A new approach to the energetics of clusters and related systems

Abstract

The work described in this thesis is concerned with cluster-species and related systems, many of which are electron deficient. The term 'electron deficient' is used to describe a polynuclear species in which there are too few valence electrons to allocate a localised 2-centre 2-eleotron bond to every pair of atoms which are within normal covalent bonding distance. The bonding in these systems may be rationalised instead in terms of the relationship between the total number of skeletal electrons provided by the skeletal cluster units, and the total number of skeletal atoms. The aim of this work is to suggest new ways in which bond enthalpy contributions can be allocated to individual 2-centre links in cluster systems. In order to obtain energy terms (E) which reflect changes in bond length, (d), relationships of the form E α d(^-k)(where k=constant; 2<k≤5) are proposed. Such empirical correlations are shown to be appropriate for simple main group systems and are applied in turn to boron hydrides, borane anions, transition metal carbonyls and to complexes containing multiple metal-metal bonds. Similar relationships are used to suggest possible bond orders in some systems. Finally, the extent to which skeletal electron counting methods may be used to rationalise the bonding in boranes, carboranes, transition metal complexes and small clusters, metal π-hydrocarbon complexes and small cyclic hydrocarbons is discussed.