This chapter gives a summary of four different projects in which the reaction mechanisms of transition metal homogeneously catalyzed reactions have been studied with quantum chemical methods. The geometries of the reacting agents, intermediates and transition states are optimized at the DFT level of theory (B3LYP) using relativistic effective core potentials for the metals and valence basis set of DZ+P quality.

The equilibrium geometries and transition states for interconversion of the CSiH2 isomers in the singlet electronic ground state are optimized at the MP2 and CCSD(T) levels of theory using a TZ2P basis set. The heats of formation, vibrational frequencies, infrared intensities, and rotational constants are also predicted. There are three energy minima on the CSiH2 potential energy surface. Energy calculations at CCSD(T)/TZ2P(fd) + ZPE predict that the global energy minimum is silavinylidene (1), which is 34.1 kcal mol−1 lower in energy than trans‐bent silaacetylene (2) and 84.1 kcal mol−1 more stable than the vinylidene isomer (3). The barrier for rearrangement 2→1 is calculated at the same level of theory to be 5.1 kcal mol−1, while for the rearrangement 3→2 a barrier of 2.7 kcal mol−1 is predicted. The natural bond orbital (NBO) population scheme indicates a clear polarization of the C(SINGLE BOND)Si bonds toward the carbon end. A significant ionic contribution to the C(SINGLE BOND)Si bonds of 1 and 2 is suggested by the NBO analysis. The C(SINGLE BOND)Si bond length of trans‐bent silaacetylene (2) is longer than previously calculated [1.665 Å at CCSD(T)/TZ2P)]. The calculated carbon‐silicon bond length of 2 is in the middle between the C(SINGLE BOND)Si double bond length of 1 (1.721 Å) and the C(SINGLE BOND)Si triple bond of the linear form HCSiH (4), which is 1.604 Å. Structure 4 is a higher‐order saddle point on the potential energy surface. © 1996 by John Wiley & Sons, Inc.

The title compound has been prepared by the reduction of N,N'-dimethyl-4,4'-bipyridinium dichloride with dilithium tritelluride in dimethylformamide solution. It forms deep blue crystal needles, which were characterized by a crystal structure determination. Space group P na 2(1), Z = 8, 3450 observed unique reflections, R = 0.045. Lattice dimensions at - 70-degrees-C: a = 1707.6(8), b = 2323.9(11), c = 664.6(3) pm. The N,N'-dimethyl-4,4'-bipyridinium radical cations are planar and form ion pairs in the lattice. The chloride ions together with the water molecules form an infinite network of hydrogen bridges along the crystallographic a-axis. The structural parameters of the radical cation are in excellent agreement with ab initio calculations.

The geometries of the acetylene complexes MX4C2H2 and the vinylidene isomers MX4CCH2 (M = W, MO; X = F, Cl) are theoretically predicted using quantum mechanical ab initio methods at the Hartree-Fock level of theory and relativistic effective core potentials for the transition metals. The optimized geometries and energies of the anionic complexes MX5C2H2- are also reported. The optimization of WX5C2H2- gives geometries for the chloro and fluoro complexes, which are in good agreement with experiment. The corresponding MoX5C2H2- structures are not minima on the potential energy surface. The geometries and energies of the alkyne complexes are compared with the optimized structures of the vinylidene complexes. The vinylidene complexes are calculated to be slightly higher in energy than the alkyne complexes, but they become clearly more stable than the alkyne complexes when hydrogen is substituted by fluorine. The electronic structure of the complexes is investigated using the natural bond orbital population analysis and the topological analysis of the wave function. Detailed information is given about the nature of the metal-carbon bonds and the hybridization and atomic population of the transition metals in the alkene and vinylidene complexes.

The title compound has been prepared by the reduction of N,N'-dimethyl-4,4'-bipyridinium dichloride with dilithium tritelluride in dimethylformamide solution. It forms deep blue crystal needles, which were characterized by a crystal structure determination. Space group Pna2 1 , Z=8,3450 observed unique reflections, R = 0.045. Lattice dimensions at -70 o C:a=1707.6(8), b=2323.9(11), c=664.6(3) pm. The N,N'-dimethyl-4,4'-bipyridinium radical cations are Planar and form ion pairs in the lattice. The chloride ions together with the water molecules form an infinite network of hydrogen bridges along the crystallographic a-axis

The title compound has been prepared by the reduction of N,N′-dimethyl-4,4′-bipyridinium dichloride with dilithium tritelluride in dimethylformamide solution. It forms deep blue crystal needles, which were characterized by a crystal structure determination. Space group Pna 21,Z = 8,3450 observed unique reflections, R = 0.045. Lattice dimensions at -70°C: a =1707.6(8), b = 2323.9(11), c = 664.6(3) pm. The N,N′-dimethyl-4,4′-bipyridinium radical cations are planar and form ion pairs in the lattice. The chloride ions together with the water molecules form an infinite network of hydrogen bridges along the crystallographic a-axis. The structural parameters of the radical cation are in excellent agreement with ab initio calculations.