Structural Distortions in Six-Coordinate Adducts of Niobium(V) and Tantalum(V)

Article abstract of DOI:10.1021/ic970331l

The mixed chloro - aryloxide compounds [M(OC₆H₃Pr₂ⁱ-2,6)₂Cl ₃]₂ (M = Nb (1a). Ta (1b); both structurally characterized) and [M(OC₆Pr₂ⁱ-2,6)₃

Full text of DOI:10.1021/ic970331l

Inorg. Chem. 1997, 36, 3623-3631
3623
Structural Distortions in Six-Coordinate Adducts of Niobium(V) and Tantalum(V)
Janet R. Clark,† April L. Pulvirenti,† Phillip E. Fanwick,† Michaelis Sigalas,‡
Odile Eisenstein,*,§ and Ian P. Rothwell*,†
Department of Chemistry, 1393 Brown Building, Purdue University,
West Lafayette, Indiana 47907-1393, Laboratory for Applied Quantum Chemistry, University
of Thessaloniki, Thessaloniki, Greece, and Laboratoire de Chimie-The´orique, Baˆtiment 490,
Universite´ de Paris-Sud, 91405 Orsay, France
ReceiVed March 20, 1997X
The mixed chloro-aryloxide compounds [M(OC6H3Pri2-2,6)2Cl3]2 (M ) Nb (1a), Ta (1b); both structurally
characterized) and [M(OC6H3Pri2-2,6)3Cl2] (M ) Nb (2a), Ta (2b)) react with pyridine (py) and PMe2Ph to produce
a series of adducts cis-mer-[MCl3(OC6H3Pri2-2,6)2(L)] (M, L: Nb, py (3a); Ta, py (3b); Nb, PMe2Ph (4a); Ta,
PMe2Ph (4b)) and trans-mer-[MCl2(OC6H3Pri2-2,6)3(L)] (M, L: Nb, py (5); Nb, PMe2Ph (6a); Ta, PMe2Ph (6b)).
The assigned geometric arrangement of ligands is based upon 1H NMR studies and single-crystal X-ray diffraction
analyses of 3a, 3b, 4, 6a, and 6b. The salt complex [HPMe2Ph]+[mer-NbCl3(OC6H3Pri2-2,6)3]- (7) has also
been isolated and structurally characterized. The structural parameters for the neutral adducts are compared with
those of previously reported hydrido aryloxides of tantalum. A small but consistent distortion away from octahedral
geometry involving the bending of mutually trans anionic ligands toward the neutral donor group is observed.
Theoretical analysis at several levels of theory (RHF, MP2, and DFT) on model compounds [Ta(OH)2(H)2(PH3)-
(X)] (X ) Cl, OH, H) show a distortion involving bending of the trans-hydride groups toward the PH3 ligand for
X ) Cl and OH. This distortion can be accounted for in terms of an improvement in both X p to metal d
π-bonding and Ta-H σ-bonding. The contribution of σ-bonding effects is clearly shown in the case of X ) H,
where again a bend of the two hydride ligands toward the Ta-P bond is calculated. A smaller distortion of the
Cl ligands in trans-mer-[Ta(OH)3(Cl)2(PH3)] is also predicted.
Introduction
of electronic factors, in particular the π-accepting/donating
capacity of the coordinated ligands.10-14 Recent studies of the
The sovereignty of the octahedral geometry in transition metal
six-coordination has slowly been undermined. Besides the
burgeoning class of trigonal prismatic structures,1-9 large
deformations from octahedral geometry have been observed for
distinct categories of complexes containing formally dn, n < 6
electron counts.10-13 A particularly significant subset of these
molecules is those containing a d4 configuration. Theoretical
studies have rationalized the observed deformations in terms
six-coordinate dihydride compounds trans-mer-[Ta(H)2(OC6H3-
Pri2-2,6)3(L)] and trans-trans-[TaCl(H)2(OC6H3But2-2,6)2(L)]
have shown an interesting coordination environment about the
metal center.15 Specifically, the two hydride ligands in these
d0 molecules are bent toward the donor ligand L, a distortion
that cannot be justified on steric grounds. The infrared spectra
of these compounds confirm the X-ray diffraction data.15
† Purdue University.
‡ University of Thessaloniki.
§ Universite´ de Paris-Sud. Present address: Laboratoire de Structure et
Dynamique des Syste`mes Mole´culaires et Solides, UMR 5636, Universite´
de Montpellier 2, 34095 Montpellier Cedex 5, France.
X Abstract published in AdVance ACS Abstracts, July 1, 1997.
(1) (a) Pfennig, V.; Seppelt, K. Science 1996, 271, 616. (b) Seppelt, K.
Science 1996, 272, 182. (c) Haaland, A.; Hammel, A.; Rypdal, K.;
Volden, H. V. J. Am. Chem. Soc. 1990, 112, 4547. (d) Shortland, A.
J.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1973, 872.
(2) Morse, P. M.; Girolami, G. S. J. Am. Chem. Soc. 1989, 111, 4114.
(3) Demolliens, A.; Jean, Y.; Eisenstein, O. Organometallics 1986, 5,
1457.
(4) Kang, S. K.; Albright, T. A.; Eisenstein, O. Inorg. Chem. 1989, 28,
1611.
(5) Zyubin, A. S.; Musaev, D. G.; Charkin, O. P. Zhu. Neorgan. Khim.
1992, 37, 2345.
(6) Shen, M. Z.; Schaefer, H. F.; Partridge, H. J. Chem. Phys. 1993, 98,
508.
As an extension of this work, we report here our investigation
of the reaction of the chloride compounds [M(OC6H3Pri2-2,6)2-
Cl3]2 and [M(OC6H3Pri2-2,6)3Cl2] (M ) Nb, Ta with various
donor ligands). An evaluation of the solid state structures of
the simple pyridine and phosphine adducts has been undertaken
to quantitate any significant distortions from octahedral geom-
etry. Theoretical analysis of a number of these systems has
also been undertaken in an attempt to identify the underlying
electronic principles that account for the observed deformations.
(7) Kang, S. K.; Tang, H.; Albright, T. A. J. Am. Chem. Soc. 1993, 115,
1971.
Results and Discussion
(8) Kaupp, M. J. Am. Chem. Soc. 1996, 118, 3018.
(9) Landis, C. R.; Cleveland, T.; Firman, T. K. Science 1996, 272, 182.
(10) Kubacek, P.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 4320.
(11) Kamata, M.; Hirotsu, K.; Higuchi, T.; Tatsumi, K.; Hoffmann, R.;
Yoshida, T.; Otsuka, S. J. Am. Chem. Soc. 1981, 103, 5772.
(12) Gusev, D. G.; Kuhlman, R.; Sini, G.; Eisenstein, O.; Caulton, K. G.
J. Am. Chem. Soc. 1994, 116, 2685.
Synthesis and Spectroscopic Properties of New Com-
pounds. The mixed chloro aryloxides of Nb(V) and Ta(V) are
important starting materials for studying the organometallic
(14) Caulton, K. G. New J. Chem. 1994, 18, 25.
(15) Parkin, B. C.; Clark, J. C.; Visciglio, V. M.; Fanwick, P. E.; Rothwell,
I. P. Organometallics 1995, 14, 3002.
(13) Gusev, D. G.; Kuhlman, R.; Rambo, J. R.; Berke, H.; Eisenstein, O.;
Caulton, K. G. J. Am. Chem. Soc. 1995, 117, 281.
S0020-1669(97)00331-5 CCC: $14.00 © 1997 American Chemical Society

3624 Inorganic Chemistry, Vol. 36, No. 17, 1997
Clark et al.
Scheme 1
chemistry of these metals.16,17 The pentachlorides [MCl5]2 (M
) Nb, Ta) will react with 2,6-diisopropylphenol (2 equiv) in
hydrocarbon solvents to generate the corresponding trichloride
compounds [M(OC6H3Pri2-2,6)2Cl3]2 (M ) Nb (1a), Ta (1b)16b
)
(Scheme 1). The solid state structures of the niobium compound
(Figure 1, Table 1) and isomorphous tantalum compound (Table
1) show them to adopt a dimeric structure with bridging chloride
ligands. The dimerization results in a pseudooctahedral envi-
ronment about each metal center (Figure 1) with the aryloxide
ligands mutually cis and trans to the two bridging chloride
groups. The solid state structure of 1 contrasts with that
observed previously for the trichloride compounds [Ta(OC6H3-
(16) (a) Chamberlain, L. R.; Keddington, J. L.; Rothwell, I. P. Organo-
metallics 1992, 1, 1098. (b) Chamberlain, L. R.; Rothwell, I. P.;
Huffman, J. C. Inorg. Chem. 1984, 23, 2575. (c) Chamberlain, L. R.;
Rothwell, I. P.; Huffman, J. C. J. Am. Chem. Soc. 1986, 108, 1502.
