Investigation of the electronic structure of transition metal compounds by photoelectron spectroscopy with variable photon energy

Relative partial photoionisation cross section (RPPICS) data have been obtained for a variety of transition metal compounds using synchrotron radiation in the incident photon energy range 17-115 eV. Cross section features such as p→d giant resonances, delayed maxima, Cooper minima and molecular shape resonances have been identified and interpreted in terms of the localisation properties of the ionising electrons.
The RPPICS behaviour of the photoelectron (PE) band corresponding to ionisation of the 1a1g highest occupied molecular orbital (HOMO) of [Moη-C6H5Me)2] indicates that it is essentially metal-localised. The p→d giant resonant absorption enhancement of the cross section is found to be almost coincident in photon energy with a molecular shape resonance. Similar features in the 1e2g-1 and 1e1g-1 bands provide strong evidence for metal-ligand covalency in the associated MOs. In contrast, the monotonic fall off with increasing photon energy of the RPPICS of the 1e1u-1 band is typical of ligand-localised MOs. Studies of the closely related [Mη-C7H7)(η-C5H4R)] (M=Ti, Nb, Mo; R=H: M=Ta; R=Me) reveal a significant degree of both metal and ligand character to the 1e2 MOs, suggesting that neither the +1 nor the -3 formalism for the charge on the cycloheptatrienyl ring in its complexes is a good one, as they imply a metal- and ligand-localised 1e2 level respectively. Analysis of the occurrence and magnitude of shape resonances in all of these sandwich molecules suggests that they are governed chiefly by the amount of metal d orbital character in an MO.
The 5e MOs of PF3, traditionally regarded as fluorine 2p lone pair MOs, have been shown to possess significant phosphorus 3p atomic orbital (AO) character. Comparison of the data obtained on PF3 with those on [Ni(PF3)4] indicates that the π back donation from the nickel 3d orbitals to the 7e lowest unoccupied MO of PF3 also affects the composition of the 5e derived orbitals.
A combined experimental and theoretical study has resulted in an assignment of the PE spectrum of OsO4 in which an osmium np AO (n=5 and/or 6) derived spin-orbit splitting of the lower ionisation energy (IE) 2T2 primary ion state is found. RPPICS results obtained on TiCl4, TiBr4 and ZrBr4 indicate that their PE spectra may be interpreted within a framework in which the primary ion state ordering is preserved on moving from chloride to bromide and from first to second row transition metal. Bromine-centred spin-orbit coupling produces the more complicated spectra of TiBr4 and ZrBr4. In all cases, the highest IE band shows RPPICS behaviour which indicates that it arises from the 2a1 MO, resolving the debate as to the assignment of the PE spectrum of TiCl4. The He I and He II spectra of [(CH3)ReO3] have been measured and Extended Hückel MO calculations performed upon both this molecule and ReO3Cl. A consistent assignment of their spectra has been achieved, based upon that of OsO4, and it is found that the methyl group represents a much greater perturbation to tetrahedral symmetry than does a chlorine atom.
The bonding between the metal and dithiolene fragments in [Fe2(CO)6S2C2(CF3)2] has been shown to be primarily due to an interaction between the b1 symmetry Fe2(CO)
6 HOMO and the n. MO of the dithiolene, which is doubly reduced. In [Ni(S,sub>2C2(CF3)2)2], however, the closer energy match of metal and ligand valence orbitals results in monoanionic dithiolene groups and a more complex PE spectrum.