Crystal structure and molecular geometry of homogeneous hydrogenation catalyst .mu.-chloro-.mu.-hydrido-dichlorobis(pentamethylcyclopentadienyl)diiridium(III) and of its di-.mu.-chloro-dichlorobis- precursor. Direct comparison of .mu.-hydrido-.mu.-chloro-diiridium and di-.mu.-chloro-diiridium bridging systems