant tantalum dπ-π* backbonding to
the coordinated CNXyl groups. A single-crystal X-ray study
on 2 was carried out to verify its formulation in the solid state,
and it revealed the presence of discrete [Ta(CNXyl)7]+ cations
and [BF4]- anions, where both are well separated in the
crystalline lattice. Interestingly, 1 and 2 represent the only
known 18 electron homoleptic complexes of Nb(I) and Ta(I).
To the best of our knowledge, the only other related species is
the 16 electron [Nb(mesitylene)2]+, mesitylene ) 1,3,5-trim-
ethylbenzene, but this highly reactive substance has not been
fully characterized.26
[M(CO)6]- + 2 AgBF4
+ 7 CNXyl
M ) Nb, Ta
THF
[M(CNXyl) ][BF ]
yields: ca. 80%
-60 to +20 °C8
(1)
7
4
-2 Ag, -6 CO
-
-BF4
Satisfactory elemental analyses were obtained in support of the
formulations of 1 and 2. The latter are moderately air sensitive
solids but are quite thermally robust; i.e., they slowly darken at
about 170 °C under an argon atmosphere. Both compounds are
diamagnetic and have nearly superimposable IR and NMR
spectra, indicating that they likely have very similar structures.
Infrared spectra of 1 and 2 in THF, in the isocyanide CN
stretching frequency region, show intense absorptions at about
2030 cm-1, with a shoulder at ca. 1994 cm-1. The positions of
the major absorptions are very similar to that previously reported
for [V(CNXyl)6][PF6].12 Because the IR ν(CN) absorptions for
1 and 2 are “red-shifted” by about 86 cm-1 relative to free
CNXyl, which absorbs at 2116 cm-1 in THF, it is clear that the
metals in these complexes effectively participate in back-
bonding to the coordinated isocyanides. NMR spectra of 1 and
2 feature signals attributable to only one CNXyl environment,
indicating that these seven coordinate species are fluxional at
room temperature. Although the 13C resonance of the terminal
isocyanide carbons in 1 could not be observed, likely due to
extensive broadening by the quadrupolar 93Nb nucleus (I ) 9/2,
100% abundant),25 the corresponding signal in 2 was easily
The molecular structure of the cation in 2 is shown in Figure
1. The mean isocyanide C-N bond length of 1.168(11) Å for
2 is essentially identical to the corresponding distance of 1.169-
(6) Å found for [V(CNXyl)6]+.12 This result is also consistent
with the observation that the IR active isocyanide C-N
stretching frequencies for both species are very similar. All Ta-
C-N units are practically linear and average 176(2)°. Complex
2 is very crowded with the shortest Me-Me contact being only
3.593(8) Å (between C(27) and C(53)). The other Me-Me
distances under 4.0 Å (the doubled Van der Waals radius of a
methyl group) are C(35)-C(54) ) 3.625(8) Å, C(18)-C(45)
) 3.752(10) Å, C(8)-C(18) ) 3.855(11) Å, C(8)-C(36) )
3.860(10) Å, and C(35)-C(45) ) 3.881(9) Å.
Seven discrete CNXyl ligands are present in 2, and a detailed
analysis of the geometry of its TaC7 core involving interligand
angles, shape-determining dihedral angles, and the Ta-C bond
length pattern (see Supporting Information) indicates that it is
best described as a capped octahedron slightly distorted toward
a capped trigonal prism. The Ta-C bond distances (Å) fall into
the 1:3:3 pattern expected of a capped octahedron, where C(37)
(23) (a) Collazo, C.; Rodewald, D.; Schmidt, H.; Rehder, D. Organometallics
1996, 15, 4884. (b) Rehder, D.; Bu¨ttcher, C.; Collazo, C.; Hedelt, R.;
Schmidt, H. J. Organomet. Chem. 1999, 585, 294.
(25) Rehder, D. In Multinuclear NMR; Mason, J., Ed., Plenum: New York,
1987; Chapter 19, pp 479-519.
(26) Calderazzo, F.; Pampaloni, G.; Rocchi, L.; Englert, U. Organometallics
1994, 13, 2592.
(24) Carnahan, E. M.; Lippard, S. J. J. Am. Chem. Soc. 1990, 112, 3230.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1143

A R T I C L E S
Barybin et al.
Scheme 1. Syntheses of M(CNXyl)6I, M ) Nb, Ta
caps a trigonal face defined by C(10), C(28), and C(46), i.e.,
Ta-C(37) 2.103(5) (cap); Ta-C(10) 2.163(6), Ta-C(28) 2.144-
(5), Ta-C(46) 2.155(5) (capped face); and Ta-C(1) 2.194(5),
Ta-C(19) 2.200(5), Ta-C(55) 2.214(6) (uncapped face). Also,
the capping Ta-C(37) bond vector is practically perpendicular
(89°) to the capped trigonal face, where an angle of 90° is
expected for an ideal capped octahedron.27 Although the
arrangement of the CNXyl ligands about the tantalum is likely
to be influenced by steric repulsions of these bulky substituents,
it is interesting to note that the geometry of the TaC7 core in 2
is strikingly similar to that of the MoC7 core of [Mo(CNMe)7]2+
.
The latter complex, isolated as a [BF4]- salt, was described as
“a slightly distorted capped octahedron.”28 Very few chemical
reactions of 1 and 2 have been examined to date. The reactions
of 2 with I- and KC8 will be described below.
Figure 2. Molecular structure of 4: 50% thermal ellipsoids; hydrogens
omitted for clarity. Selected bond lengths (Å) and angles (deg): Ta-I
2.9422(3), Ta-C(37) 2.075(3), Ta-C(28) 2.093(3), Ta-C(46) 2.159(3),
Ta-C(1) 2.162(3), Ta-C(19) 2.168(3), Ta-C(10) 2.180(3), C(37)-N(5)
1.198(4), C(28)-N(4) 1.186(4), C(46)-N(6) 1.168(4), C(1)-N(1) 1.166-
(4), C(19)-N(3) 1.171(4), C(10)-N(2) 1.163(4), av N-Xyl 1.393(5), av
Ta-C-N 1.76(3), C(1)-N(1)-C(2) 173.3(3), C(10)-N(2)-C(11) 169.0-
(3), C(19)-N(3)-C(20) 175.0(4), C(28)-N(4)-C(29) 162.5(4), C(37)-
N(5)-C(38) 152.4(3), C(46)-N(6)-C(47) 171.2(3), av C-N-C 167(8).
