Spin-Crossing in an Organometallic Pt-Benzene Complex

The interaction between the Pt atom and the benzene (Bz) molecule was investigated theoretically, using a symmetric Pt-Bz half-sandwich complex. Various levels of wave function theory (HF, MP2, CCSD(T), CASPT2, multistate (MS) MS-CASPT2) together with several functionals (PBE, PBE+D3, PBE+vdW, PBE/EE+vdW) of the density functional theory (DFT) were complemented by quantum Monte Carlo (QMC) calculations. Spin-orbit coupling (SOC) effects were also taken into account at the CASPT2 and DFT levels of theory. The inclusion of dynamic correlation energy was found to be essential to maintaining the stability of the complex. The dative type of bonding was identified to be responsible for the Pt-Bz binding in the ground state. Single-reference CCSD(T) and MP2 as well as multireference CASPT2 and MS-CASPT2 methods reveal that the interaction curve has a single energy minimum (corresponding to the (1)S0 state) and a shoulder (arising from the crossing between the (1)S0 and (3)D3 states) at longer distances. The inclusion of SOC at the CASPT2 level leads to the appearance of another well-separated minimum, which corresponds to the triplet state (3)F4. The PBE/EE+vdW functional, which includes a fraction of exact-exchange (EE) and nonlocal electron correlation, shows the best qualitative agreement with respect to the CCSD(T) data among all DFT functionals used. Large-scale QMC calculations, based on DFT wave functions constructed using TPSSh and M11 functionals, were confronted with the CCSD(T) results. The QMC-TPSSh protocol favorably agrees with the CCSD(T) data, suggesting its possible use in other Pt-containing organometallic systems. All of the methods used (except for HF) show that the Pt-benzene binding leads to the quenching of the Pt high-spin triplet ground state, and the low-spin closed-shell singlet state is found to be preferred in the ground-state of the complex.