Extracting absolute titanium-alkyl and -hydride bond enthalpies from relative D(TiR(H)) in (silox)2(tBu3SiNH)TiR:: electronegativity and ECT models

Relative D(Ti-R)(rel) have been measured previously for (silox)(2)((Bu3SiNH)-Bu-t)TiR (1-R), Pauling and Matcha equations of D(MR) that contain elements of electronegativity, D(MM), D(HH) and D(RH) were used to estimate absolute D(TiR) in 1 with minimal success. The electronegativity approaches need a variety of substituents with a wide spread in chi(R), hence examination of 1-H and I-R are heavily and incorrectly weighted by D(TiH)-D(TiR). Absolute D(TiR) generated in this manner approach those of related systems only when chi(Ti) = 2.9, a value shown to be irrational, Drago's ECT parameters were used in the hope that the three parameter fit would compensate for inadequacies in electronegativity-based methods. The limited set of alkyl parameters available, and a lack of distinguishing parameters that can significantly differentiate one alkyl from another, render this potentially useful method imprecise. (C) 1998 Elsevier Science S.A.