Synthesis and Properties of M(CNXyl)6I, M ) Nb (3), Ta-
(4). Treatment of [Et4N][M(CO)6], M ) Nb, Ta with 1 equiv
of I2 in THF followed by 7 equiv of CNXyl at -78 °C, and
slow warming of the reaction mixture to room temperature,
produced extensive evolution of gaseous CO, precipitation of
colorless [Et4N][I], and deep maroon solutions. The reaction
times of ca. 60 h were crucial for the reactions to go to
completion, otherwise mixed carbonyl isocyanide complexes
were present. Workup of the resulting deep red-maroon solutions
afforded neutral complexes 3 and 4 as analytically pure, air-
sensitive, dark maroon solids in 66% and 80% yields, respec-
tively. The isolation of both compounds required vigorous
trituration of the initially somewhat oily products in pentane.
Complexes 3 and 4 are very soluble in toluene and slightly
soluble in saturated hydrocarbons, consistent with their formula-
tion as neutral substances. As in the case of 1 and 2, compounds
3 and 4 decompose in acetonitrile with liberation of free CNXyl.
Prior to the addition of the CNXyl, IR spectra in the ν(CO)
region established that the iodination of [M(CO)6]- provided
mango-colored solutions of the respective dinuclear [M2(CO)8-
(µ-I)3]-, which were first described by Calderazzo and co-
workers.20 Addition of the CNXyl ultimately provided 3 and 4
respectively, Scheme 1.
THF show two intense isocyanide ν(CN) peaks in positions
nearly identical to those observed for 2, 2032 and 1996 cm-1
.
However, two weaker shoulders at lower energies, 1969 cm-1
and 1856 cm-1, are also present. The 13C signal for the terminal
isocyanide carbon atoms in 4 appears as a single broad
resonance at 199 ppm, about 12 and 34 ppm downfield of
corresponding peaks observed for 2 (187.5 ppm), and free
CNXyl (165.4 ppm), respectively. Both IR and 13C NMR spectra
indicate that tantalum to isocyanide back-donation increases
slightly in proceeding from cationic 2 to neutral 4, a well-
precedented phenomenon in metal isocyanide chemistry.29
The solid-state structure of 4 is shown in Figure 2. Analysis
of the interligand angles and the metal-ligand bond distance
pattern for 4 (see Supporting Information) indicates that its
“TaIC6” core is best described as a capped trigonal prism, with
the iodine ligand capping the prism’s face, which is composed
of the atoms C(1), C(10), C(19), and C(46). The Ta-L distances
in 4 fall into a 1:4:2 pattern. The two shortest of these involve
the atoms C(28) and C(37), which form the unique edge of the
capped trigonal prism. Notably, the most acute C-N-C angles
in the structure of 4 involve these two carbon atoms as well.
Syntheses and Properties of Cs[Nb(CNXyl)6] (5), [K(crypt
2.2.2)][Ta(CNXyl)6] (6), and Cs[Ta(CNXyl)6] (7). Compounds
5 and 7 were best prepared by the reduction of 3 and 4,
respectively, with about a 2-fold excess (4.0-4.5 equiv) of CsC8
in THF at -78 °C, followed by a filtration at -20 to 0 °C to
give deep brown solutions, eq 3.
The above reactions involved initial formation of mixed
carbonyl isocyanide complexes, such as the previously isolated
Nb(CO)2(CNXyl)4I,23b but no attempts were made to character-
ize any intermediates. An alternative route to 4 involved the
reaction of 2 with a soluble source of I- and demonstrates the
ease with which a xylyl isocyanide ligand is displaced from
the crowded [Ta(CNXyl)7]+, eq 2.
[Ta(CNXyl)7][BF4] + [Bu4N]I THF 8 Ta(CNXyl)6I +
20 °C
[Bu4N][BF4] + CNXyl (2)
Spectroscopic properties of 3 and 4 are nearly identical, so
only those of 4 will be addressed. IR spectra of the latter in
M(CNXyl)6I + xs CsC8 THF, -78 °C8 Cs[M(CNXyl)6]
-CsI
(3)
(27) (a) Kepert, D. L. Prog. Inorg. Chem. 1979, 25, 41. (b) Kepert, D. L.
Inorganic Stereochemistry; Springer-Verlag: Berlin, 1982.
By careful layering of excess pentane or a heptane-pentane
mixture on these solutions, single crystals of X-ray quality were
obtained. Both 5 and 7 were extremely air sensitive, and attempts
(28) Brant, P.; Cotton, F. A.; Sekutowski, J. C.; Wood, T. E.; Walton, R. A.
J. Am. Chem. Soc. 1979, 101, 6588.
(29) Singleton, E.; Oosthuizen, H. E. AdV. Organomet. Chem. 1983, 22, 209.
9
1144 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Homoleptic Isocyanidemetalates of 4d/5d Transition Metals
A R T I C L E S
to obtain satisfactory analyses were not successful. In addition,
5 was so thermally unstable in solution that its isolated yield
was only about 1%. Although the compositions of 5 and 7 in
the solid state were established only by X-ray studies on single
crystals, spectroscopic properties of the bulk samples of 5 and
7 were entirely consistent with the assigned formulations, vide
infra.
Reduction of 4 by excess KC8 in THF at low temperature
provided a species that was significantly less thermally stable
than the cesium salt 7, in solution or in the solid state, but
addition of crypt 2.2.2 afforded poorly soluble iridescent green
microcrystals of [K(crypt 2.2.2)][Ta(CNXyl)6], 6, in 41% yield.
