title: Structural Distortions in Six-Coordinate Adducts of Niobium(V) and Tantalum(V).
abstract: The mixed chloro-aryloxide compounds [M(OC(6)H(3)Pr(i)(2)-2,6)(2)Cl(3)](2) (M = Nb (1a), Ta (1b); both structurally characterized) and [M(OC(6)H(3)Pr(i)(2)-2,6)(3)Cl(2)] (M = Nb (2a), Ta (2b)) react with pyridine (py) and PMe(2)Ph to produce a series of adducts cis-mer-[MCl(3)(OC(6)H(3)Pr(i)(2)-2,6)(2)(L)] (M, L: Nb, py (3a); Ta, py (3b); Nb, PMe(2)Ph (4a); Ta, PMe(2)Ph (4b)) and trans-mer-[MCl(2)(OC(6)H(3)Pr(i)(2)-2,6)(3)(L)] (M, L: Nb, py (5); Nb, PMe(2)Ph (6a); Ta, PMe(2)Ph (6b)). The assigned geometric arrangement of ligands is based upon (1)H NMR studies and single-crystal X-ray diffraction analyses of 3a, 3b, 4, 6a, and 6b. The salt complex [HPMe(2)Ph](+)[mer-NbCl(3)(OC(6)H(3)Pr(i)(2)-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)(PH(3))(X)] (X = Cl, OH, H) show a distortion involving bending of the trans-hydride groups toward the PH(3) ligand for X = Cl and OH. This distortion can be accounted for in terms of an improvement in both X p to metal d pi-bonding and Ta-H sigma-bonding. The contribution of sigma-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)(PH(3))] is also predicted.
authors.affiliation: 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-Théorique, Bâtiment 490, Université de Paris-Sud, 91405 Orsay, France.
authors.forename: Janet R.
authors.initials: JR
authors.lastname: Clark
authors.forename: April L.
authors.initials: AL
authors.lastname: Pulvirenti
authors.forename: Phillip E.
authors.initials: PE
authors.lastname: Fanwick
authors.forename: Michaelis
authors.initials: M
authors.lastname: Sigalas
authors.forename: Odile
authors.initials: O
authors.lastname: Eisenstein
authors.forename: Ian P.
authors.initials: IP
authors.lastname: Rothwell