Hyperstoichiometric and Nonstoichiometric Molecules and Clusters

The octet rule dictates that atoms of the first and second rows are most stable when they are surrounded by the eight calence electrons. Based on this rule, one can predict the maximum stoichiometry of hydrides to be: CH4, NH3, H20, and HF. However, when electropostive lignads such as alkali metal atoms, alkaline earth atoms, aluminum, or transitions metal are present, thermodynamically stable hyperstoichiometric molecules such as Li6O, Mg2, Mg3C, Al4O with the number of ligands larger than the octet rule dictates, have been predicted computationally and some of them have been recently experimentally observed. The substantial stabillity of these molecules is due to the high degree of ioinic character between the central atom and the ligands as well as bonding interactions among the ligand atoms. In a sense, hyper stoichiometric species can be regarded as metal clusters bound ionically to a centrally located "impurity" heteroatom.

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