(d) Chamberlain, L. R.; Rothwell, I. P.; Folting, K.; Huffman, J. C. J.
Chem. Soc., Dalton Trans. 1987, 155. (e) Chamberlain, L. R.;
Rothwell, I. P. J. Chem. Soc., Dalton Trans. 1987, 163. (f) Chesnut,
R. W.; Durfee, L. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1987,
6, 2019. (g) Chamberlain, L. R.; Kerscher, J. L.; Rothwell, A. P.;
Rothwell, I. P.; Huffman, J. C. J. Am. Chem. Soc. 1987, 109, 6471.
(h) Chamberlain, L. R.; Steffey, B. D.; Rothwell, I. P. Polyhedron
1989, 8, 341. (i) Coffindaffer, T. W.; Steffy, B. D.; Rothwell, I. P.;
Folting, K.; Huffman, J. C.; Streib, W. E. J. Am. Chem. Soc. 1989,
111, 4742. (j) Steffey, B. D.; Chesnut, R. W.; Kerschner, J. L.;
Pellechia, P. J.; Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc.
1989, 111, 378. (k) Steffey, B. D.; Chamberlain, L. R.; Chesnut, R.
W.; Chebi, D. E.; Fanwick, P. E.; Rothwell, I. P. Organometallics
1989, 8, 1419. (l) Chesnut, R. W.; Yu, J. S.; Fanwick, P. E.; Rothwell,
I. P. Polyhedron 1990, 8, 1051. (m) Steffey, B. D.; Fanwick, P. E.;
Rothwell, I. P. Polyhedron 1990, 9, 963. (n) Yu, J. S.; Fanwick, P.
E.; Rothwell, I. P. J. Am. Chem. Soc. 1990, 112, 8171. (o) Chesnut,
R. W.; Jacob, G. G.; Yu, J. S.; Fanwick, P. E.; Rothwell, I. P.
Organometallics 1991, 10, 321. (p) Clark, J. R.; Fanwick, P. E.;
Rothwell, I. P. J. Chem. Soc., Chem. Commun. 1993, 1233. (q)
Lockwood, M. A.; Potyen, M. C.; Steffey, B. D.; Fanwick, P. E.;
Rothwell, I. P. Polyhedron 1995, 14, 3293.
Figure 1. Molecular structure of [NbCl3(OC6H3Pri2-2,6)2]2 (1a). An
identical labeling scheme is adopted for isomorphous 1b, Table 1.
But2-2,6)2Cl3]16b and [Nb(OC6HPh4-2,3,5,6)2Cl3],16q which were
found to be mononuclear in the solid state, adopting a square
pyramidal geometry about the metal center with one axial and
one equatorial aryloxide ligand. The binuclear structure ob-
served for the diisopropylphenoxide 1a presumably is a result
of the slightly lower steric requirements of this aryloxide ligand
compared to the more bulky 2,6-di-tert-butylphenoxide and
2,3,5,6-tetraphenylphenoxide ligands. The substitution of a third
aryloxide into the coordination sphere of the metal occurs when
1 is refluxed with a further 1 equiv of 2,6-diisopropylphenol in
benzene solvent (Scheme 1) to produce [M(OC6H3Pri2-2,6)3-
Cl2] (M ) Nb (2a), Ta (2b)). Although the exact structure of
2 is unknown, a related compound [Ta(OC6H3But2-2,6)3Cl2] has
been shown to adopt a square pyramidal structure in the solid
state with an axial aryloxide and two mutually trans basal
aryloxide ligands.18
(17) (a) Bruck, M. A.; Copenhaver, A. S.; Wigley, D. E. J. Am. Chem.
Soc. 1987, 109, 6525. (b) Strickler, J. R.; Bruck, M. A.; Wexler, P.
A.; Wigley, D. E. Organometallics 1990, 9, 266. (c) Arney, D. J.;
Wexler, P. A.; Wigley, D. E. Organometallics 1990, 9, 1282. (d) Gray,
S. D.; Smith, D. P.; Bruck, M. A.; Wigley, D. E. J. Am. Chem. Soc.
1992, 114, 5462. (e) Gray, S. D.; Fox, P. A.; Kingsborough, R. P.;
Bruck, M. A.; Wigley, D. E. Prepr.sACS DiV. Pet. Chem. 1993, 39,
706. (f) Allen, K. D.; Bruck, M. A.; Gray, S. D.; Kingsborough, R.
P.; Smith, D. P.; Weller, K. J.; Wigley, D. E. Polyhedron 1995, 14,
3315.
The trichloride compounds 1 react rapidly in hydrocarbon
solvents with a variety of oxygen,19 nitrogen,19 and phosphorus
donor ligands to generate the corresponding adducts. In the
case of pyridine (py) and the phosphine ligand PMe2Ph, the
(18) Clark, G. R.; Nielson, A. J.; Rickard, C. E. F. Polyhedron 1987, 6,
1765.

Six-Coordinate Adducts of Nb(V) and Ta(V)
Inorganic Chemistry, Vol. 36, No. 17, 1997 3625
Table 1. Selected Bond Distaces (Å) and Angles (deg) for
[MCl3(OC6H3Pri2-2,6)2]2 (M ) Nb (1a), Ta (1b))
1a
1b
M-O(10)
1.816(7)
1.816(7)
2.347(3)
2.359(3)
2.550(3)
2.622(3)
101.1(3)
97.0(3)
95.7(3)
90.2(3)
168.6(3)
94.1(3)
91.6(3)
1.815(12)
1.791(14)
2.335(6)
2.352(6)
2.533(6)
2.613(6)
101.0(6)
95.1(5)
97.0(5)
90.3(4)
168.1(4)
93.7(5)
91.7(5)
168.7(4)
90.9(4)
165.6(2)
86.4(2)
83.6(2)
85.7(2)
82.9(2)
77.8(2)
102.2(2)
173.0(14)
170.5(16)
M-O(20)
M-Cl(2)
M-Cl(3)
M-Cl(1)
M-Cl(1)′
O(20)-M-O(10)
O(20)-M-Cl(2)
O(20)-M-Cl(3)
O(20)-M-Cl(1)
O(20)-M-Cl(1)′
O(10)-M-Cl(2)
O(10)-M-Cl(3)
O(10)-M-Cl(1)
O(10)-M-Cl(1)′
Cl(2)-M-Cl(3)
Cl(2)-M-Cl(1)
Cl(2)-M-Cl(1)′
Cl(3)-M-Cl(1)
Cl(3)-M-Cl(1)′
Cl(1)-M-Cl(1)′
M-Cl(1)-M
168.6(3)
90.3(3)
Figure 2. Molecular structure of cis-mer-[NbCl3(OC6H3Pri2-2,6)2(py)]
(3a). An identical labeling scheme is adopted for isomorphous 3b, Table
2.
164.82(12)
86.22(11)
83.26(11)
85.43(11)
82.66(11)
78.41(10)
101.59(10)
172.8(8)
168.6(8)
Table 2. Selected Bond Distances (Å) and Angles (deg) for
cis-mer-[MCl3(OC6H3Pri2-2,6)2(py)] (M ) Nb (3a), Ta (3b))
3a
3b
MJ-O(10)-C(11)
M-O(20)-C(21)
M-O(10)
1.832(3)
1.852(3)
2.379(1)
2.373(1)
2.403(1)
2.331(4)
164.34(5)
85.95(5)
97.1(1)
92.9(1)
82.8(1)
85.96(5)
96.9(1)
92.9(1)
83.3(1)
94.2(1)
170.4(1)
86.1(1)
95.4(1)
179.6(1)
84.3(1)
173.0(3)
174.8(3)
1.841(4)
1.848(5)
2.370(2)
2.366(2)
2.399(2)
2.315(6)
164.90(7)
86.03(8)
97.1(2)
92.6(2)
82.8(2)
86.12(8)
96.3(2)
93.1(2)
83.8(2)
94.3(2)
170.6(2)
86.1(2)
95.0(2)
179.6(2)
84.6(2)
173.8(5)
174.0(5)
M-O(20)
M-Cl(1)
Chart 1. Possible Isomers of [M(OAr)2Cl3(L)] and
[M(OAr)3Cl2(L)]
M-Cl(2)
M-Cl(3)
M-N(31)
Cl(1)-M-Cl(2)
Cl(1)-M-Cl(3)
Cl(1)-M-O(10)
Cl(1)-M-O(20)
Cl(1)-M-N(31)
Cl(2)-M-Cl(3)
Cl(2)-M-O(10)
Cl(2)-M-O(20)
Cl(2)-M-N(31)
Cl(3)-M-O(10)
Cl(3)-M-O(20)
Cl(3)-M-N(31)
O(10)-M-O(20)
O(10)-M-N(31)
O(20)-M-N(31)
M-O(10)-C(11)
M-O(20)-C(21)
products 3 and 4 (Scheme 1) can be obtained as orange (niobium
compounds) or yellow (tantalum compounds) crystalline materi-
als. Spectroscopic (NMR) studies of adducts 3 and 4 show that
they adopt a cis-mer structure in solution. The 1H NMR spectra
of the pyridine adducts 3, show well resolved, sharp signals
for the pyridine ligand and two nonequivalent aryloxide groups.
