Stabilization of phosphinidenes by metal complexation: A theoretical study of Cr(CO)5–PH

Ab initio MO and DFT calculations have been performed on the Cr(CO)5–PH complex to probe the interaction between a phosphinidene ligand and a transition-metal fragment. The formation of a Cr–P bond essentially arises from a ligand → metal charge transfer. In the lowest-lying singlet and triplet states, the complex exhibits a staggered conformation. CASSCF/CASPT2 calculations, performed with ANO basis sets, indicate a closed-shell singlet ground state for Cr(CO)5–PH, which lies about 35 kJ/mol below the triplet state. The binding energy between Cr(CO)5 and PH is 172 kJ/mol as calculated by CASPT2 (138 kJ/mol by DFT/B3LYP). While HF and MP2, either in restricted or unrestricted formulation, CASSCF(2/2) and CASPT2(2/2) fail to predict a correct qualitative picture, B3LYP-DFT yields reasonable qualitative and quantitative results as compared to CASPT2(12/12).