An overview of the electronic structure in trigonal bipyramidal clusters of main elements or mixed with transition metals

Equatorial/apical bond repartitioning in TBP clusters, formed by main group elements only (e.g., B5H5 (2-), B3H3N2, etc.) or mixed with metal atoms (e.g., [(L2M)(3)S-2] (n) , [(L3M)(3)S-2] (n) , [(CpM)(3)S-2] (n) ), is critically analyzed from the horizontal comparison of the experimental structures and in terms of basic MO concepts. The ideas are double-checked through specific DFT calculations or existing ab initio results. Based on the Wade's rules for closo-clusters, the five vertices systems are normally characterized by six skeletal electron pairs. In the main group clusters, the electron delocalization at the equatorial edges depends on the electronegativity of the apical groups and for the B-eq-B-eq bonds an inverse relationship between bond strength and bond length is remarked. In [(L2M)(3)S-2] (n) compounds with a total electron count (TEC) of 48, the six bonding electron pairs localize at the apical M-S bonds. In [(L3M)(3)S-2] (n) or [(CpM)(3)S-2] (n) , also with TEC = 48, the effective atomic number rule predicts three single M-M single bonds besides the six M-S apical ones. In actuality, only partial M-M bonding can be considered due to the intermediation of the capping sulphur atoms, that help shifting the antibonding character of populated radial levels with that of the vacant tangential ones (bonding). The qualitative MO arguments are supported by the topological nature of the calculated DFT wave functions. Moreover, the MO nature of three lowest LUMOs for 48e(-) species help to rationalize experimental structural trends observed for the addition of up to five electrons.