Furthermore neither of the aryloxide CHMe2 groups is diaste-
reotopic. These data are consistent with a solution structure in
which one of the aryloxide ligands is trans to the pyridine donor
group while the other aryloxide ligand is cis and rules out both
the trans-mer and cis-fac isomeric forms for the solution
structures of 3. (See Chart 1.)
spectra become sharp, and at -20 °C, well-resolved signals are
obtained for the aryloxide and phosphine ligands very similar
to those observed for the tantalum analog. This result indicates
that, in the case of the niobium phosphine adduct, stereochemical
nonrigidity is present, allowing exchange of the nonequivalent
aryloxide ligands. Furthermore, it can be shown by 31P NMR
that rapid exchange of coordinated and free phosphine occurs
at 30 °C for solutions of compound 3a to which has been added
PMe2Ph. Single-crystal X-ray diffraction analyses of the
isomorphous pyridine adducts 3a and 3b (Figure 2, Table 2)
and the niobium-phosphine adduct 4a (Figure 3, Table 3) have
been carried out and confirm that the cis-mer arrangement of
ligands inferred from solution spectroscopic studies is present
in the solid state. These structures are discussed in more detail
below.
The tantalum phosphine adduct 4b also shows a sharp set of
1
well-resolved ligand signals in the solution H NMR spectra.
Again two nonequivalent aryloxides are observed, indicating
that a structure similar to that of the pyridine adduct is present.
However, for the niobium compound 4a, the signals due to the
1
aryloxide ligands are broad at ambient temperatures in the H
NMR spectrum. Upon cooling of the solution (toluene-d8), the
(19) A series of diethyl ether complexes of tantalum were reported by
Wigley et al. (see refs 17a-c), e.g. cis-mer-[TaCl3(OAr)2(OEt2)] and
trans-mer-[TaCl2(OAr)3(OEt2)] (OAr ) 2,6-dimethyl- and 2,6-diiso-
propylphenoxide). A series of quinoline (quin) complexes cis-mer-
[TaCl3(OC6H3Pri2-2,6)2(quin)] and trans-mer-[TaCl2(OC6H3Pri2-2, 6)3-
(quin)] were also reported by Wigley et al. (see refs 17d,f). A number
of tetrahydrofuran complexes of mixed chloro aryloxides of niobium
were structurally characterized by Nakamura et al.: (a) Kanehisa, N.;
Kai, N.; Yasuda, H.; Nakayama, Y.; Takei, K.; Nakamura, A. Chem.
Lett. 1990, 2167. (b) Yasuda, H.; Nakayama, Y.; Takei, K.; Nakamura,
A.; Kai, N.; Kanehisa, N. J. Organomet. Chem. 1994, 473, 105.
The addition of pyridine or PMe2Ph to hydrocarbon solutions
of the dichlorides 2 also leads to the formation of 1:1 adducts
(Scheme 1).19 These compounds are formulated as trans-mer-
[Nb(OC6H3Pri2-2,6)3Cl2(py)] 5 and trans-mer-[M(OC6H3Pri2-
2,6)3Cl2(PMe2Ph)] (M ) Nb (6a), Ta (6b)) on the basis of their
1H NMR spectra and X-ray diffraction studies (Figure 4, Table
4). In the case of the pyridine complex 5 and the tantalum
complex 6b, two sharp sets of nondiastereotopic aryloxide

3626 Inorganic Chemistry, Vol. 36, No. 17, 1997
Clark et al.
Figure 3. Molecular structure of cis-mer-[NbCl3(OC6H3Pri2-2,6)2(PMe2-
Ph)] (4a).
Figure 5. Molecular structure of mer-[HPMe2Ph]+[NbCl3(OC6H3Pri2-
2,6)3]- (7).
Table 4. Selected Bond Distances (Å) and Angles (deg) for
trans-mer-[MCl2(OC6H3Pri2-2,6)3(PMe2Ph)] (M ) Nb (6a), Ta (6b))
6a
6b
M-O(10)
1.913(2)
1.868(2)
1.894(2)
2.3823(9)
2.3792(9)
2.7666(9)
158.74(3)
88.63(6)
102.53(7)
89.17(7)
81.00(3)
89.92(7)
98.70(7)
91.60(7)
77.76(3)
90.83(9)
177.36(9)
88.89(6)
91.07(9)
89.32(7)
89.32(7)
159.9(2)
175.8(2)
165.5(2)
1.913(2)
1.863(2)
1.897(2)
2.3713(8)
2.3690(8)
2.7574(9)
159.87(3)
88.50(7)
101.86(7)
89.15(7)
81.61(3)
89.93(6)
98.24(7)
91.51(6)
78.28(3)
91.43(10)
176.77(10)
88.26(7)
91.22(10)
89.20(7)
89.20(7)
158.9(2)
175.7(2)
164.9(2)
M-O(20)
M-O(30)
M-Cl(1)
M-Cl(2)
M-P(1)
Cl(1)-M-Cl(2)
Cl(1)-M-O(10)
Cl(1)-M-O(20)
Cl(1)-M-O(30)
Cl(1)-M-P(1)
Cl(2)-M-O(10)
Cl(2)-M-O(20)
Cl(2)-M-O(30)
Cl(2)-M-P(1)
O(10)-M-O(20)
O(10)-M-O(30)
O(10)-M-P(1)
O(20)-M-O(30)
O(20)-M-P(1)
O(30)-M-P(1)
M-O(10)-C(11)
M-O(20)-C(21)
M-O(30)-C(31)
Figure 4. Molecular structure of trans-mer-[NbCl2(OC6H3Pri2-
2,6)3(PMe2Ph)] (6a). An identical labeling scheme is adopted for
isomorphous 6b, Table 4.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
cis-mer-[NbCl3(OC6H3Pri2-2,6)2(PMe2Ph)] (4a)
Nb-O(10)
Nb-O(20)
Nb-Cl(1)
1.83(2)
1.82(2)
2.366(8)
Nb-Cl(2)
Nb-Cl(3)
Nb-P(1)
2.425(7)
2.374(7)
2.742(8)
Table 5. Selected Bond Distances (Å) and Angles (deg) for
mer-[HPMe2Ph]+[NbCl3(OC6H3Pri2-2,6)3]- (7)
Cl(1)-Nb-Cl(2)
Cl(1)-Nb-Cl(3)
Cl(1)-Nb-O(10)
Cl(1)-Nb-O(20)
Cl(1)-Nb-P(1)
Cl(2)-Nb-Cl(3)
Cl(2)-Nb-O(10)
Cl(2)-Nb-O(20)
Cl(2)-Nb-P(1)
86.2(3)
161.5(3)
92.7(6)
98.5(6)
80.5(3)
86.1(3)
171.3(5)
92.0(6)
79.8(3)
Cl(3)-Nb-O(10)
Cl(3)-Nb-O(20)
Cl(3)-Nb-P(1)
O(10)-Nb-O(20)
O(10)-Nb-P(1)
O(20)-Nb-P(1)
Nb-O(10)-C(11)
Nb-O(20)-C(21)
92.4(6)
98.5(6)
81.6(2)
96.6(7)
91.6(5)
171.8(6)
172.6(16)
172.0(19)
Nb-O(10)
Nb-O(20)
Nb-O(30)
1.884(3)
1.914(3)
1.865(3)
Nb-Cl(1)
Nb-Cl(2)
Nb-Cl(3)
2.407(1)
2.492(1)
2.440(1)
Cl(1)-Nb-Cl(2)
Cl(1)-Nb-Cl(3)
Cl(1)-Nb-O(10)
Cl(1)-Nb-O(20)
Cl(1)-Nb-O(30)
Cl(2)-Nb-Cl(3)
Cl(2)-Nb-O(10)
Cl(2)-Nb-O(20)
Cl(2)-Nb-O(30)
85.61(4)
169.84(4)
89.55(9)
90.86(9)
95.8(1)
84.24(4)
90.02(9)
89.14(9)
178.4(1)
Cl(3)-Nb-O(10)
Cl(3)-Nb-O(20)
Cl(3)-Nb-O(30)
O(10)-Nb-O(20)
O(10)-Nb-O(30)
O(20)-Nb-O(30)
Nb-O(10)-C(11)
Nb-O(20)-C(21)
Nb-O(30)-C(31)
90.03(9)
89.42(9)
94.4(1)
179.0(1)
90.7(1)
90.2(1)
CHMe2 resonances in the ratio of 2:1 are present in the 1H NMR
spectrum. In contrast, the corresponding resonances for 6a are
broad, implying that, as with 4a above, exchange of nonequiva-
lent aryloxide ligands can occur on the NMR time scale possibly
via phosphine dissociation.
