Transition metal polyhydride complexes.: 9.: The effect of ligand σ- and π-bonding on the H-Ta-H bond angle in six-coordinate tantalum(V) dihydride complexes

In the known structures of the pseudo-octahedral complexes [Ta(OR)(2)L'(H)(2)L](+), with L = PR3 and L'= Cl-, OR-, the "trans" hydrides are bent away from the sterically less demanding ligand (L') toward the bulkier phosphine group (L). Here, ab initio calculations on a series of analogous model complexes (R = H; L, L' = OH-, F-; Cl-, Br-, CO CN-, CS, BF, N-2, CH3-, NH3, PH3, CH3CN, NH2- and H-) are used to examine the effect of the sigma- and pi-bonding of the ligands L and L' on the direction of H-Ta-H bending. While the trend in the trans influence, which dominates the Ta-L bond length of the ligand trans L', originates primarily from sigma-bonding, the direction of the distortion of the "trans" hydrides originates primarily from the relative pi-acceptor ability of the ligand L to that of the ligand L'. Generally, as the ligands are-varied, the hydrides will bend toward the stronger pi-acceptor (L vs L'); With certain ligand pairs the possibility of two isomers exists, one with hydrides bent toward L and another with hydrides bent toward L'.