A π-Donor Spectrochemical Series for X in (Me5C5)2TiX, and β-Agostic Interactions in X = Et and N(Me)Ph

The EPR and electronic spectra of d 1 -bent metallocene compounds of the type Cp* 2 TiX, where X is halide, alkoxide, amide, alkyl, or hydride and Cp* = Me 5 C 5, have been studied. Several of these compounds are new, and those with X = N(Me)H and F were characterized by X-ray crystallography. The crystal structure of Cp* 2 TiN(Me)H showed that the N(Me)H ligand lies on the plane defined by Cp*(centroid)−Ti−Cp*(centroid). This is the sterically most unfavorable conformation but allows maximum Ti−N π-bonding. The anisotropic frozen solution EPR spectra were analyzed by the method used by Petersen and Dahl for the d 1 -metallocenes, Cp 2 VX 2, which gives g x, g y, and g z . Although the values of g x and g z are relatively constant throughout the series, the value of g y varies with the π-donor ability of X. The π-donor series is N(Me)H ≈ NH 2 ≈ OMe > OPh ≈ F > N(Me)Ph > Cl > Br > I > H. Among the known alkyls, the π-donor ability was Et > Me > n -Pr ≈ CH 2 CMe 3 > CH 2 C 6 H 5, which is rationalized, in part, by a β-agostic interaction in the case of Et. The β-agostic interaction in Cp* 2 TiEt and in Cp* 2 TiN(Me)Ph was investigated by variable-temperature EPR spectroscopy giving an enthalpy and entropy for the agostic interaction. For Cp* 2 TiEt the parameters for the agostic interaction are Δ H ° = −1.93(3) kcal/mol and Δ S ° = −6.3(2) eu, and for Cp* 2 TiN(Me)Ph, Δ H ° = −1.5(1) kcal/mol and Δ S ° = −7.9(5) eu.