On the validity of formal electron counting rules in lithium silicides |
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Affiliation: | 1. Núcleo de Matemáticas, Física y Estadística, Facultad de Estudios Interdisciplinarios, Universidad Mayor, Chile;2. Grupo de NanoMateriales, Departamento de Física, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile;1. Department of Chemical Engineering, Imperial College London, UK;2. Department of Chemistry, Imperial College London, UK |
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Abstract: | The validity of widely used electron counting rules (i.e. (8-N) rule) is investigated in two of the four stable crystalline lithium silicides (Li12Si7 and Li21Si5) by means of semiempirical MO calculations of the INDO-type on the stage of localized orbitals which are derived from the canonical one-particle set by a unitary transformation. This representation allows for a transparent interpretation of the nature of the chemical bond in the employed systems. Finite cluster models are used in both cases. The system Li12Si7 is approximated by [Li21Si4]9+ units, the latter one by [Li22Si4]4+ fragments. It is demonstrated that the extension of the octet configuration is only pretended in these systems. The close cage of Li atoms allows for the formation of an additional bonding one-electron state formed by the 2s AO's at the metal centers. This cage orbital is highly independent from the Si subspace. It can be occupied by two electrons that are not considered in formal counting rules. The geometric condition supporting the formation of such a function is a close cage of Li atoms. This solid-state effect has a molecular counter part in hyperlithiated molecules (i.e. [CLi4]2−, [CLi5]−, CLi6). It is shown that other crystalline lithium silicides are stabilized by Li 2s cage orbitals too. |
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