These were significantly less sensitive than 7 and provided
satisfactory analyses. Another route to [Ta(CNXyl)6]- involved
the reaction of highly labile tris(η4-naphthalene)tantalate(1-),
as the previously characterized [Na(crypt 2.2.2)]+ salt15 with 6
equiv of CNXyl. During this reaction, much of the free
isocyanide appeared to be polymerized by the strongly reducing
naphthalenetantalate(1-). However, about a 4% isolated yield
was obtained for iridescent green microcrystalline [Na(crypt
2.2.2)][Ta(CNXyl)6], eq 4. The potassium analogue 6 and this
substance had superimposable IR spectra in the isocyanide
stretching frequency region as mineral oil mulls.
Figure 3. IR spectra of 7 in THF in the absence (fine line) and presence
(thick line) of 2 equiv of 18-crown-6.
atoms increases in the order [Ta(CNXyl)7]+ (187.5 ppm) < TaI-
(CNXyl)6 (199 ppm) < [Ta(CNXyl)6]- (210.4), a result fully
consistent with the infrared spectral data for these complexes,
vide supra.
As in the case of Cs[V(CNXyl)6], addition of 2 equiv of 18-
crown-6 to THF solutions of 7 caused the ν(CN) bands to
become sharper (Figure 3), due to the formation of [Cs(18-
crown-6)2]+,32 which presumably interacts more weakly with
and causes less perturbation of [Ta(CNXyl)6]- than Cs+.33 The
peaks at 2021 vw br, 1870 sh, and 1817 vs br cm-1 in the
infrared spectrum of [Cs(18-crown-6)2][Ta(CNXyl)6] were
assigned to A1g, Eg, and T1U νCN modes of vibration, respec-
tively, assuming ideal Oh symmetry of 7. Indeed, the ratio (20212
- 18702)/(18702 - 18172) ) 3.01, which is essentially identical
to the required value of 3.0 for an approximately octahedral
molecule of this type.34 The formally IR-forbidden A1g and Eg
modes have nonzero intensities because, as suggested above,
the true symmetry of 7 in solution is expected to be substantially
lower than Oh.
Crystallization of 5 and 7 from THF/pentane afforded dark
brown needles of composition Cs[M(CNXyl)6] and space group
R3h, with the metal atoms lying on inversion centers within the
3h axis. This results in one independent metal-isocyanide unit
for both structures, depicted for the niobate in Figure 4. Anions
of 5 and 7 proved to be almost isostructural with nearly identical
interatomic distances and angles. Details of the structure of 7
will be emphasized in this discussion because the tantalate is
also well characterized in solution by IR and NMR spectra.
Selected interatomic data for free CNXyl, [Ta(CNXyl)7]+, 2,
and [Ta(CNXyl)6]-, 7, are collected in Table 1 and establish
that Ta to isocyanide backbonding increases on proceeding from
2 to 7, the same trend established earlier from spectral data. In
particular, average Ta-C distances decrease and isocyanide
C-N distances increase from 2 to 7, respectively, in accord
with the classic Dewar-Chatt-Duncanson model for back-
4
+ 6 CNXyl
[Na(L)][Ta(η -C10H8)3]
L ) crypt 2.2.2
THF
-78 to 20°8
[Na(L)][Ta(CNXyl) ]
(4)
6
ca. 4% yield
-3 C10H8
Infrared spectra of 5 and 7 in THF in the isocyanide ν(CN)
region consist of quite broad bands centered at about 1820 cm-1
,
approximately the same shape and position as those of the
corresponding absorption previously observed for Cs[V(C-
NXyl)6].12 These peaks are shifted about 200 cm-1 to lower
energy compared to the isocyanide stretching frequencies of the
cationic 1 and 2, consistent with the expected greater metal to
isocyanide back-bonding in proceeding from homoleptic iso-
cyanide M(I) to M(I-) species. Due to the poor thermal stability
of 5, which decomposed within minutes at 20 °C in THF,
additional spectral studies have only been carried out on 7. 1H
and 13C NMR spectra of 7 in THF-d8 showed the same pattern
of resonances in very similar positions to those previously
observed for Cs[V(CNXyl)6].12 However, 13C NMR spectra of
7 also revealed a well-defined singlet at 210.4 ppm, which, until
recently, was the most downfield resonance known for a
terminal isocyanide ligated carbon in a diamagnetic metal
isocyanide complex.30 Interestingly, this shift is practically
identical to that observed for the carbon atoms of [Ta(13CO)6]-,
211 ppm.11 However, the 13C peak for 7 is relatively narrow,
∆ν1/2 ) 23 Hz, compared to the corresponding signal observed
for hexacarbonyltantalate, ∆ν1/2 ) 820 Hz, likely due to
unresolved 13C-181Ta coupling, where 181Ta is 99.99% abundant
7
with I ) /2.31 This feature is yet another indication that the
effective symmetry of [Ta(CNXyl)6]- in solution is much lower
than that of the nearly octahedral [Ta(CO)6]-.25 As expected,
the magnitude of the 13C chemical shift of the ligating carbon
(32) [Cs(18-crown-6)2]+ is well established in several salts and has been
previously employed in the isolation of [V(CNXyl)6]-.12 See: (a) Vidal,
J. L.; Troup, J. M. J. Organomet. Chem. 1981, 213, 351. (b) Tinkham,
M. L.; Dye, J. L. J. Am. Chem. Soc. 1985, 107, 6129.
(30) Very recently, [Fe(CNXyl)4]2- was reported to have a 13C resonance at
238.7 ppm for the ligated isocyanide carbon. Also, the dianionic iron
complex has an average isocyanide C-N-C angle of 144(3)°.1
(31) Emsley, J. The Elements, 3rd ed.; Clarendon Press: Oxford, U.K., 1998;
pp 140, 200.
(33) Similar changes in the IR spectra of metal carbonyl anions in the ν(CO)
region in THF are observed when an alkali metal cation is replaced by a
larger and less perturbing counterion. See: Darensbourg, M. Y. Prog. Inorg.
