INFLUENCE OF A CIS HYDRIDE ON A COORDINATED H-2 LIGAND - ABINITIO CALCULATIONS

The H-2 molecule is a fascinating ligand in transition(~)metal complexes because it is affected by small changes in the nature of thc coordinating metal moiety. It is well accepted that back-donation from the metal determines the orientation of H-2. Here we show by ab-initio calculations that a cis hydride can play an important role in favoring the alignment of the metal-H and H-2 bonds (cis effect introduced in 1990). The experimentally observed staggered orientation of H-2 in H2Fe(PEtPh2)2(H-2) is thus suggested to come from a competition between the well-known back-donation and the more novel cis effect.