Combined DV-Xα and Gas-Phase UV Photoelectron Spectroscopic Investigation of the Electronic Structures of Tetravalent Titanium, Zirconium, Molybdenum, and Thorium 1-Sila-3-metallacyclobutane Metallocene Complexes

The electronic structures of the 1-sila-3-metallacyclobutane complexes, Cp₂M(CH₂SiMe₂CH₂), where Cp = η⁵-C₅H₅ and M = Ti, Zr, Mo, Th, have been investigated by a combination of SCF Hartree-Fock-Slater discrete variational Xα calculations and He I, He II UV photoelectron spectroscopy. Photoelectron data are completely consistent with the energy sequences and valence orbital atomic compositions determined by the theoretical calculations. It is found that these “stabilized” metallacyclobutane complexes are best described electronically as heterodinuclear molecules containing bridging μ-CH₂ groups rather than simple strained hydrocarbyl derivatives. The formation of the four-membered ring involves bonding interactions analogous to those found in cyclobutane. The metal-ligand bonding involves stabilizing interactions between higher-lying empty orbitals of the bridging μ-CH₂ groups and appropriate metal orbitals of the metallocene fragment The resulting higher-lying molecular orbitals representing the M-C bonds have energies modulated by the relative amount of metal participation. More internal metallacycle molecular orbitals also provide a bonding contribution, although to a smaller extent. The population of these orbitals causes a redistribution of electron densities and, in spite of the high formal metal oxidation state (+4), partially restores the d² metal configuration. The 2 + 2 reactivity modes in the related metallacyclobutenes can be accounted for by analyzing the evolution of correlated molecular orbitals of Cp₂Ti(CH₂SiMe₂CH₂), Cp₂TiCH₂CH₂CH₂, Cp₂Ti==CH₂(C₂H₄), and of noninteracting Cp₂Ti═CH₂ + C₂H₄ molecules. The titanacyclobutane complex, Cp₂TiCH₂CH₂CH₂, can be considered as a latent olefin complex even though the metallacyclic structure is thermodynamically favored. Variation in orbital character and decreasing metal-d or -f covalency (with Ti ≥ Zr ≫ Th) of the LUMO in the corresponding 1-sila-3-metallacyclobutanes is closely connected with differences in reactivity.