Bimetallic complexes of metallacyclopentynes: cis versus trans and planarity versus nonplanarity

Abstract

Density functional theory calculations show that the Cp₂M in cis-dimetallabicycles of metallacyclopentynes, Cp₂M[μ-(η⁴:η²-H₂C₄H₂)]M'L₂ (M = Ti, Zr and M' = Ti, Zr), deviates from the C4 plane. Both the metal fragments deviate from the C4 plane in the nickel complexes of metallacyclopentynes (3Ti-Ni and 3Zr-Ni). The nonplanarity of Ni(PH₃)₂ from the C4 plane reduces the antibonding interaction between nickel orbitals and the p-MO at the C2-C3 bond, whereas that of the Cp₂M acts mainly to reduce the antibonding interaction between C1 and C2. The energetics of the isodesmic equations show that the nickel complexes 3Ti-Ni and 3Zr-Ni are more stable than the homodimetallabicycles, 3Zr-Zr and 3Ti-Ti. The electron deficiency on the cis-homodimetallabicycles due to the vacant d-orbital on η²-M' can be decreased by accepting electrons from a Lewis base or by flipping into trans geometry. This is reflected in the experimental realization of cis-Cp₂Zr[μ-(η⁴:η²-H₂C₄H₂)]ZrCp₂(PMe₃) and the trans geometry for Cp₂Ti[μ-(η³:η³-H₂C₄H₂)]TiCp₂ and Cp₂Zr[μ-(η³:η³-H₂C₄H₂)]ZrCp₂.