During studies of the reaction of the niobium trichloride [Nb-
(OC6H3Pri2-2,6)2Cl3]2 (1a) with PMe2Ph, a few crystals of a
new product were isolated and identified by X-ray diffraction
as the salt complex mer-[HPMe2Ph][Nb(OC6H3Pri2-2,6)3Cl3] (7)
(Figure 5, Table 5).
168.2(3)
174.8(3)
176.7(3)
compounds obtained in this study are comparable to those found
in other aryloxide and chloride derivatives of Nb(V) and Ta-
(V).16m It is interesting to note that the tantalum-ligand
distances are consistently shorter than the corresponding nio-
bium-ligand distances in the isomorphous pairs of compounds
1 and 3 (Tables 1 and 2) although the differences are within 3σ
of the experimental errors. There are presently far fewer
structurally characterized pyridine and related N-heterocyclic
donor ligand complexes of Nb and Ta than phosphine com-
Solid State Structures. The M-O (aryloxide) and M-Cl
distances as well as the M-O-C (aryloxide) angles in the

Six-Coordinate Adducts of Nb(V) and Ta(V)
Inorganic Chemistry, Vol. 36, No. 17, 1997 3627
Table 6. Selected Structural Parameters for Six-Coordinate Derivatives of Nb and Ta
species
L-M-X(trans), deg
L-M-X(cis), deg
∑R, deg
ref
[NbCl5(NCH)]
178.52(7)
178.1(2)
177.5(1)
180.0
82.06(5), 83.49(5), 82.06(5), 83.49(5)
83.5(3), 85.5(3), 87.4(4), 88.0(4)
83.7(1), 84.2(1), 84.7(1), 86.8(1)
84.9(1), 84.9(1), 84.9(1), 84.9(1)
84.5(2), 84.7(2), 85.0(2), 86.3(2)
83.0(2), 88.1(1),
82.8(1), 83.3(1), 84.3(1), 86.1(1)
82.8(2), 83.8(2), 84.6(2), 86.1(2)
79.8(3), 80.5(3), 81.6(2), 91.6(5)
78.5(1), 79.2(2), 89.6(1), 91.0(2)
56(2), 62(2), 92.7(1), 94.6(1)
-28.9
-15.5
-20.6
-19.4
-19.4
-19.4
-23.5
-22.6
-26.4
-21.6
-54.7
-23.0
-22.7
-50.3
-11.0
29
30
31
19
19
19
a
a
a
32
15
a
[NbCl5(PMe2Ph)]-
trans-[TaCl4{OSi(C6H4Me-2)3}(OEt2)]
trans-[NbCl4(OC6H3Me2-2,6)(THF)]
cis-mer-[NbCl3(OC6H3Me2-2,6)2(THF)]
cis-mer-[NbCl3(OC6H3Ph2-2,6)2(THF)]
cis-mer-[NbCl3(OC6H3Pri2-2,6)2(py)] (3a)
cis-mer-[TaCl3(OC6H3Pri2-2,6)2(py)] (3b)
cis-mer-[NbCl3(OC6H3Pri2-2,6)2(PMe2Ph)] (4a)
cis-mer-[TaCl3(NMe2)2(HNMe2)]
177.4(3)
172.0(2)
179.6(1)
179.6(2)
171.8(6)
169.3(2)
174.96(5)
176.45(7)
176.51(7)
174.5(1)
178.4(1)
trans-trans-[TaCl(H)2(OC6H3But2-2,6)2(PMePh2)]
trans-mer-[NbCl2(OC6H3Pri2-2,6)3(PMe2Ph)] (6a)
trans-mer-[TaCl2(OC6H3Pri2-2,6)3(PMe2Ph)] (6b)
trans-mer-[Ta(H)2(OC6H3Pri2-2,6)3(PMe2Ph)]
77.76(3), 81.00(3), 88.89(6), 89.32(7)
78.28(3), 81.61(3), 88.26(7), 89.20(7)
69(1), 66(2), 89.3(1), 85.4(1)
a
15
a
mer-[NbCl3(OC6H3Pri2-2,6)3]-
7
84.24(4), 85.61(4), 89.14(9), 90.02(9)
a This work.
plexes. The M-N(py) distances of 2.331(4) and 2.315(6) Å
in 3a and 3b can be compared with values of 2.291(21) Å in
[TaCl2Me3(bipy)],20 2.420(12) Å in [NbBr4(CF3CCCF3)(py)]-,21
2.479(4) Å in [Nb(NBut)Cl4(py)]-,22 2.28(2) Å in [Ta{C(Me)C-
(Me)CHCMe3}(OC6H3Pri2-2,6)3(py)],23 2.348(6) and 2.408(6)
Å in [Ta(NAr)(OC6H3Pri2-2,6)Cl2(py)2],24 and 2.316(14) and
2.324(15) Å in [Ta(NSiBut3)2Me(py)2].25 The Nb-P distances
of 2.742(8) and 2.7666(9) Å in 4a and 6 are among the longest
niobium-phosphine distances so far reported. They are only
slightly shorter than one of the Nb-P distances of 2.787(1) Å
in [CpNbCl3(dppe)]26 and compare with values of 2.74-2.75
Å found in a series of phosphine adducts of niobium cluster
compounds.27,28 The adducts 3a, 3b, 4a, 6a, and 6b (Figures
2-4) structurally characterized in this study are examples of a
class of six-coordinate derivatives of Nb and Ta of general
formula [MX5L] where X represents either a single type of
monoanionic ligand or a set of monoanionic ligands, e.g. halide,
hydride, alkoxide, aryloxide, siloxide, amide groups, etc. The
previously characterized six-coordinate hydride compounds
trans-mer-[Ta(H)2(OC6H3Pri2-2,6)3(PMe2Ph)] and trans-trans-
[TaCl(H)2(OC6H3But2-2,6)2(PMePh2)] are highly distorted in the
solid state with acute H-Ta-P angles of 56(2)-69(1)°.16q In
Table 6 are collected the L-M-X angles (cis and trans) for
these compounds along with the corresponding parameters for
the new molecules obtained in this study and certain related
Nb and Ta derivatives in the literature.15,19,10-32 It can be seen
that there is a consistent distortion in which some or all of the
cis groups are bent toward the donor ligand. This is clearly
evident in the simple formonitrile complex [NbCl5(NCH)]29 (all
ligands being sterically very innocent) where all four cis-chloride
groups form angles of 82-83° with the nitrile ligand (Table
6). The resulting structure can hence be best described as a
square pyramidal arrangement of chloride groups to which a
nitrile group is bound axially. Similar, but less pronounced
distortions are evident in the d1 anion [NbCl5(PMe2Ph)]- and
the monosiloxide trans-[TaCl4{OSi(C6H4Me-2)3}(OEt2)]31 (Table
6). The pyridine adducts cis-mer-[MCl3(OC6H3Pri2-2,6)2(py)]
(M ) Nb (3a), Ta (3b)) obtained in this study and the 2,6-
dimethylphenoxide complex cis-mer-[NbCl3(OC6H3Me2-2,6)2-
(THF)]19 possess solid state structures in which the three chloride
ligands and one aryloxide oxygen atom are bent toward the
neutral donor group. In the related complex cis-mer-
[NbCl3(OC6H3Pri2-2,6)2(PMe2Ph)] (4a), all three chloride ligands
form angles of 80-82° with the cis-phosphine ligand while the
angle formed with the aryloxide group is 91.6(5)o. In trans-
mer-[MCl2(OC6H3Pri2-2,6)3(PMe2Ph)] (6), the two trans-
chloride ligands are distinctly bent toward the PMe2Ph group
although not as dramatically as the hydride groups in trans-
mer-[Ta(H)2(OC6H3Pri2-2,6)3(PMe2Ph)] or trans-trans-[TaCl-
(H)2(OC6H3But2-2,6)2(PMePh2)] (Table 6).
