A unified view of ethylene polymerization by d0 and d0fn transition metals.: 1.: Precursor compounds and olefin uptake energetics

A systematic study has been carried out on the complexation of ethylene to a number of d(0) [L]MC2H50,+,2+ fragments [M = Sc(III), Y(III), La(III), Lu(III), Ti(IV), Zr(IV), Hf(IV), Ce-(IV), Th(IV), and V(V); L = NH(CH)(2)NH2- (1), N(BH2)(CH)(2)(BH2)N2- (2), O(CH)(3)O- (3), Cp-2(2-) (4), NHSi(H-2)C5H42- (5), [(OXO)(O(CH)(3)O)](3-) (6), (NH2)(2)(2-) (7), (OH)(2)(2-) (8), (CH3)(2)(2-) (9), and NH(CH2)(3)NH2-(10)1, where a hydrogen on the beta-carbon of the ethyl unit is bound to the metal in an agostic interaction (beta-agostic bond). It is shown that the complexation energy of an ethylene molecule to a [L]MC2H5n+ precursor can be predicted to within +/-20 kJ/mol by simple empirical rules, based on the accessible metal surface of the [L]MC2H5n+ fragment and its gross charge. Discussions are also given of the relative preference for front side (ethylene syn to beta-agostic bond) versus backside (ethylene anti to beta-agostic bond) coordination by the olefin as a function of the central atom, the auxiliary ligand set L and the strength of the beta-agostic bond. It is finally shown that the beta-agostic bond strength in the [L]MC2H5nf precursor follows the order Ti approximate to Zr > Th > Hf for [L]MC2H5+ and Sc approximate to Y L La > Lu for [L]MC2H5 for L = 7-9, with agostic interactions for uncharged precursor complexes [L]MC2H5 generally being weaker than for charged precursor complexes.