STRUCTURAL DISTORTIONS IN 6-COORDINATE ADDUCTS OF NIOBIUM(V) AND TANTALUM(V)

The mixed chloro-aryloxide compounds [M(OC6H3Pr2i-2,6)(2)Cl-3](2) (M = Nb (1a), Ta (1b); both structurally characterized) and [M(OC6H3Pr2i-2 ,6)(3)Cl-2] (M = Nb (2a), Ta (2b)) react with pyridine (py) and PMe2Ph to produce a series of adducts cis-mer-[MCl3(OC6H3Pr2i-2,6)(2)(L)] (M , L: Nb, py (3a); Ta, py (3b); Nb, PMe2Ph (4a); Ta, PMe2Ph (4b)) and t rans-mer-[MCl2(OC6H3Pr2i-2,6)(3)(L)] (M, L: Nb, py (5); Nb, PMe2Ph (6a ); Ta, PMe2Ph (6b)). The assigned geometric arrangement of ligands is based upon H-1 NMR studies and single-crystal X-ray diffraction analys es of 3a, 3b, 4, 6a, and 6b. The salt complex [HPMe2Ph](+)[mer-NbCl3(O C6H3Pr2i-2,6)(3)](-) (7) has also been isolated and structurally chara cterized. The structural parameters for the neutral adducts are compar ed with those of previously reported hydride aryloxides of tantalum. A small but consistent distortion away from octahedral geometry involvi ng the bending of mutually trans anionic ligands toward the neutral do nor group is observed. Theoretical analysis at several levels of theor y (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 b e accounted for in terms of an improvement in both X p to metal d pi-b onding and Ta-H sigma-bonding. The contribution of sigma-bonding effec ts is clearly shown in the case of X = H, where again a bend of the tw o hydride ligands toward the Ta-P bond is calculated. A smaller distor tion of the Cl ligands in trans-mer-[Ta(OH)(3)(Cl)(2)(PH3)] is also pr edicted.