SITE PREFERENCE OF LIGAND AND METAL SUBSTITUTION IN TRIGONAL-BIPYRAMIDAL METAL-CARBONYL CLUSTERS

Gimarc's topological charge stabilization rule (TCS), for addressing s ite preference, has been extended to the realm of metal carbonyl clust ers using Alien's energy indexes (EIs) instead of charges. EIs have be en computed within the extended Huckel (EH) approximation and, in orde r to assess an internal electronegativity scale for transition metals and to allow comparisons across the periodic table, a homogeneous set of EH parameters has been determined. EIs have been shown to behave si milarly to charges on ''clusters'' with a rigid ligand stereochemistry like carboranes but, when one deals with metal carbonyl clusters and their intriguing ligand mobility, EIs are definitely superior to charg es. EIs do address both ''skeletal'' and ''ligand'' site preferences a ccording to the following rules of thumb: (i) more electronegative met al atoms occupy higher EI sites of the uniform reference frame (URF) ( i.e. the one with the lower local electronegativity) and (ii) (nucleop hilic) ligand substitution preferentially occurs on the lowest energy index site of the URF (i.e. the one with the largest electrophilic cha racter). Previous attempts to rationalize site preference in metal clu sters were mainly concerned with the relative strengths of metal-metal and metal-ligand bonds and substantially disregarded electronegativit y differences between different metal atoms. In contrast, we have show n that the latter are important whenever the actual URF allows for dif ferent EIs on different metal sites. In particular, differences in ele ctronegativity have been shown to be important for M5L15 and M5L14 but not for M5L12 bipyramidal metal carbonyl clusters.