Iron-57 NMR and Structural Study of [Fe(η5‑Cp)(SnPh3)(CO)(PR3)] (PR3 = Phosphine, Phosphite). Separation of Steric and Electronic σ and π Effects

The complexes [Fe­(Cp)­(SnPh3)­(CO)­(PR3)] (PR3 = PMe3 (1), PnBu3 (2), PCy3 (3), PMe2Ph (4), PMePh2 (5), P­(CH2Ph)3 (6), PPh3 (7), P­(4-MeC6H4)3 (8), P­(4-MeOC6H4)3 (9), P­(4-FC6H4)3 (10), P­(4-CF3C6H4)3 (11), P­(NMe2)3 (12), P­(OMe)3 (13), P­(OPh)3 (14)), which have been characterized by X-ray crystallography (except for 1 and 4), infrared spectroscopy (carbonyl stretching frequency, νCO), and NMR spectroscopy (13C, 31P, 57Fe, 119Sn) offer some insight into the response of the iron nucleus to changes in the electronic and steric properties of the PR3 ligand. A fairly good correlation is found between the 57Fe chemical shift and the Tolman cone angle θ for PR3 and a rather poorer correlation between δ­(57Fe) and νCO. However, for the subseries of complexes 7–11 having PR3 = P­(4-XC6H4)3 (X = H, Me, MeO, F, CF3), the correlation between δ­(57Fe) and νCO is very good. Since the steric properties of these ligands, from the point of view of the metal, are identical (θ = 145°), this provides a means of separating the steric and electronic contributions of PR3 to δ­(57Fe). The electronic contribution of PR3 to δ­(57Fe) can be further separated into σ and π components by making use of the finding that the π component of the Fe–P bond has a negligible influence on δ­(57Fe), unlike its influence on νCO. The ligands PMe3, PnBu3, PCy3, PMe2Ph, PMePh2, and P­(NMe2)3 are found to be “pure” σ donors, P­(OMe)3 and P­(OPh)3 are found to be π acceptors of differing strength, and P­(4-XC6H4)3 is found to show weak but clearly distinguishable π acceptor properties.