Ligand effects in transition metal dihydrogen complexes: A theoretical study

Ab initio calculations at the MP2 and CCSD(T) level of theory using effective core potentials for the heavy atoms have been carried out in order to investigate systematically the trans influence of different ligands L upon H-2 coordination in the dihydrogen complexes ML(CO)(4)(H-2) (M = Cr, Mo, W; L = CO, SiO, CS, CN-, NC-, NO+, N-2, H-, F-, Cl-, PH3). The optimized geometries and the theoretically predicted M-H-2 bond dissociation energies are reported. The ligands L are characterized in terms of their donor and acceptor capability using the CDA partitioning scheme. Strong sigma-donor ligands in the trans position enhance the M-H-2 bonding, while strong pi-acceptors weaken the M-H-2 bond. There is a good correlation between the calculated acceptor strength of L and the distance r(H-H) and the M-(H-2) bond dissociation energy D-e, respectively. Finally, the dihydrogen complexes are compared with the related classical hydrides ML(CO)(4)(H)(2).