Cluster compounds and remarks on the 2e, 2c bonding scheme

2 Cluster compounds and remarks on the 2e, 2c bonding scheme . A formal metal-metal bond designation may be introduced having as reference the simple bonding schemes based on the inert gas rules ( octet rule). 

A reference picture for the bonding between atoms is indeed related to the electrons which are needed for an atom to obtain the electron configuration of the next noble gas in the Periodic Table. In the specific case of a pair of electrons shared by two atoms, each atom gains one electron in its valence shell. The energy decrease of the shared electron pair links the two atoms (the two centres) and we speak of a two electron-two centre bond (2e, 2c bond). 

In this case the total number of valence electrons (Ee) in the cluster will be given by Ee = 8n — 2B (main group elements) 

In a ring cluster (a ring molecule) the number of edges is equal to the number of vertices (atoms) B = n, so we have  

For triangular clusters, for instance, this means 18 or 48 electrons. A classic example may be the Os3 triangular group in the carbonyl compound Os3(CO)12 in which 4 CO are connected to each Os (to each triangle vertex). Each Os atom contributes 8 valence electrons whereas the CO group acts as a 2-electron donor so  

---