Introduction

Cluster and ring compounds arise in many areas of chemistry. Rings are most often built by nonmetallic elements with directional covalent bonding. They add homoelement rings such as benzene C6H6 and S8 and ones with heteroelement bonding such as S2N2, the silicate ion [Si3O9] and borazine B3N3H6.

Clusters are polyhedral arrangements of atoms found vary openly in the periodic table: nonmetals (e.g. P4 3 and boranes discussed below), main-group metals (e.g. [Pb5]2- 4) and transition metals (often with ligands such as CO. Heteroelement bonding is also possible as in the A4B4 structure 5, include by S4N4 (where A=S and B=N) and As4S4 (where A=As and B=S).