A critical evaluation of the location and refinement of a bridging hydrogen atom in a transition-metal hydride, .mu.-hydrido-.mu.-dimethylphosphido-bis(.eta.5-cyclopentadienyldicarbonylmolybdenum), by x-ray diffraction

Abstract

A comparison of the structural parameters obtained by x-ray and neutron diffraction for the transition-metal hydride complex Mo/sub 2/(eta/sup 5/-C/sub 5/H/sub 5/)/sub 2/(CO)/sub 4/(..mu..-H)(..mu..-P(CH/sub 3/)/sub 2/) was performed to examine the discrepancies between the ''x-ray-determined'' and ''neutron-determined'' values for the structural parameters associated with the bridging hydride ligand. Within the experimental limitations of the x-ray analysis, the x-ray-determined hydrogen position reflects the covalent character of the bent Mo--H--Mo bond with the estimated region of maximum overlap displaced ca. 0.2 A from the hydrogen nucleus along the Mo--H--Mo bisector toward the centroid of the Mo--H--Mo triangle. The consequences of this displacement are (1) an apparent shortening of 0.1 A in the average x-ray-determined Mo--H distance of 1.79 (15) A compared to the average neutron-determined Mo--H distance of 1.86 (1) A and (2) a large increase in the Mo--H--Mo bond angle from the neutron-determined value of 122.9 (2)/sup 0/ to the x-ray-determined value of 133 (3)/sup 0/. For closed-type, bent metal--hydrogen--metal bonds, one should then expect to find shorter average M--H bond distances and larger M--H--M bond angles by x-ray diffraction than by neutron diffraction.