Bimetallic Complexes of Metallacyclopentynes:  cis versus trans and Planarity versus Nonplanarity

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 π-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.