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_π 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 σ and π 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 π-acceptors and even stronger σ-donors. In the case of unsubstituted PH complexes, the ground state is a singlet due to strong preferential stabilization by π-back-donation into the empty phosphorus p_π orbital. When substituents are present, the singlet state is already relatively stabilized in the free phosphinidene due to π-donation from the substituent. The π-back-donation from the metal fragment decreases accordingly due to competition with this substituent π-donation but remains effective in further preferentially stabilizating the singlet.