Chem. 1984, 33, 221.
(34) Cotton, F. A.; Kraihanzel, C. S. J. Am. Chem. Soc. 1962, 84, 4432.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1145

A R T I C L E S
Barybin et al.
Figure 5. Packing diagram of 7, parallel to the ab plane, viewed down
extended chains of linear Cs-Ta units.
frequencies of 2 and 7 and corresponding Ta-C and isocyanide
C-N distances unequivocally show that strengthening of the
Ta-C bonds and weakening of the isocyanide C-N bonds are
correlated with a dramatic bending of the CNC unit. Although
we cannot rule out the possibility that cesium-isocyanide
interactions also contribute to the distinctly nonlinear CNC units
present in 7 and 5, Lippard and co-workers have observed
similar trends in an unusual neutral seven-coordinate Ta(I)
species, Ta(CNMe)2(dmpe)2Cl. Remarkably, the latter substance
appears to possess the most nonlinear CNC angles previously
recorded for terminal isocyanide ligands, 122.1(7)°, 121.8(7)°,
and also has very short Ta-C distances, 2.022(7), 2.012(7)Å,
Figure 4. Molecular structure of the anion in 5: 50% thermal ellipsoids;
hydrogens omitted for clarity. Selected bond lengths (Å) and angles (deg):
Nb-C(1) 2.141(3), C(1)-N(1) 1.192(4), N(1)-C(2) 1.401(4), Nb-C(1)-
N(1) 178.3(3), C(1)-N(1)-C(2) 157.0(3), C(1)-Nb-C(1C) 84.8(1), C(1)-
Nb-C(1A) 95.2(1), C(1)-Nb-C(1B) 180.0.
Table 1. Selected Structural Data for Free CNXyl, [Ta(CNXyl)7]+,
2, and [Ta(CNXyl)6]-, 7
CNXyla
2b
7b
Ta-C (Å)
-
2.15(1)c
1.17(1)
1.401(7)
176(2)
2.127(3)
1.199(4)
1.393(4)
178.4(3)
155.7(3)
C-NXyl (Å)
N-CXyl (Å)
Ta-C-N (deg)
C-N-C (deg)
1.160(3)
1.399(2)
-
and quite low ν(CN) values of 1737, 1695 cm-1 38
.
179.4(2)
173(5)
The tantalum and cesium atoms in 7 form infinite linear
chains with a Ta-Cs distance of 3.886(3) Å and with Cs-Ta-
Cs and Ta-Cs-Ta angles of 180°, as shown in Figure 5.
Cesium cations interact identically with three facial isocyanides
with Cs-C(1) and Cs-N(1) distances of 3.304(3) and 3.547-
(3)Å, respectively. These close contacts, shown in Figure 6,
are best described as arising from electrostatic attractive forces
and are responsible for appreciable distortion of the octahedral
TaC6 cores in 7. Thus, the cis-C-Ta-C angles on opposite faces
of the octahedron closest to the cesium counterions are 94.8°,
while those on the other six faces average 88.4°, smaller than
the ideal 90° value. As required by symmetry, the trans-C-
Ta-C angles are 180.0°.
a Reference 35. b This work; average values for 2. c Average Ta-C
distance for the three RNC groups on the capped face of 2, which has an
approximately capped octahedral structure with three significantly different
isocyanide environments in a 1:3:3 ratio. The chosen Ta-C distance is
closest to the overall, but much less precise, average distance of 2.17(4) Å,
for all seven isocyanide groups.
bonding.36 The average C-NXyl bond length of 1.999(4) Å
for 7 is indistinguishable from analogous values of 1.20(2), 1.20-
(2), and 1.20(3) Å, reported for the only other crystallo-
graphically characterized homoleptic isocyanidemetalates,
[V(CNXyl)6]-,12 [Mn(CNXyl)5]-,7 and [Co(CNXyl)4]-,8 re-
spectively. Also, the Nb-C and Ta-C distances of 2.141(3)
and 2.127(3) Å in 5 and 7, respectively, are significantly longer,
likely for both steric and electronic reasons, than corresponding
values found for [Nb(CO)6]-, 2.089(5) Å, and [Ta(CO)6]-,
2.083(6) Å. Incidentally, the latter species, with [(Ph3P)2N]+
counterions, also afford crystals of the identical R3h space group
as 5 and 7.37
Synthesis and Properties of [Ta(CO)5(CNXyl)]- (8). Com-
pound 8 was prepared to permit a comparison of the spectral
properties of the coordinated aryl isocyanide in homoleptic anion
7, with the identical ligand in a closely related mixed isocyanide
carbonyl complex. It was obtained in about 60% yield from a
one-pot reaction sequence, shown in Scheme 2, involving an
initial reduction of [Ta(CO)6]- by sodium metal in liquid
ammonia to produce Na3[Ta(CO)5],11 followed by protonation
Of particular interest is the CNC bend angle of metal
isocyanides, which is nearly linear in the free ligand and 2 but
substantially bent, 155.7(3)°, in 7 (and a similar angle of 157.0-
(3)° in 5).30 Comparison of the IR active isocyanide stretching
with NH4 to generate the quite labile [Ta(CO)5NH3]-.39 The
+
latter is a useful synthon for the 16 electron [Ta(CO)5]- group
and readily reacts with CNXyl at about 0 °C in THF-NH3 to
(35) Mathieson, T.; Schier, A.; Schmidbaur, H. J. Chem. Soc., Dalton Trans.
2001, 1196.
(36) See: Elschenbroich, C. Organometallics, 3rd ed.; Wiley-VCH: Weinheim,
2006; p 399.
(37) Calderazzo, F.; Englert, U.; Pampaloni, G.; Pelizzi, G.; Zamboni, R. Inorg.
Chem. 1983, 22, 1865.
(38) Carnahan, E. M.; Rardin, R. L.; Bott, S. G.; Lippard, S. J. Inorg. Chem.
1992, 31, 5193.