In order to more readily quantitate this specific distortion from
octahedral geometry we introduce the parameter ∑R defined
as follows: ∑R ) ∑[(cis-L-M-X) - 90] (Table 6). All of
the group 5 metal derivatives can be seen to have negative values
of ∑R (as defined), consistent with the bending of these groups
toward the donor ligand. We have also evaluated this parameter
for all of the species [MCl5(L)]n- contained in the Cambridge
Crystallographic Database (Table 7).33-49 A definite trend is
evident, with the value of ∑R becoming less negative as the
(20) Drew, M. G. B.; Wilkins, J. D. J. Chem. Soc., Dalton Trans. 1973,
1830.
(21) Felten, C.; Olbrich, F.; Rehder, D. Organometallics 1993, 12, 982.
(22) Clegg, W.; Errington, R. J.; Hockless, D. C. R.; Redshaw, C.
Polyhedron 1991, 10, 1959.
(23) Wallace, K. C.; Liu, A. H.; Davis, W. M.; Schrock, R. R. Organo-
metallics 1989, 8, 644.
(24) Chao, Y. W.; Wexler, P. A.; Wigley, D. E. Inorg. Chem. 1989, 28,
3860.
(25) Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1993, 32, 131.
(26) Prout, K.; Daran, J.-C. Acta Crystallogr., Sect. B 1979, 35, 2882.
(27) Cotton, F. A.; Diebold, M. P.; Feng, X.; Roth, W. J. Inorg. Chem.
1988, 27, 3413.
(33) Sobota, P.; Utko, J.; Lis, T. J. Chem. Soc., Dalton Trans. 1984, 2077.
(34) Prinz, H.; Bott, S. G.; Atwood, J. L. J. Am. Chem. Soc. 1986, 108,
2113.
(35) Scholz, J.; Richter, B.; Goddard, R.; Kruger, C. Chem. Ber. 1993,
126, 57.
(36) Biagini, P.; Calderazzo, F.; Pampaloni, G.; Zanazzi, P. F. Gazz. Chim.
Ital. 1987, 117, 27.
(37) Brencic, J. V.; Ceh, B.; Leban, I. Z. Anorg. Allg. Chem. 1988, 565,
163.
(38) Lis, T. Acta Crystallogr., Sect. B 1980, 36, 2782.
(39) Stenger, H.; Weller, F.; Dehnicke, K. Z. Anorg. Allg. Chem. 1991,
606, 109.
(28) Imoto, H.; Hayakawa, S.; Morita, N.; Saito, T. Inorg. Chem. 1990,
29, 2007.
(29) Chavant, C.; Constant, G.; Jeannin, Y.; Morancho, R. Acta Crystallogr.,
Sect. B 1975, 31, 1823.
(40) Bucknor, S.; Cotton, F. A.; Falvello, L. R.; Reid, A. H., Jr.;
Schmulbach, C. D. Inorg. Chem. 1987, 26, 2954.
(41) Rochon, F. D.; Melanson, R.; Pi-Chang Kong. Acta Crystallogr., Sect.
C 1991, 47, 732.
(30) Bott, S. G.; Mazid, M. A.; Hursthouse, M. B.; Sullivan, A. C. J. Chem.
Soc., Dalton Trans. 1991, 355.
(31) Cotton, F. A.; Diebold, M. P.; Roth, W. J. Acta Crystallogr., Sect. C
1990, 46, 1624.
(32) Chisholm, M. H.; Huffman, J. C.; Tan, L.-S. Inorg. Chem. 1981, 20,
1859.
(42) Bandoli, G.; Clemente, D. A.; Mazzi, U.; Roncari, E. J. Chem. Soc.,
Dalton Trans. 1982, 1381.
(43) Subramanian, S.; Zaworotko, M. J. Can. J. Chem. 1993, 71, 433.
(44) Keppler, B. K.; Wehe, D.; Endres, H.; Rupp, W. Inorg. Chem. 1987,
26, 844.
(45) Bonnet, J.-J.; Jeannin, Y. J. Inorg. Nucl. Chem. 1973, 35, 4103.

3628 Inorganic Chemistry, Vol. 36, No. 17, 1997
Clark et al.
Figure 6. Optimized structure (DFT/B3LYP) in Cs symmetry for [Ta-
(OH)2(H)2(PH3)(Cl)]. Distances and angles are contained in Table 8.
Figure 7. Optimized structure (DFT/B3LYP) in Cs symmetry for [Ta-
(OH)3(H)2(PH3)]. Distances and angles are contained in Table 8.
Table 7
dn
cis-L-M-Cl, deg
∑R, deg ref
is not reproduced by the calculations and is undoubtedly a
consequence of the X-ray diffraction technique. The calculated
bond angles are also in excellent agreement with the solid state
structures. The predicted Ta-O-H angles at the axial hydrox-
ides (aryloxides) are as large as those found in the experimental
structures, but the Ta-O-H angle at the equatorial O in [Ta-
(OH)2(H)2(PH3)(OH)] is too small. Since the angle at O must
be due to a combination of electronic (π bonds between Ta
and O) and steric factors which are not properly incorporated
here, one should consider these results with caution.
compd/anion
[NbCl5(NCH)]
[TiCl5(THF)]-
[ZrCl5(THF)]-
[ZrCl5(THF)]-
[HfCl5(THF)]-
[NbCl5(PMe2Ph)]-
[MoCl5(OH2)]2-
[ReCl5(OH2)]-
[ReCl5(NCMe)]-
[ReCl5(PEt3)]-
[TcCl5(PEt3)]-
[TcCl5(PPh3)]-
[FeCl5(OH2)]2-
d0 82.0, 82.0, 83.5, 83.5
d0 85.1, 85.2, 85.8, 86.5
d0 84.0, 85.4, 86.0, 88.4
d0 84.3, 85.3, 85.8, 88.8
d0 86.1, 86.1, 86.2, 87.7
d1 83.5, 85.5, 87.3, 88.0
d3 86.2, 87.6, 87.6, 92.9
d3 87.5, 87.5, 88.5, 88.5
d3 86.3, 87.1, 87.7, 89.4
d3 87.8, 87.8, 98.9, 89.9
d3 85.9, 86.8, 89.8, 90.1
d3 85.3, 87.8, 90.3, 91.3
d5 85.7, 86.3, 86.6, 90.2
-29.0
-17.4
-16.2
-15.8
-13.9
-15.7
-5.7
-8.0
-9.5
-4.6
-7.4
-5.3
-11.2
-0.6
+0.9
-0.8
-5.9
-1.1
+4.1
29
33
34
35
36
31
37
38
39
40
41
42
43
44
45
46
47
48
49
Since the structure adopted by several electron-deficient
species has often been attributed to the presence of π-donor
ligands,14 it was of special interest to calculate the structure of
[Ta(OH)2H2(PH3)(H)] for a better understanding of the results.
The structure was found to be essentially identical to that
obtained for X ) Cl or OH (Table 8). This demonstrates
without ambiguity that the presence of a π donor at the X site
is not the dominant factor which makes this present structure
favorable.
[RuCl5(imidazole)]2- d5 89.7, 89.7, 90.0, 90.0
[IrCl5(pyrazine)]2-
[PtCl5(OH2)]-
[PtCl5(NH3)]-
[PtCl5(py)]-
d6 89.8, 89.8, 90.0, 91.3
d6 88.0, 88.0, 91.6, 91.6
d6 87.3, 88.1, 89.2, 89.5
d6 88.1, 89.6, 90.4, 90.8
d6 89.7, 90.3, 90.7, 93.4
[PtCl5(PEt3)]-
number of electrons in the formally nonbonding orbitals (t2g in
Oh symmetry) increases.
