Electron spin resonance studies of some organometallic complexes.

An e.s.r. study has been made of the formation of some intermediate dicyclopentadienyltitanium (III) complexes. For comparison, some V(IV) and Nb(IV) cyclopentadienyl complexes have been examined. The use of a paramagnetic probe, Nb(allyl)4, has been investigated, for studying the surface hydroxyl groups of metal oxides. The e.s.r. spectra of a series of (C5H5)2 Ti dihydride derivatives have been examined, with particular attention to the line-width trends. An approximately tetrahedral configuration is suggested for the titanium atom, with the unpaired electron in the 3dz2 orbital which lies along the symmetry axis between the cyclopentadienyl rings and bisects the H-Ti-H angle. The results also suggest that on going across the series from the ionic [(C5H5)2TiH2] -Na+ to the covalent (C5H5)2TiH2A1H2, the H-Ti-H angle steadily decreases. Group IV and Group V derivatives of titanocene have been prepared and their e.s.r. spectra recorded. All the complexes have an intermediate g-component of 1.988 0.006, and it is postulated that this value is associated with the cyclopentadienyl groups, whilst the low g-value is connected with the remaining ligands. The trends in Aiso (Ti) and g (average) are explained in terms of the electron-donor character of the ligands and the resulting s-orbital expansion. Spin-polarisation is believed to be the dominant mechanism for the acquisition of spin by the ligands, although for the Group V derivatives this negative spin density is reduced by direct overlap between the lone-pair orbitals and the titanium 3dz2 orbital cortaining the unpaired electron. The e.s.r. spectra of alkyl, aryl, and allyl complexes of titanocene, and the identity of the species responsible for them are considered. It has been shown that by using Nb(allyl)4 as a probe, it is possible to distinguish between the different types of hydroxyl sites on the surface of silica.