(39) Ellis, J. E.; Fjare, K. L.; Warnock, G. F. Inorg. Chim. Acta 1995, 240,
379.
9
1146 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Homoleptic Isocyanidemetalates of 4d/5d Transition Metals
A R T I C L E S
Figure 6. Cation-anion interactions within the unit cell of 7; xylyl substituents are omitted for clarity. See text for relevant distance and angle data.
Scheme 2. Synthesis of [Et4N][Ta(CO)5(CNXyl)], 8
216.2 ppm, which establishes that CNXyl is a better donor and
weaker acceptor than CO in 8. A similar 13C NMR spectrum
has been reported for [Ta(CO)5NH3]-, δ 225.0 (1C, axCO),
219.4 (4C, eq CO),39 where the 3-4 ppm downfield shifts of
the axial and equatorial CO resonances, compared to those of
8, are due to the better donor ability of NH3 relative to CNXyl.43
Another indication CO is a better acceptor ligand than CNXyl
in 8 is that its isocyanide carbon resonance, δC ) 182.4 ppm,
is quite upfield of the analogous shift observed for the
homoleptic anion 7, δC ) 210.4 ppm. Nevertheless, the 13C
resonance of the ligating isocyanide carbon in 8 is appreciably
give the desired product. To the best of our knowledge, the
only previously established mixed isocyanidecarbonyltantalates
are [Ta(CO)5(CNtBu)]-40 and cis-[Ta(CO)(CNMe)(dmpe)2]-.41
Microcrystalline red-orange 8 proved to be moderately air-
sensitive both in solution and in the solid state and was obtained
as a satisfactorily pure substance.
deshielded relative to that observed for free CNXyl, δC ) 165.4
ppm,44 indicating a substantial dπ (Ta) to pπ* (CNXyl) back-
bonding in 8, a conclusion in accord with IR spectral data, vide
supra.
Synthesis and Properties of a Tantalum Mononitrosyl
Complex, Ta(CNXyl)5NO (9). Compound 9 was an attractive
Solution and Nujol mull infrared spectra of 8 in the ν(CN,-
CO) region are consistent with the C4V structure of its [ML5L′]z
core. The highest energy band at 2044 cm-1 is due to the ν-
(CN) vibration of the CNXyl ligand and is shifted about 72
cm-1 to lower energy than that of free CNXyl. Absorptions at
1930 and 1840 cm-1 correspond to one of the A1 and E
stretching modes of the carbonyls, respectively. As for many
other analogous complexes,42 the other A1 ν(CO) band in the
THF solution spectrum of 8 appears to be obscured by a very
intense E peak. This conclusion is supported by the presence
of a band of medium intensity at 1880 cm-1 in the Nujol mull
spectrum of 8.
target because only one well-characterized tantalum nitrosyl had
been previously reported, (trmpsi)Ta(CO)2NO, trmpsi ) tBuSi-
(CH2PMe2)3, by Legzdins and co-workers, when we began this
research.45 The latter species was prepared by nitrosylation of
the very electron-rich anionic precursor, [(trmpsi)Ta(CO)3]-,
so there was considerable interest in determining whether an
equivalent reaction of [Ta(CNXyl)6]-, 7, would afford an
isocyanide analogue of the unknown Ta(CO)5NO.46 More
recently, (trmpsi)Ta(NO)I2(PMe3), (trmpsi)Ta(NO)2Cl, and cor-
responding niobium complexes have been described.47
Treatment of homoleptic anion 7 with N-methyl-N-nitroso-
p-toluenesulfonamide, or Diazald, a weakly oxidizing source
of NO+,48 at 0 °C gave, within seconds, a blood red solution,
from which satisfactorily pure, air-sensitive, and deep red-
1H and 13C NMR data also support the formulation of 8. In
particular, two carbonyl 13C environments are present in a 1:4
ratio, where the unique axial CO, δC ) 221.1 ppm, is about 5
ppm downfield of the value for the equatorial carbonyls, δC )
(43) Chisholm, M. H.; Godleski, S. Prog. Inorg. Chem. 1976, 20, 299.
(44) Minelli, M.; Maley, W. J. Inorg. Chem. 1989, 28, 2954.
(45) (a) Daff, P. J.; Legzdins, P.; Rettig, S. J. J. Am. Chem. Soc. 1998, 120,
2688. (b) Hayton, T. W.; Daff, P. J.; Legzdins, P.; Rettig, S. J.; Patrick,
B. O. Inorg. Chem. 2002, 41, 4114.
(46) Fjare, K. L.; Ellis, J. E. J. Am. Chem. Soc. 1983, 105, 2303.
(47) Hayton, T. W.; Legzdins, P.; Patrick, B. O. Inorg. Chem. 2002, 41, 5388.
(48) Crease, A. E.; Legzdins, P. J. Chem. Soc. 1973, 1501.
(40) Warnock, G. F.; Sprague, J.; Fjare, K. L.; Ellis, J. E. J. Am. Chem. Soc.
1983, 105, 672.
(41) Carnahan, E. M.; Lippard, S. J. J. Am. Chem. Soc. 1992, 114, 4166.
(42) Darensbourg, M. Y.; Hanckel, J. M. Organometallics 1982, 1, 82.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1147

A R T I C L E S
Barybin et al.
maroon microcrystalline 9 was isolated in about 60% yield, eq
5, where R ) p-tolyl.
of significant amounts of 1 and 2, the reactions were conducted
at -60 to -70 °C, followed by careful slow warming, eq 6.
[M(CO)6]- + 2 [NO][BF4] + 4 CNXyl
THF
[Ta(CNXyl)6]- + RSO2N(NO)Me
8
0 °C
-CNXyl
1. CH2Cl2, -60 to +20 °C
2. DME
Ta(CNXyl)5NO + RSO2NMe- (5)
cis-[M(CNXyl) (NO) ][BF ]
8
(6)
4
2
4
-BF4-, -6 CO
M ) Nb, Ta
IR spectra of 9 in THF show an intense broad isocyanide ν-
(CN) peak at 1964 cm-1, as well as weak shoulders at 2060
and 2010 cm-1, appropriate for a molecule of approximate C4V
symmetry. Also, a quite low ν(NO) band at 1542 cm-1 indicates
a high degree of dπ(Ta) to pπ*(NO) back-bonding in 9.
