Phosphinidene complexes M(CO)5-PR:: A density functional study on structures and electronic states

Density functional studies have been carried out for the equilibrium structures of the phosphinidene transition-metal complexes M(CO)(5)-PR, with M = Cr, Mo, and W and R = H, Ph, OH, and NH2. The free phosphinidenes P-R have triplet ground states, but their M(CO)(5) complexes prefer singlet states because of the substantial stabilization of the unoccupied phosphorus p(pi) acceptor orbital. This follows from calculations based on the local density approximation, including nonlocal corrections for correlation and exchange self-consistently. The M(CO)(5)-PR bond is investigated using a bond energy analysis in terms of electrostatic interaction, Pauli repulsion, and orbital interaction. A symmetry decomposition scheme affords a quantitative estimate of the sigma and pi bond strengths, which gives an interpretation of the donor-acceptor complexes within the Dewar-Chatt-Duncanson model. It is shown that the investigated ligands are strong pi-acceptors and even stronger sigma-donors. In the case of unsubstituted PH complexes, the ground state is a singlet due to strong preferential stabilization by pi-back-donation into the empty phosphorus p(pi) orbital. When substituents are present, the singlet state is already relatively stabilized in the free phosphinidene due to pi-donation from the substituent. The pi-back-donatian from the metal fragment decreases accordingly due to competition with this substituent pi-donation but remains effective in further preferentially stabilizating the singlet.