SI2H2 AND CSIH2 ISOMERS AS LIGANDS IN HIGH-VALENT TRANSITION-METAL COMPLEXES

Quantum mechanical ab initio calculations using relativistic effective core potentials for tungsten are reported for isomers of WF4(Si2H2) a nd WF4(CSiH2). The geometries are optimized at the Hartree-Fock level, and the metal-ligand bond energies are calculated at MP2 and CCSD(T) using valence basis sets of DZP quality. Four energy minimum structure s are predicted for WF4(Si2H2). The energetically lowest lying isomer is the disilaacetylene complex C4. Structure C4 is 10.1 kcal/mol lower in energy than Cia, which has the most stable doubly bridged Si2H2 fo rm as a side-on bonded ligand. Two other isomeric forms C1b and C2 are clearly higher in energy. Three energy minimum structures are predict ed for the WF4(CSiH2) complex. The global energy minimum is the silavi nylidene complex C7, which has a very short and strong W-C double bond . The silaacetylene complex C9 is 9.9 kcal/mol higher in energy. The t hird isomer is the silylidene complex C8, which is 33.2 kcal/mol less stable than C7. The electronic structure of the complexes is analyzed using the NBO partitioning scheme and the topological analysis of the electron density distribution. The results show that the polarization of the W-Si bonds can be quite different among different molecules and that it is very different from related complexes with W-C bonds. The W-Si bonds are more polarized toward the tungsten atom.