Interestingly, the only previously known tantalum mononitrosyls,
(trmpsi)Ta(CO)2NO45 and (trmpsi)Ta(NO)I2(PMe3),47 exhibit ν-
Under these conditions, 50-55% isolated yields of bright orange
microcrystalline 10 and 11 were obtained. It is also important
to employ [Bu4N][M(CO)6], rather than analogous [Et4N]+ salts,
to facilitate the purification of 10 and 11, as the latter are easily
separated from the byproduct [Bu4N][BF4], but not from
analogous [Et4N][BF4].
Compounds 10 and 11 are indefinitely stable at 20 °C in the
solid state under an inert atmosphere. In contrast, the only other
dinitrosyls of these elements, (trmpsi)M(NO)2Cl, M ) Nb, Ta,
reportedly decompose as solids above -30 °C.47 Although 10
and 11 are quite soluble and afford relatively thermally stable
but quite air sensitive solutions in CH2Cl2 and CHCl3, they
decompose in the presence of more polar solvents, e.g., slowly
in THF or DME, or rapidly in acetonitrile, to give presently
uncharacterized substances.52
(NO) bands at even lower energies of 1515 and 1450 cm-1
respectively.
,
1H and 13C NMR spectra of 9 show two discrete sets of
signals for the unique axial and four equatorial CNXyl ligands.
Of particular interest is the 27 ppm difference in the 13C
resonance positions of the axial and equatorial isocyanide
ligating carbons, δC ) 167.1 (1C, ax CNXyl), 194.3 (4C, eq
CNXyl) ppm, due to the large trans influence of the NO ligand.49
Confirmation of the formulation of 9 was obtained by a
single-crystal X-ray study,13a which showed its TaNC5 core to
be very close to octahedral. The molecular structure has
essentially linear Ta-N-O and Ta-C-N bond angles, 178-
(1)° and av ) 175(2)°, respectively, and slightly bent C-N-C
units, av ) 167(7)°. As expected, based on the outstanding
acceptor character of coordinated NO+,49 the Ta-N distance,
1.90(1)Å, is much shorter than the axial Ta-C distance, 2.23-
(2)Å, which in turn is appreciably longer than the average
equatorial Ta-C distance, 2.16(3)Å. Thus, structural parameters
are in accord with 13C NMR data, which indicate that the
equatorial isocyanides participate more than the axial isocyanide
group in back-bonding with tantalum. Attempts to obtain the
niobium analogue of 9 have not been carried out to date.
Structural characterization of the aforementioned (trmpsi)Ta-
(NO)I2(PMe3) revealed that its Ta-N distance, 1.824(5)Å,47 is
significantly shorter than that in 9, consistent with the fact that
the infrared NO stretching frequency observed for the trmpsi
complex, 1450 cm-1, is nearly 90 cm-1 lower in energy than
the corresponding value for 9.
Infrared spectra of 10 and 11 in CH2Cl2 and CHCl3 are nearly
identical and are consistent with the anticipated cis- geometry
of the [M(CNXyl)4(NO)2]+ cations. Indeed, the ON-M-NO
angle in both cases is calculated to be about 91° from the relative
intensities of the bands corresponding to the symmetric (A1)
and antisymmetric (B1) N-O stretching vibrations.53 The
energies of these bands (values given for 11): ν(NO): 1685,
1616 cm-1, respectively, are well below the ν(NO) value for
free NO, 1860 cm-1, but significantly higher in energy than
those reported for the thermally unstable dinitrosyl (trmpsi)Ta-
(NO)2Cl, ν(NO): 1605, 1515 cm-1 47
.
In contrast to mononitrosyl 9, the isocyanide ν(CN) positions
for 10 and 11 cm-1 are at higher energy than the corresponding
ν(CN) for free CNXyl, 2120 cm-1 in CHCl3, indicating that
the isocyanides are functioning mainly as donors, in the presence
of the two strong acceptor nitrosyl ligands. This circumstance
likely explains why carbonyl analogues of 10 and 11, e.g., [Ta-
(CO)4(NO)2]+, are unknown, due to the anticipated great lability
of the coordinated CO groups in such a molecule.
The ν(CN) bands for 11 occur at 2164 and 2138 cm-1, where
the intense lower energy peak is attributed to the antisymmetric
(B1) C-N stretching mode of the mutual trans-CNXyl ligands,
on the basis of intensity arguments outlined by Orgel.54 The
isocyanide ligands opposite the nitrosyl groups are expected to
produce two IR-allowed vibrations of A1′′ and B2 symmetries
of similar intensities. These appear to be nearly degenerate and
Syntheses and Properties of cis-[M(CNXyl)4(NO)2]+, M
) Nb (10), Ta (11). Previous efforts to obtain the tantalum
analogue of the well-known V(CO)5NO, or substituted versions
thereof, involved the reactions of [Ta(CO)6]- with 1 equiv of a
[NO]+ salt, in the absence or presence of appropriate donors
and, unlike related reactions with [V(CO)6]-, did not lead to
the isolation of any well-defined metal nitrosyls.46 However,
due to the robust character of several vanadium dinitrosyl
are assigned to the somewhat broad peak at 2164 cm-1
.
Interestingly, analogous ν(CN) bands of the closely related
dicarbonyl complexes, cis-[Mn(CO)2(CNPh)4]+55 and cis-
W(CO)2(CNXyl)4,56 also coincide. Finally, the symmetric (A1′)
50
complexes,50,51 including cis-[V(CNtBu)4(NO)2]+,50 the only
previously known cationic isocyanide group 5 complex of this
type, it was decided to examine corresponding reactions of
[M(CO)6]-, M ) Nb, Ta, with 2 equiv of [NO]+. In this article,
only the results with CNXyl are reported. To prevent formation
(52) Barybin, M. V. Ph.D. Thesis, University of Minnesota, 1999.
