Experimental and density functional theory studies of ethyltrichlorotitanium, EtTiCl3 and of the ethyl group in EtTiCl3 complexes: Vibrational properties of a beta-agostic ethyl ligand

Infrared spectra are reported for the range 4000-400 cm(-1) for CH3CH2TiCl3 ,CH2DCH2TiCl3, CH3CD2TiCl3, CHd(2)CD(2),TiCl3 and CD3CD2TiCl3, and the dmpe adducts of some of these species (dmpe = Me2PCH2CH2PMe2). Density function al theory is used to calculate force fields for EtTiCl3, EtCl, EtTiCl3(dmpe ) and the model complex EtTiCl3(dhpe)(dhpe = H2PCH2CH2PH2). Scaling of the free EtTiCl3 force field allows many features of the spectra to be assigned . Comparison of the scaled force constants with similar ones for EtCl enabl es changes in the ethyl group force constants to be identified. The develop ment of beta -agostic behaviour on passing from EtTiCl3 to the complex is c haracterised in the geometry by a switch from a staggered to an eclipsed st ructure for the ethyl group, a small TiCC angle and a short Ti . . .H dista nce. In the vibrational spectra it is characterised by the appearance of th e lowest isolated stretching frequency, v(is)CH, observed to date, 2585 cm( -1), an unusually high methyl bending force constant, and marked changes in the magnitude and direction of the dipole derivative partial derivative mu /partial derivativer for the agostic C-H bond. A small Fermi resonance affl ecting the above v(is)CH value is diagnosed and an r(0) value of 1.131 Angs trom is predicted for the agostic C-H bond. Force constants and electrical properties associated with the other ethyl C-H bonds and with the Ti-Cl bon ds are compared between the base-free and complexed species.