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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry