An Investigation of the Electronic Structure of Bis(η5-cyclopentadienyl) Dicarbonyl Complexes of Titanium(II) and Zirconium(II). Discrete Variational Xa Calculation and Gas-Phase Photoelectron Spectroscopy

The electronic structure of the unique group IVB metal carbonyls M(Cp)2(CO)2 has been studied by using the SCF Hartree-Fock-Slater first-principle discrete variational Xα method. Theoretical results nicely fit both experimental ionization energies and relative intensity data taken from He I versus He II photoelectron spectra. Furthermore, they provide an accurate description of the M-CO bonding that appears strongly dominated by π-back-bonding interactions. No evidence has been found either of σ or π donation in accordance with other physicochemical data. The back-donation to π* carbonyl orbitals mainly involves the metal-based HOMO, and the values of the clearly resolved vibrational interval associated with ionization from this orbital provide experimental support for these theoretical findings. The intriguing HOMO-LUMO relationships might be of relevance to explain reactivities and substitution kinetic data of the complexes.