Infrared spectra are reported for the range 4000–400 cm−1 for CH3CH2TiCl3, CH2DCH2TiCl3 , CH3CD2TiCl3, CHD2CD2TiCl3 and CD3CD2TiCl3, and the dmpe adducts of some of these species (dmpe = Me2PCH2CH2PMe2). Density functional 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 enables changes in the ethyl group force constants to be identified. The development of β-agostic behaviour on passing from EtTiCl3 to the complex is characterised in the geometry by a switch from a staggered to an eclipsed structure for the ethyl group, a small TiCC angle and a short Ti···H distance. In the vibrational spectra it is characterised by the appearance of the lowest isolated stretching frequency, νisCH, 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 ∂μ/∂r for the agostic C–H bond. A small Fermi resonance affecting the above νisCH value is diagnosed and an r0 value of 1.131 A 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 bonds are compared between the base-free and complexed species.