Phosphinidene Transition Metal Complexes: A Combined Ab Initio MO-DFT Study of Cr(CO)5-PR

Ab initio MO and DFT calculations have been performed on phosphinidene complexes of the type Cr(CO)5-PR, with R = H, CH3, SiH3, NH2, PH2, OH, and SH. The formation of the Cr-P bond essentially arises from a ligand-to-metal charge transfer. While a significant (pi) backdonation is also observed for the Cr(CO)5-PH, Cr(CO)5-PCH3, and Cr(CO)5-PSiH3 complexes, this is less the case for Cr(CO)5-POH, Cr(CO)5-PSH, and Cr(CO)5-PNH2, and the backbonding almost disappears for Cr(CO)5-PPH2. In both the lowest lying singlet and triplet states, all complexes exhibit a staggered conformation. CASSCF/CASPT2 calculations performed with the ANO basis sets indicate a closed-shell singlet ground state along the whole series. The binding energy between Cr(CO)5 and PR ranges from 216 kJ/mol for Cr(CO)5-PNH2 to 127 kJ/mol for Cr(CO)5-PSiH3 (B3LYP values). In general, the B3LYP-DFT scheme yields reasonable qualitative and quantitative results when compared with CASPT2(12/12).