UNDERSTANDING REACTIVITY TRENDS BY STRUCTURAL AND THEORETICAL-STUDIES OF DISTORTIONS IN GROUND-STATE REAGENTS

Distortions of the carbonyl ligands in RM(CO)n (M = Mn, n = 5; M = Co, n = 4) are examined by studying published structural data. The more electropositive the R group, the more the equatorial CO groups are found to bend toward the R group. The results can be understood in terms of a simple model: for example, in RMn(CO)5 the more electropositive the R group, the more the Mn(CO)5 group departs from the octahedral structure expected for a d6 system and approaches the distorted trigonal bipyramidal (TBP) or square pyramidal (SP) geometry expected for a d8 configuration. In a similar way the TBP RCo(CO)4 undergoes distortions leading toward d10 tetrahedral geometry when R is electropositive. The effect is found to be more pronounced for late transition metals and for TBP over O(h) coordination modes. EHT calculations validate and amplify this simple model and are used to identify the main orbital interactions. On Burgi-Dunitz ideas, the above distortions are correlated with the carbonyl insertion into a M-R bond for which larger reaction rates are found for more electropositive R groups. These results suggest that the electronic factors that favor distortion also favor the insertion reaction. Distortion analysis allows us to understand why nucleophiles sometimes substitute and sometimes abstract R in RCo(CO)3.