Structure and bonding energy analysis of M-Ga bonds in dihalogallyl complexes trans-[X(PMe3)2M(GaX2)] (M = Ni, Pd, Pt; X = Cl, Br, I).

Geometry, electronic structure, and bonding analysis of the terminal neutral dihalogallyl complexes of nickel, palladium, and platinum trans-[X(PMe(3))(2)M(GaX(2))] (M = Ni, Pd, Pt; X = Cl, Br, I) were investigated at the BP86 level of theory. The calculated geometries of platinum gallyl complexes trans-[X(PMe(3))(2)Pt(GaX(2))] (X = Br, I) are in excellent agreement with structurally characterized platinum complexes trans-[X(PCy(3))(2)M(GaX(2))]. In the gallyl complexes of nickel and palladium, the M-Ga sigma bonding orbital is slightly polarized toward the gallium atom, while in the platinum gallyl complexes, the M-Ga sigma bonding orbital is slightly polarized toward the platinum atom. It is significant to note that gallium atoms along the M-Ga sigma bonds have large p character, which is always >51% of the total AO contributions, while along the Ga-X sigma bonds, the p character varies from 72% to 73%. The short M-Ga bond distances, in spite of the significantly small M-Ga pi bonding, are due to the large s character of gallium (approximately 45-48%) along the M-Ga bonds. The calculated NPA charge distributions indicate that the metal atom carries negative charge and the Ga atom carries significantly large positive charge. The contributions of the electrostatic interaction terms, DeltaE(elstat), are significantly larger in all gallyl complexes than the covalent bonding DeltaE(orb) term. Thus, the [M]-GaX(2) bond in the studied gallyl complexes of Ni, Pd, and Pt has a greater degree of ionic character (65.7-72.5%). The pi-bonding contribution is, in all complexes, significantly smaller than the sigma bonding contribution. In the GaX(2) ligands, gallium dominantly behaves as a sigma donor. The interaction energy increases in all three sets of complexes via order of Ni < Pd < Pt, and the absolute value of DeltaE(Pauli), DeltaE(int), and DeltaE(elstat) contributions to the M-Ga bonds decreases via X = Cl < Br < I in all three sets of complexes.