(53) θ ) 2arctan[{I(B1)/I(A1)}1/2], where θ is the ON-M-NO angle, and I(B1)
and I(A1) are intensities of the antisymmetric and symmetric stretching
vibrations of NO, respectively. See: Cotton, F. A.; Wilkinson, G. AdVanced
Inorganic Chemistry, 3rd ed.; Wiley-Interscience: New York, 1972; pp
696-698.
(49) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University
Press: New York, 1992.
(50) Herberhold, M.; Trampisch, J. Inorg. Chim. Acta 1983, 70, 143.
(51) (a) Na¨umann, F.; Rehder, D. Z. Naturforsch 1984, 39B, 1647, 1654. (b)
Mallik, M.; Ghosh, P. N.; Bhattacharyya, R. J. Chem. Soc., Dalton Trans.
1993, 1733 and references cited therein.
(54) Orgel, L. E. Inorg. Chem. 1962, 1, 25.
(55) Treichel, P. M.; Firisch, D. W.; Essenmacher, G. P. Inorg. Chem. 1979,
18, 2405.
(56) Coville, N. J.; Albers, M. O. Inorg. Chim. Acta 1982, 65, L7.
9
1148 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Homoleptic Isocyanidemetalates of 4d/5d Transition Metals
A R T I C L E S
Table 2. Summary of Vibrational and 13C NMR Data for
Octahedral Ta(1-) Isocyanidesa
b
νCN
,
KCN
,
δ
c (CNXyl),
ppm
1
1
complex
cm-
mdynes Å-
[Ta(CNXyl)6]-, 7
2021 (a1g)
1870 (Eg)
1817 (T1U
2044
2114 (A1′)
2010 (A1′′)
1964 (E)
2164 br
k ) 13.31
ki ) 0.37
210.4
)
[Ta(CO)5(CNXyl)]-, 8
Ta(CNXyl)5NO, 9
k ) 15.90
k1 ) 16.97
k2 ) 14.92
ki ) 0.12
182.4
167.1
194.3
cis-[Ta(CNXyl)4(NO)2]+,
11
k1 ) 18.03
156.0
163.7
(A1′′ + B1)
2138 (B2)
k2 ) 17.82
ki ) 0.21
a The IR and 13C NMR data for 7, 8, and 9 were obtained in THF and
THF-d8, respectively. Corresponding data for 11 were obtained in CHCl3
and CDCl3, respectively. b The reciprocal of the reduced mass of the CN
group used in the calculations is 0.154 76; k1 is kCN for CNXyl trans to
NO, k2 is kCN for CNXyl cis to NO, and ki is the interaction force constant
(see ref 34).
Figure 7. Plot of 13C NMR chemical shifts of the ligating CNXyl carbons
vs C-N stretching force constants for octahedral low-spin d6 tantalum
isocyanides, 7, 8, 9, and 11. The position of free CNXyl is indicated by an
open circle.
CNCy ) cyclohexylisocyanide, cis-Mo(CO)4(CNCy)2, and fac-
Mo(CO)3(CNCy)3.60 However, for a quantitative treatment,
C-N force constants are better indicators of the extent of dπ-
(M) f pπ*(CO) back-bonding than C-N stretching frequencies,
particularly when complexes of different symmetries are
compared.
The availability of the structurally similar compounds 7, 8,
9, and 11 prompted an examination of the relationship between
the isocyanide ligating carbon 13C resonance positions and
corresponding C-N force constant data. We believe this
represents the first such quantitative analysis for a series of
related isocyanide complexes and is of particular interest because
it includes a homoleptic isocyanidemetalate.
Table 2 summarizes the C-N force constants and the
corresponding 13C NMR chemical shifts of the isocyanide
carbons for the aforementioned tantalum complexes. The force
constants were calculated using the C-K approximation, which
assumes that kt ) 2 kc ) ki (kt and kc are interaction force
constants between pairs of trans and cis ligands, respectively).34
Despite the well-founded criticism61,62 of this simplified ap-
proach, its use is justified because all compounds considered
here are closely related;63,64 i.e., they are all octahedral low-
spin d6 tantalum isocyanides. Also, due to limited available
vibrational information, possible interactions between different
types of oscillators in the mixed complexes (i.e., CN-CO and
CN-NO) were not taken into account. The neglect of such
interactions in similar systems (mixed isocyanide-carbonyl
complexes) was shown to lead to only marginal errors in the
magnitudes of the principal force constants (<0.1 mdynes
Å-1).64
C-N vibration of the mutually trans-CNXyl groups is predicted
to be IR forbidden, in the absence of coupling between the A1′
and A1′′ modes and is not observed in either 10 or 11.
1H and 13C NMR spectra exhibit two sets of signals
corresponding to two different CNXyl environments. Of par-
ticular interest are the terminal isocyanide 13C resonances for
11,57 δ 163.7 (2C, CNXyl cis to NO), 156.0 (2C, CNXyl trans
to NO) ppm, which are both upfield of the corresponding
position for free CNXyl (δC ) 165.4 ppm). These results are
in accord with IR data, showing that all of the CNXyl groups
function primarily as donors in 11. Single-crystal X-ray structure
determinations on 10 and 11 have been previously discussed13b
and show that both monocations, [M(CNXyl)4(NO)2]+, are close
to octahedral, with a cis-arrangement of the practically linear
NO groups. Thus, the isolated cations and those observed in
solution appear to be essentially identical and provide convincing
evidence that back-bonding primarily involves the M(NO)2
fragments in these complexes.
Relationship Between Infrared and 13C NMR Spectral
Data for a Series of Octahedral Formal Ta(I-) Isocyanides.
