An assessment of theoretical methods for the study of transition metal carbonyl complexes: [Cl2Rh(CO)(2)](-) and [Cl2Rh(CO)](-) as case studies

A comparative theoretical study of the equilibrium geometries, vibrational frequencies, and reaction energetics of Rh-CO bond activation in the [Cl2Rh (CO)(2)](-) and [Cl2Rh(CO)](-) complexes has been performed using the B3LYP density functional method and the traditional correlated second-order Moll er-Plesset (MP2), quadratic configuration interaction with single;and doubl e substitutions (QCISD) and coupled-cluster single double (triple) [CCSD(T) ] methods. CCSD(T) is employed herein as a benchmark method to examine the validity of the B3LYP and MP2 methods in studies of transition metal comple xes. The results show that the geometries and energies obtained with the B3 LYP method agree quite well with the QCISD and CCSD(T) results. The convent ional MP2 method yields poor results for all geometries, vibrational freque ncies, and reaction energies. It is noteworthy that the relative reaction e nergies calculated at the highest correlated CCSD(T) level using the B3LYP and MP2 geometries are comparable even though the B3LYP and MP2 geometries differ significantly from each other. The absolute energies calculated with the QCISD and CCSD(T) methods at the B3LYP geometries are all lower than t hose obtained with the MP2 geometries, indicating that the B3LYP method cou ld be more reliable for the study of the geometries and energetics of the c atalytic reactions. Basis set effects have been examined by B3LYP calculati ons. The present results provide a comprehensive assessment of the widely u sed ab initio theoretical methods for the study of transition metal carbony l complexes. Physics. [S0021-9606(00)31345-9].