The structures of d0 complexes have been the subject of
numerous controversies. While d6 saturated hexacoordinated
complexes have a structure which is essentially octahedral, this
is far from being the case with lesser electrons in the d shell.
Disregarding the case of closed-shell d2 or d4 complexes, where
distortions away from octahedral usually occur10-13 in part
because of the necessity for opening a HOMO-LUMO gap,
the case of d0 ML6 complexes is considerably more fascinating.
In an octahedral geometry, the HOMO-LUMO gap is already
significant since it is associated with the energy gap between
the occupied M-L σ bond and the empty “t2g” orbitals of the
metal. The gap depends on the nature of the complex, and one
can foresee that second-order Jahn-Teller distortion may occur
for specific cases leading to nonoctahedral complexes. The
requirement for such distortion is that the energy gap be small
in the octahedral geometry. This means that the energy of the
M-L σ bonds must be high and/or the energy of the t2g orbitals
must be low and calls for ligands which are bound to the metal
by atoms which are not too electronegative. It also calls for
ligands which have no π-donor orbitals, since this would raise
the energy of the t2g set. The ideal case is thus WMe6.1d It
was proposed several years ago that the model d0 MH6 complex
could have a C3V geometry with nonequivalent triangular faces
within a trigonal prismatic structure.3,4 Electron diffraction
Theoretical Analysis and Discussion. Calculations were
carried out on the model compounds [Ta(OH)2(H)2(PH3)(X)]
(X ) OH, Cl) in which the two hydroxides and hydrides are
maintained in a mutually transoid geometry. The OH groups
(called axial in the following discussion) were maintained in
the plane containing Ta, P, and X.
The calculated structures (Figures 6 and 7) are in excellent
agreement with the experimental structures (Table 8). The
notable feature is the strong deviation from the octahedral
geometry with a P-Ta-H angle of around 60°. The Ta-ligand
bond lengths are well reproduced in the theoretical structures
with the exception of the Ta-P bond, which is too long (Table
8). This discrepancy is often observed in the calculation of
phosphine complexes and does not bear any influence on the
other properties of the molecules. The Ta-H bond lengths of
1.76-1.79 Å are well within the average expected values. The
significant shortening of one of the Ta-H bond observed in
the X-ray structure of [Ta(OC6H3But2-2,6)2Cl(H)2(PMePh2)]15
measurements1c on [WMe6] followed by more recent ab initio
7-9
calculations on [CrH6],5 [WH6],6 and [WMe6]
agreed with
(46) Fanizzi, F. P.; Natile, G.; Maresca, L.; Lanfredi, A. M. M.; Tiripicchio,
A. J. Chem. Soc., Dalton Trans. 1984, 1467.
the earlier predictions. Very recently, an X-ray determination
of [WMe6] (as well as [ReMe6])1a gave a definitive answer to
the structure of this complex.
(47) Kukushkin, V.; Zenkevich, I. G.; Belsky, V. K.; Konovalov, V. E.;
Moiseev, A. I.; Sidorov, E. O. Inorg. Chim. Acta 1989, 166, 79.
(48) Kukushkin, V.; Zenkevich, I. G.; Belsky, V. K.; Konovalov, V. E.;
Moiseev, A. I.; Sidorov, E. O. Inorg. Chim. Acta 1989, 166, 79.
(49) Albinati, A.; Kaufmann, W.; Venanzi, L. M. Inorg. Chim. Acta 1991,
188, 145.
It is of interest to return briefly to the origin of the distortion
in WMe6, since a similar argument will be used for the tantalum

Six-Coordinate Adducts of Nb(V) and Ta(V)
Inorganic Chemistry, Vol. 36, No. 17, 1997 3629
Table 8. Comparison of Optimized Theoretical and Experimental15 Bond Distances (Å) and Angles (deg)
[Ta(OH)2(H)2(PH3)(Cl)]
[Ta(OAr)2(H)2(PMePh2)(Cl)]
[Ta(OH)3(H)2(PH3)]
[Ta(OAr)3(H)2(PMe2Ph)]
Ta-Cl
2.42
1.91
2.72
1.76, 1.77
66.1, 63.4
86.7
2.397(1)
1.896(3), 1.888(3)
2.655(1)
1.73(5), 1.54(5)
56(2), 62(2)
86.5(1), 86.5(1)
160.8(1)
Ta-O(3)
1.95
1.90
2.82
1.78, 1.79
64.8, 66.6
92.3
168.7
114.3
1.912(4)
1.897(3), 1.907(3)
2.650(1)
1.74(4), 1.83(6)
66(2), 69(1)
92.0(2), 93.6(2)
160.7(3), 169.5(4)
153.6(3)
Ta-O
Ta-O(1)
Ta-P
Ta-H
P-Ta-H
Cl-Ta-O
Ta-O-H
Ta-P
Ta-H
P-Ta-H
O(3)-Ta-O(2)
Ta-O(1)-H
Ta-O(3)-H
140.6
Table 9. Crystal Data and Data Collection Parameters
1a
1b
3a
3b
formula
fw
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å
Nb2Cl6O4C48H68
1107.60
P21/n (No. 14)
12.735(2)
15.527(3)
13.620(2)
90
90.607(14)
90
2693.0(14)
2
Ta2Cl6O4C48H68
1283.68
P21/n (No. 14)
12.757(6)
12.757(6)
13.625(9)
90
90.83(6)
90
2703(5)
2
NbCl3O2NC29H39
632.90
P212121 (No. 19)
11.622(2)
13.283(5)
20.666(7)
90
90
90
3190(3)
4
1.318
293
TaCl3O2NC29H39
720.95
P212121 (No. 19)
11.648(2)
13.273(3)
20.624(6)
90
90
90
3188(2)
4
1.502
293
Z
F
calc, g cm-3
1.366
295
1.577
297
temp, K
radiation (λ, Å)
R
Cu KR (1.541 84)
0.062
Mo KR (0.710 73)
0.072
Mo KR (0.710 73)
0.031
Mo KR (0.710 73)
0.029
Rw
0.121
0.160
0.036
0.036
4
6a
6b
7‚C6H6
formula
fw
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å
NbCl3PO2C32H45
691.95
Pna21 (No. 33)
19.153(4)
17.372(5)
10.461(8)
90
90
90
3480(4)
4
1.320
297
NbCl2PO3C44H62
833.77
TaCl2PO3C44H62
921.81
NbCl3PO3C50H69
948.34
P1 (No. 2)
13.1862(9)
13.3274(12)
13.4391(16)
85.553(9)
73.433(9)
89.794(7)
2256.5(5)
2
P1 (No. 2)
13.1685(15)
13.337(2)
13.4210(16)
85.553(12)
73.584(10)
89.735(12)
2253.8(7)
2
P1 (No. 2)
10.505(2)
12.967(4)
19.289(3)
83.17(2)
89.35(1)
89.20(2)
2608(2)
2
Z
F
calc, g cm-3
1.227
296
1.357
295
1.207
295
temp, K
radiation (λ, Å)
R
Mo KR (0.710 73)
0.062
Mo KR (0.710 73)
0.039
Mo KR (0.710 73)
0.036
Mo KR (0.710 73)
0.046
Rw
0.114
0.086
0.028
0.052
complexes in this study. In pure Oh geometry, the empty t2g
set is not involved in W-C bonding. A change of geometry
which could permit the overlap between those nonbonding
orbitals and the ligands can thus stabilize the complex by
lowering the energy of some of the orbitals that describe the
W-C bonds. This stabilization may only occur if the loss of
W-C bonding associated with the diminished overlap between
the ligands and the metal orbitals (p orbitals of t1u and d orbitals
of eg symmetry) used in the octahedral bonding is not too large.9
Calculations are thus required to determine which is the
dominant effect, but it is clear that a low energy of the t2g set
of orbitals is central to the story. The absence of π-donor
ligands is thus an important requirement. For this reason, WMe6
is a distorted trigonal prism while [WCl6],50 [WF6],51 and
[CrF6]52 are octahedral.
increased stabilization of these two Ta-H bonds. This is thus
a phenomenon related to that occurring in WMe6. When X is
a π donor, the energy of dxy is raised but the orbital is still at
low energy. Moving the hydrides as in Figure 8 now has two
effects: reinforcing the Ta-H bonds and improving the Ta-X
π bonds. The movement of the hydrides forces mixing of dxy
and py, which thus creates a hybrid orbital that has improved
capacity for Ta-X π-bonding. This was previously shown to
be the case in the molecules [OsH3L2(X)].12 Theoretical studies
of the trihydride species [Ta(OH)2H2(PH3)(H)] are highly
significant. The fact that this molecule is also predicted to
distort (Table 8) implies that the mixing of dxy and py orbitals
increases the σ-bonding of the two hydride ligands cis to the
phosphine group.
