Relativistic effects in the metal-carbon bond. A comparative ab initio and DFT study of M=CH2+ and M-CH3+ (M=Cu, Ag and Au)

Standard quasi-relativistic (QR) and non-relativistic (NR) effective core potential (ECP) ab initio molecular orbital MO (MP2 and CCSD(T)) and hybrid Hartree-Fock density functional (DFT, B3LYP) methods have been applied to cationic complexes M-X+ (X=CH2 and CH3) of the group 11 transition metals (M=Cu, Ag, and Au), Calculated bond dissociation energies, optimized geometries and vibrational frequencies are reported. A comparison of these results with earlier high-level ab initio calculations and experimental data is carried out in order to assess the reliability of the hybrid DFT method as a practical tool in organometallic chemistry. The relativistic effect in the bonding was evaluated by comparing the QR and NR results. Large relativistic stabilization was found for both the gold-containing systems. The CCSD(T)-calculated bond dissociation energies (BDE) amount to 242 kJ mol(-1) (Cu=CH2+), 168 kJ mol(-1) (Ag=CH2+), 396 kJ mol(-1) (Au=CH2+), 142 kJ mol(-1) (Cu-CH3+), 99 kJ mol(-1) (Ag-CH3+), and 198 kJ mol(-1) (Au-CH3+). The B3LYP calculated BDE values are higher by ca. 12%. The unscaled B3LYP frequencies are smaller by ca. 5% as compared with those calculated by the MP2 method.