AB-INITIO MOLECULAR-ORBITAL STUDY OF ELECTRONIC AND GEOMETRICAL STRUCTURES OF MCH(2)+ AND MSIH(2)+ COMPLEXES (M = CO, RH, AND IR)

Complete active space self-consistent field (CASSCF) and internally co ntracted single and double configuration interaction from the CASSCF r eference functions (MR-SD-CI-CASSCF) methods have been used to calcula te electronic and geometrical structures of low-lying 3A1, 3A2, B-3(1) , B-3(2), and 1A1 states of methylene, MCH2+, and silylene, MSiH2+, co mplexes for M = Co+, Rh+, and Ir+. The ground state of CoCH2+, IrCH2and all considered MSiH2+ complexes is the nearly degenerate 3A1 and 3 A2 states. For RhCH2+ the ground state is 1A1, with 3A1 and 3A2 states lying only 4-5 kcal/mol higher. The M+=CH2 bond is approximately 10-2 0 kcal/mol stronger than the M+=SiH2 bond. Our best calculated binding energies are 80.3 for CoCH2+, 78.3 for RhCH2+, 113.1 for IRCH2+, 61.5 for CoSiH2+, 69.3 for RhSiH2+, and 98.7 kcal/mol for IrSiH2+, which a re in general in good agreement with experimental values. By using the MC/LMO/CI technique has been shown that all present silylene complexe s have mostly Fischer-type character with donor-acceptor sigma and pi bond. In contrary, the analogous methylene complexes are mostly Schroc k-type complexes with covalent sigma and pi bond, except 3A1, 3B2, and 3B1 states of RhCH2+, which have mostly Fischer-type character.