Bonding Interactions of a Molecular Pair of Tweezers with Transition Metals. Theoretical Study of Bis(η²-alkyne) Complexes of Copper (I), Silver (I), and Gold (I)

Quantum chemical calculations at the DFT level of theory using the three-parameter fit of the potential B3LYP in conjunction with relativistic effective core potentials and large valence basis sets are reported for the model compounds [Cl₂Ti(C≡CH)₂]MCH₃ (M = Cu, Ag, Au), [CuCH₃(HCCH)_n] (n = 1, 2), and [H₂X(C≡CH)₂]CuCH₃ (X = Si, C). Theoretically predicted geometries and MCH₃ binding energies are given, and the nature of the binding interactions has been analyzed with the help of the NBO partitioning scheme and the topological analysis of the electron density distribution. The copper complex [Cl₂Ti(C≡CH)₂]CuCH₃ has a much higher bond energy (D_e = 39.2 kcal/mol) than the analogous silver (D_e = 21.2 kcal/mol) and gold compound (D_e -17.3 kcal/mol). There is significant metal → acetylide charge donation in the complexes, which induces strong Coulomb attraction between MCH₃ and the tweezer ligand. The role of the bridging TiY₂ moiety in the bisalkyne complexes is a twofold one. It serves to position the alkyne groups in a proper position for tricoordination around the group 11 metals, and it enhances the Lewis acidity of the ligands.