Electronic and steric effects in fragmentation reactions of Os3(CO)9(μ-C4Ph4)

Abstract

P-Donor nucleophiles of cone angle ≥145° are known to react with Os₃(CO)₉(μ-C₄Ph₄) quite differently from those with cone angles ≤143°. A detailed and systematic kinetic study of the rather slow kinetics of reactions with 11 of the larger nucleophiles is described, and the results are analysed according to standard QALE protocols. (QALE = quantitative analysis of ligand effects). The results are compared with those previously reported for 17 of the smaller phosphines, which react much more rapidly. The pronounced difference in behaviour between these two groups of nucleophiles is ascribable to major differences between the ease of formation of different intermediate Os₃(CO)₉L(μ-C₄Ph₄) adducts. The cluster is proposed to be capable of opening up to form a nido structure that has a small, well defined opening capable of accommodating any of the smaller nucleophiles quite easily. However, this opened nido structure is concluded to be extremely rigid and incapable of further opening to accommodate any of the larger nucleophiles. These have to take advantage of an alternative nido structure that contains a much larger opening but that requires considerably more energy for it to be formed. These results are compared with those for the clusters M₅C(CO)₁₅ (M = Ru or Fe), which show somewhat similar behaviour. This is ascribed to the complexity of the structures of the clusters that allows them to form alternative nido structures of the sort described, something that more symmetrical clusters are apparently incapable of doing.