In 1971, Gansow and co-workers established a linear relationship
between CO stretching force constants and appropriate 13C
chemical shifts for a series of octahedral tungsten carbonyls of
the general formula W(CO)5L.58 They calculated the force
constants according to the Cotton-Kraihanzel (C-K) ap-
proximation34 and showed that as the CO force constant
decreased for a given vibrational mode, the corresponding 13
C
chemical shifts moved downfield, due to dominance of para-
magnetic contributions in the Saika-Slichter screening equa-
tion.59 On this basis, the authors concluded that metal to CO
The data shown in Table 2 are graphically represented in
Figure 7. Compounds 9 and 11 provided two data points each,
because they both contain two different isocyanide environ-
ments. As indicated, the 13C δc value of the isocyanide carbon
is essentially directly proportional to the corresponding kCN force
constant. Taking into account the rather crude assumptions
π-donation or back-bonding played a significant role in the 13
C
shift values of the carbonyl ligands. Unfortunately, the ranges
of the C-O force constants and 13C chemical shifts were rather
narrow (15.12 e kCO e 16.08 mdynes Å-1; 192.1 e δC e 201.9
ppm) in this study.58 A qualitatively similar result was noted in
a correlation of the ν(CN) (A1 mode) stretching frequency and
the isocyanide carbon 13C chemical shift for Mo(CO)5(CNCy),
(60) Cronin, D. L.; Wilkinson, J. R.; Todd, L. J. J. Magn. Reson. 1975, 17,
353.
(61) (a) Bower, L. M.; Stiddard, M. H. B. Inorg. Chim. Acta 1967, 1, 231.
(b) Jones, L. H. Inorg. Chem. 1968, 7, 1681.
(57) Corresponding resonances were not observed for 11, likely because they
were extremely broad due to 13C-93Nb coupling. See ref 25 for more details.
(58) Gansow, O. A.; Kimura, B. Y.; Dobson, G. R.; Brown, R. A. J. Am. Chem.
Soc. 1971, 73, 5922.
(62) Drago, R. S. Physical Methods in Chemistry; W. B. Saunders: Philadelphia,
PA, 1977; Chapter 12, p 142.
(63) Cotton, F. A. Inorg. Chem. 1968, 7, 1683.
(64) Sarapu, A. C.; Fenske, R. F. Inorg. Chem. 1975, 14, 247.
(59) Grim, S. O.; Wheatland, D. A. Inorg. Chem. 1969, 8, 1716.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1149

A R T I C L E S
Barybin et al.
employed in calculating the kCN values, vide supra, the degree
of linearity in the correlation in Figure 7 is surprisingly good.65
It is also noteworthy, in contrast to a similar but rather limited
analysis involving metal carbonyls,58 that this plot covers wide
ranges of kCN (13.31 e kCN e 18.03 mdynes Å-1) and δC (156.0
e δC e 210.4 ppm) values. Infrared and 13C NMR data
emphasize that the CNXyl ligand is a strong π-acceptor in anion
7 but functions primarily as a σ-donor in cation 11. Thus, in
compound 7, CNXyl plays the role of CO in stabilizing a very
electron-rich metal center, whereas, in 11, it functions more like
a simple Lewis base in binding to a relatively electron-poor
metal center, due to the presence of two linear nitrosyls, which
are unusually strong π-acceptors in this molecule.
the isocyanide CNC unit due to metal to isocyanide back-
bonding interactions. 13C NMR spectra of the tantalum isocya-
nide complexes prepared in this study revealed well-defined
resonances for ligated carbons, including the first to be observed
for a homoleptic isocyanidemetalate. A plot of these 13C shifts
with appropriate isocyanide force constants showed a linear
relationship between these NMR and infrared data. Thus, δ(CN)
and k(CN) values are both equally good measures of the relative
acceptor ability of the CNXyl ligand in these octahedral,
formally d6 Ta(I-) complexes.
Acknowledgment. This work was supported by the National
Science Foundation, donors of the Petroleum Research Fund,
administered by the American Chemical Society, a University
of Minnesota doctoral dissertation fellowship (M.V.B.), and
predoctoral fellowships from Proctor and Gamble (B.E.K.), 3M
Company (B.E.K.), and the National Science Foundation
(V.J.S.). J.E.E. thanks Professor Wolfgang Beck for the op-
portunity to prepare compound 8 in his laboratory at the Institut
fu¨r Anorganische Chemie, Ludwig-Maximilian-Universita¨t, Mu-
nich, Germany, with support from the Alexander von Humboldt
Stiftung. Finally, we greatly thank Ms. Christine Lundby for
her expert assistance in the preparation of the manuscript.
Conclusion
Two electron oxidations of [M(CO)6]-, M ) Nb, Ta, by
appropriate amounts of AgBF4 or I2 in the presence of a slight
excess of CNXyl, afford the homoleptic isocyanide cations,
[M(CNXyl)7]+, the only known examples for heavier group 5
metals, or the neutral iodo complexes, M(CNXyl)6I, respectively.
Analogous treatments of [M(CO)6]- with 2 equiv of [NO][BF4]
provide rare examples of niobium and tantalum nitrosyls, cis-
[M(CNXyl)4(NO)2]+. These compounds are also of interest as
the first examples of fully substituted derivatives of the unknown
group 5 carbonyl complexes, [M(CO)7]+, M(CO)6I, and [M(CO)4-
(NO)2]+, respectively. Cesium graphite, CsC8, reductions of
M(CNXyl)6I give Cs[M(CNXyl)6], the first homoleptic isocya-
nidemetalates to be isolated for the 4d- and 5d-block transition
elements. Both species have been structurally authenticated by
single-crystal X-ray studies and show substantial bending of
Supporting Information Available: Detailed analyses of the
geometries of 2 and 4 (PDF). Experimental procedures and
characterization data for compounds 1, 3, 5, 7-11 (PDF). X-ray
crystallographic methods and Data (PDF, CIF) for 2, 4, 5, and
7. This material is available free of charge via the Internet at
http://pubs.acs.org.
(65) Indeed, the average deviation of the observed δc values from the trend line
(δC ) 360.99 - 11.265 kCN) is only 1.77 ppm.
JA065964Q
9
1150 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007