THEORETICAL-STUDIES OF ORGANOMETALLIC COMPOUNDS .8. STRUCTURES AND BOND-ENERGIES OF THE METHYLCUPRATES CH3CU, (CH3CU-, (CH3)(2)CULI, (CH3)(2)CULI.H2O, [(CH3)(2)CULI](2), AND [(CH3)(2)CULI](2).2H2O

The geometries of the copper compounds CH3Cu (1), (CH3Cu- (2), (CH3)(2)CuLi (3), (CH3)2CuLi H2O (4), [(CH3)(2)CuLi](2) (5), and [(CH3)(2)CuLi](2).2H(2)O (6) are optimized at the HF and MP2 level of theory, using an. effective core potential for Cu and all-electron basis sets for the other atoms. The Cu-C bond strength is calculated,at correlated levels, and the electronic structure of; 1-6 is investigated using the topological analysis of the electron density distribution and the natural bond orbital partitioning scheme. The optimized geometries at the MP2 level appear to be reliable, while the HF optimized structures have Cu-C bond lengths which are too long. Methylcopper(1) is predicted with a Cu-C bond length of 1.923 Angstrom and a Cu-C bond strength D-0 = 45.0 kcal/mol. Dimethylcuprate anion (2) has a linear (D-3h) structure and slightly longer Cu-C bond distances (1.963 Angstrom) than 1. The global energy minimum structure of the monomeric form of Gilman's reagent (CH3)2CuLi (3d) is predicted with an open (noncyclic) geometry and a nearly linear CH3-Cu-CH3 moiety. The lithium atom in 3d is coordinated to only one methyl group. The cyclic forms with a bridging lithium atom are no energy minima on the potential energy hypersurface at the MP2 level of theory. Complexation of 3 by one water molecule at Li does not influence the geometries and relative energies of the open and-cyclic forms. The global energy minimum structure of (CH3)2CuLi.H2O (4) is 4d, which has an open (noncyclic) geometry with a nearly linear CH3-Cu-CH3 moiety. The dimeric form of Gilman's reagent [(CH3)2CuLi](2) (5) has a planar cyclic geometry with D-2h symmetry. Complexation of 5 at the lithium atoms by two water molecules gives [(CH3)2CuLi](2).2H(2)O (6). The geometry optimization of 6 yields two energetically nearly degenerate forms 6a,b with slightly twisted geometries which deviate only little from D-2h symmetry. The analysis of the electronic structure shows that the Cu-C bonds in 1-6 are strongly polarized toward the carbon atom. The polar Cu-C bonds have distinct covalent contributions, however. The copper bonding has mainly s-character (similar to 90%) as revealed by the NBO analysis.