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

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.