In the present tantalum complexes, drawn in an octahedral
geometry in Figure 8, the case of the trihydride (X ) H) is
highly significant. The dxy orbital is the lowest empty orbital,
since it is nonbonding with respect to every lone pair of the
alkoxy groups. In contrast, the dxz and dyz orbitals are
destabilized by the alkoxy lone pairs. Thus dxy is most able to
participate in the stabilization of the d0 species. Moving the
two hydrides so that they can overlap with dxy permits an
(50) Haaland, A.; Martinsen, K. G.; Shlykov, S. Acta Chem. Scand. 1992,
46, 1208.
(51) Marx, R.; Seppelt, K.; Ibberson, R. M. J. Chem. Phys. 1996, 104,
7658.
(52) (a) Marsden, C. J.; Wolynec, P. P. Inorg. Chem. 1991, 30, 1681. (b)
Neuhaus, A.; Frenking, G.; Huber, C.; Gauss, J. Inorg. Chem. 1992,
31, 5355. (c) Pierloot, K.; Roos, B. O. Inorg. Chem. 1992, 31, 5353.
(d) Jacobs, J.; Muller, H. S. P.; Willner, H.; Jacob, E.; Burger, H.
Inorg. Chem. 1992, 31, 5357. (e) Vanquickenborne, L. G.; Vinckier,
A. E.; Pierloot, K. Inorg. Chem. 1996, 35, 1305.

3630 Inorganic Chemistry, Vol. 36, No. 17, 1997
Clark et al.
(28.7 g, 161 mmol). The resulting mixture was stirred at room
temperature for 1.5 h. Removal of benzene solvent gave a red sticky
solid, which was washed with hexane and dried in Vacuo to yield the
product as a brick-red powder (31.5 g, 64%). Crystals suitable for
X-ray diffraction analysis were obtained by slow cooling of a hot,
saturated toluene solution. Anal. Calcd for C24H34O2Cl3Nb: C, 52.03;
H, 6.15; Cl, 19.24. Found: C, 51.77; H, 6.30; Cl, 19.05. 1H NMR
(C6D6, 30 °C): δ 7.03-6.86 (m, 6H, aromatics); δ 4.03 (septet, 4H,
CHMe); δ 1.21 (doublet, 24H, CHMe).
[Nb(OC6H3Pri2-2,6)3Cl2] (2a). To a suspension of NbCl5 (26 g,
96 mmol) in benzene (250 mL) was slowly added 2,6-diisopropylphenol
(55 g, 308 mmol). The resulting mixture was heated to reflux for 6 h,
cooled to room temperature, and stirred for an additional 14 h, producing
an orange-red suspension. Removal of the benzene solvent in Vacuo
yielded the crude product (60g, 89%) as a light-orange powder.
Recrystallization from warm hexane produced red needles of the pure
product. Anal. Calcd for C36H51O3Cl2Nb: C, 62.25; H, 7.35; Cl, 10.09.
Found: C, 61.96; H, 7.63; Cl, 9.94. 1H NMR (C6D6, 30 °C): δ 7.07-
6.97 (m, 9H, aromatics); δ 3.65 (septet, 6H, CHMe); δ 1.25 (d, 36H,
CHMe).
[Ta(OC6H3Pri2-2,6)3Cl2] (2b). To a solution of TaCl5 (26.07 g, 72.8
mmol) in toluene (150 mL) was slowly added 2,6-diisopropylphenol
(40.22 g, 225.6 mmol). The resulting mixture was heated to reflux
gently for 4 h and then cooled to room temperature, producing a green
solution. Removal of the toluene solvent in Vacuo yielded the crude
product as a green crystalline solid. Recrystallization from warm
hexane produced green needles of the pure product in moderate yield,
46g (77%). Anal. Calcd for C36H51O3Cl2Ta: C, 55.18; H, 6.56; Cl,
9.05. Found: C, 54.83; H, 6.89; Cl, 9.20. 1H NMR (C6D6, 30 °C):
δ 7.18-6.91 (m, 9H, aromatics); δ 3.66 (septet, 6H, CHMe); δ 1.26
(d, 36H, CHMe).
cis-mer-[Nb(OC6H3Pri2-2,6)2Cl3(py)] (3a). To a suspension of
[Nb(OC6H3Pri2-2,6)2Cl3]2 (1a) (0.5 g, 2.3 mmol) in benzene (3 mL)
was added pyridine (0.75 g, 9.5 mmol). The suspension changed to a
dark red solution within 1 min. Removal of the benzene solvent and
excess pyridine in Vacuo produced a red glassy solid. The crude solid
was taken up in benzene (7 mL), and the solution was layered with
hexane to produce bright red needles of pure product, which were
decanted, washed with hexane, and dried in Vacuo. Anal. Calcd for
C29H39O2NCl3Nb: C, 55.04; H, 6.21; N, 2.21; Cl, 16.81. Found: C,
55.22; H, 6.45; N, 2.33; Cl, 16.48. 1H NMR (C6D6, 30 °C): δ 9.15
(d, 2H, py-ortho); δ 7.13-6.92 (m, 6H, aromatics); δ 6.64 (t, 1H, py-
para); δ 6.27 (t, 2H, py-meta); δ 4.49 (septet, 2H, CHMe); δ 3.79
(septet, 2H, CHMe); δ 1.33 (d, 12H, CHMe); δ 1.07 (d, 12H, CHMe).
Figure 8. Qualitative correlation diagram (P-Ta-H decreasing from
90 to 60°) for selected molecular orbitals of [Ta(OR)2(H)2(PH3)(Cl)].
The above analysis suggests that replacement of the hydride
ligands by π-donor groups should diminish or even remove the
distortion. Theoretical calculations on trans-mer-[Ta(OH)3(Cl)2-
(PH3)], a model of trans-mer-[Ta(OC6H3Pri2-2,6)3Cl2(PMe2Ph)]-
(6b), show that in the lowest energy situation the two chloride
ligands are indeed bent toward the PH3 ligand with P-Ta-Cl
≈ 10°. Since Cl is not a strong π donor, the same effects that
are present in the hydride system are still at work here but in a
reduced fashion. The adoption of a cis arrangement of aryloxide
ligands in the adducts cis-mer-[MCl3(OC6H3Pri2-2,6)2(L)] (3a,
3b, 4a) precludes direct comparison with trans-trans-[TaCl-
(H)2(OC6H3But2-2,6)2(PMePh2)]. The small but consistent
bending of the chloride ligands toward the donor ligand in
[MCl5(L)]n- species for electron-deficient metal centers (Table
7) strongly implicates π-bonding of the axial chloride ligand
as the underlying cause. This situation is reminiscent of oxo
species [M(O)Cl4(L)]n-, where bending of the chloride ligands
away from the π-donating oxo group occurs.53
cis-mer-[Ta(OC6H3Pri2-2,6)2Cl3(py)] (3b). This compound was
obtained by a procedure analogous to that used for compound 3a to
yield yellow crystals of product. 1H NMR (C6D6, 30 °C): δ 9.19 (d,
2H, py-ortho); δ 6.86-7.12 (m, 6H, aromatics); δ 6.60 (t, 1H, py-
para); δ 6.23 (t, 2H, py-meta); δ 4.43 (septet, 2H, CHMe); δ 3.71
(septet, 2H, CHMe); δ 1.33 (d, 12H, CHMe); δ 1.07 (d, 12H, CHMe).
cis-mer-[Nb(OC6H3Pri2-2,6)2Cl3(PMe2Ph)] (4a). To a suspension
of [Nb(OC6H3Pri2-2,6)2Cl3]2 (1a) (1.0 g, 4.6 mmol) in hexane (2 mL)
was added PMe2Ph (0.75 g, 5.4 mmol). Additional hexane was added
to just dissolve the dark red product, and the solution was slowly cooled
to -10 °C for 48 h to obtain black needles of product, which were
dried in Vacuo. Anal. Calcd for C32H45O2Cl3PNb: C, 55.70; H, 6.52;
Experimental Section
All operations were carried out under a dry nitrogen atmosphere or
in vacuo either in a Vacuum Atmosphere Dri-Lab or by standard
Schlenk techniques. The compound [Ta(OC6H3Pri2-2,6)2Cl3]2 (1b) was
obtained by previously reported procedures.16b Hydrocarbon solvents
were dried by distillation from sodium/benzophenone and stored under
dry nitrogen. Dimethylphenylphosphine was purchased from Strem
Chemical Co., and pyridine was purchased from Aldrich Chemical Co.
