TRANSITION-METAL POLYHYDRIDE COMPLEXES - 9 - THE EFFECT OF LIGAND SIGMA-BONDING AND PI-BONDING ON THE H-TA-H BOND-ANGLE IN 6-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 ar e 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 th e 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', originat es primarily from sigma-bonding, the direction of the distortion of th e ''trans'' hydrides originates primarily from the relative pi-accepto r 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-acce ptor (L vs L'); With certain ligand pairs the possibility of two isome rs exists, one with hydrides bent toward L and another with hydrides b ent toward L'.