The electronic and geometric structures of the transition-metal carbyne cations ScCH+, TiCH+, VCH+, and CrCH+

Abstract

The electronic and geometric structures of several transition-metal carbyne cations have been studied by ab initio methods (MCSCF and MCSCF+1+2). CrCH/sup +/(/sup 3/..sigma../sup -/), VCH/sup +/(/sup 2/..delta..), and TiCH/sup +/(/sup 1/..sigma../sup +/) are all characterized by a metal-carbon triple bond of remarkably similar length (1.772, 1.745, 1.758 A, respectively). Relative to the spin allowed dissociation products (the metal ion in its ground state and CH in the excited /sup 4/..sigma../sup -/ state) the metal-carbon bond energies are calculated (in kcal/mol) to be CrCH/sup +/ (70.8), VCH/sup +/ (108.0), and TiCH/sup +/ (113.0). Relative to the /sup 2/II ground state of CH these bond energies become 53.7, 90.8, and 95.9 kcal/mol. The experimental VC bond energy in VCH/sup +/ is 115 kcal/mol, approximately 24 kcal/mol higher than our calculated value of 90.8. Analysis of the exchange energy loss of the metal ion upon bond formation suggests that there is an intrinsic metal-carbon bond energy for these species given by the sum of the exchange energy loss and the calculated bond energy. The /sup 2/II state of ScCH/sup +/ is characterized by a double bond between Sc and C, with a bond length of 1.940 A and D/sub e/ of 96.3/79.2 kcal/molmore » relative to the /sup 4/..sigma../sup -///sup 2/II state of CH. The electron distribution in the metal-carbon bond is analyzed for each of the titled molecules.« less