1
P, 4.48; Cl, 15.37. Found: C, 55.69; H, 6.57; P, 4.15; Cl, 15.13. H
NMR (C6D6, 30 °C): δ 7.42 (m, 2H, P-Ph ortho); δ 7.08-6.83 (m,
9H, aromatics); δ 4.07 (m, br, 4H, CHMe); δ 1.61 (d, 6H, P-Me),
2J(31P-1H) ) 9.19 Hz; δ 1.18 (s, br, 24H, CHMe). 31P NMR (C6D6,
30 °C): δ -13.35.
cis-mer-[Ta(OC6H3Pri2-2,6)2Cl3(PMe2Ph)] (4b). To a suspension
of [Ta(OC6H3Pri2-2,6)2Cl3]2 (0.5 g, 0.39 mmol) in benzene was added
dimethylphenylphosphine (0.13 g, 0.94 mmol). The resulting yellow
solution was stirred for 1 h. Removal of the benzene solvent yielded
the product as a yellow crystalline solid, which was washed with hexane
and dried in Vacuo. 1H NMR (C6D6, 30 °C): δ 7.44 (m, 2H, P-Ph
ortho); δ 6.81-7.10 (m, 9H, aromatics); δ 4.22 (septet, 2H, CHMe);
1
Both were dried over 3 Å molecular sieves prior to use. The H and
31P NMR spectra were recorded on a Varian Associates Gemini 200
and a General Electric QE-300 spectrometer and were referenced either
to protio impurities of commercial benzene-d6 or to 85% H3PO4.
Microanalyses were obtained in-house at Purdue.
[Nb(OC6H3Pri2-2,6)2Cl3]2 (1a). To a suspension of NbCl5 (21.8 g,
80 mmol) in benzene (200 mL) was slowly added 2,6-diisopropylphenol
2
δ 3.75 (septet, 2H, CHMe); δ 1.62 (d, 6H, P-Me), J(31P-1H) ) 8.87
Hz; δ 1.26 (d, 12H, CHMe); δ 1.05 (d, 12H, CHMe). 31P NMR (C6D6,
30 °C): δ -11.35.
(53) Nugent, W. A.; Mayer, J. A. Metal-Ligand Multiple Bonds;
Wiley-Interscience: New York, 1988.

Six-Coordinate Adducts of Nb(V) and Ta(V)
Inorganic Chemistry, Vol. 36, No. 17, 1997 3631
trans-mer-[Nb(OC6H3Pri2-2,6)3Cl2(py)] (5). To a suspension of
[Nb(OC6H3Pri2-2,6)3Cl2] (0.5 g, 0.72 mmol) in benzene (10 mL) was
added pyridine (0.07g, 0.9 mmol), and the resulting mixture was stirred
for 5 min, during which a slight color change from bright to dark red
was observed. Removal of the benzene solvent in Vacuo produced the
product as a bright red solid. Anal. Calcd for C41H56O3NCl2Nb: C,
63.57; H, 7.29; N, 1.81; Cl, 9.15. Found: C, 62.92; H, 7.34; N, 2.00;
Cl, 8.84. 1H NMR (C6D6, 30 °C): δ 8.83 (d, 2H, py-ortho); δ 7.17-
6.95 (m, 9H, aromatics); δ 6.70 (t, 1H, py-para); δ 6.30 (t, 2H, py-
meta); δ 4.28 (septet, 2H, CHMe); δ 4.01 (septet, 4H, CHMe); δ 1.19
(d, 12H, CHMe); δ 1.18 (d, 24H, CHMe).
Computational Details. The model systems chosen for study were
[Ta(OH)2(H)2(PH3)X] (X ) Cl, OH, H with Y ) H; X ) Cl with Y )
OH, Cl). Optimizations of the geometry were carried out with the
Gaussian 92 set of programs at the RHF, MP2, and DFT (B3LYP)
levels.54 Effective core potentials (ECPs) from Stoll et al. were chosen
for Ta.55 The ECPs for Cl and P are those of Barthelat et al.56 and
Stevens and Bash, respectively.57 A triple-ú-quality basis set55 was
used for the metal, and a double-ú for all atoms bonded to it with
polarization functions for the last.57,58 The hydrogens of OH and PH3
were calculated with a (4S)/[1S] basis set.59 Since the calculations
gave essentially the same geometry with the three levels of calculations,
only the DFT results are reported here.
trans-mer-[Nb(OC6H3Pri2-2,6)3Cl2(PMe2Ph)] (6a). To a suspension
of [Nb(OC6H3Pri2-2,6)3Cl2] (0.5 g, 0.72 mmol) in benzene (1 mL) was
added PMe2Ph (0.35 g, 2.5 mmol). After an immediate color change
from orange-red to dark bright red, the reaction mixture was covered
with a layer of hexane (15 mL). After 72 h, large, cubic, dark red
crystals of product were obtained, washed with hexane, and dried in
Vacuo. Anal. Calcd for C46H62O3Cl2PNb: C, 63.47; H, 7.44; P, 3.72;
Cl, 8.27. Found: C, 63.05; H, 7.65; P, 3.42; Cl, 8.48. 1H NMR (C6D6,
30 °C): δ 7.41 (m, P-Ph ortho); δ 7.17-6.91 (m, 12H, aromatics); δ
4.12-4.04 (m, br, 4H, CHMe); δ 4.03-3.83 (m, br, 2H, CHMe); δ
1.48 (d, 6H, P-Me), 2J(31P-1H) ) 7.70 Hz; δ 1.15 (s, br, 36H, CHMe).
31P NMR (C6D6, 30 °C): δ -17.23.
Acknowledgment. We thank the Department of Energy and
the National Science Foundation for financial support of this
research.
Supporting Information Available: X-ray crystallographic files,
in CIF format, for complexes 1a, 1b, 3a, 3b, 4, 6a, 6b, and 7 are
available on the Internet only. Access information is given on any
current masthead page.
IC970331L
trans-mer-[Ta(OC6H3Pri2-2,6)3Cl2(PMe2Ph)] (6b). To a suspension
of [Ta(OC6H3Pri2-2,6)3Cl2] (1.0 g, 1.22 mmol) in toluene (2 mL) was
added PMe2Ph (0.25 g, 1.83 mmol). After an immediate change from
a green suspension to a yellow solution, the reaction mixture was
covered with a layer of hexane (15 mL). After 72 h, yellow crystals
of product were obtained, washed with hexane, and dried in Vacuo.
Anal. Calcd for C44H62O3Cl2PTa: C, 57.33; H, 6.78; P, 3.36; Cl, 7.69.
Found: C, 56.27; H, 6.99; P, 4.94; Cl, 8.44. 1H NMR (C6D6, 30 °C):
δ 7.37 (m, 2H, P-Ph ortho); δ 7.13-6.81 (m, 12H, aromatics); δ 4.03
(septet, 4H, CHMe); δ 3.71 (septet, 2H, CHMe); δ 1.51 (d, 6H, P-Me),
2J(31P-1H) ) 7.60 Hz; δ 1.16 (d, 24H, CHMe) δ 1.09 (d, 12H, CHMe);
31P NMR (C6D6, 30 °C): δ -12.13.
(54) Frish, J. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong,
M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M.
A.; Repolge, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.;
Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.;
Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92/DFT; Gaussian,
Inc.; Pittsburgh, PA, 1993.
(55) Andrae, D.; Hau¨ssermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor.
Chim. Acta 1990, 77, 123.
(56) Bouteiller, Y.; Mijoule, C.; Nizam, M.; Barthelat, J. C.; Daudey, J.
P.; Pe´lissier, M.; Silvi, B. Mol. Phys. 1988, 65, 285.
(57) Stevens, W. J.; Bash, H.; Krauss, M. J. Chem. Phys. 1984, 81, 6026.
(58) For hydrides we have used a triple-ú basis set with polarization:
Dunning, T. H. J. Chem. Phys. 1970, 53, 2823.
(59) Huzinaga, S. J. J. Chem. Phys. 1965, 42, 1293.