Subvalent Compounds

Abstract

Abstract Subvalent compounds are those in which a main group element occurs in an oxidation state lower than its normal group oxidation state. Traditionally, these have been identified in solid‐state inorganic phases, but more recently there have been major advances in the preparation of stable, molecular compounds containing subvalent main group elements. This has occurred in some cases through the use of specially designed ligands able to stabilize low oxidation state species through electronic stabilization or steric protection using bulky substituents. Ingenious synthetic routes such as the preparation of solutions in organic solvents of metastable aluminum(I) and gallium(I) halides have allowed access to other novel species. Apart from a relatively small number of types of mononuclear complexes, most subvalent molecular compounds involve homoatomic element–element bonds in the form of rings, chains, and clusters. Many of these have unusual structural and electronic properties that challenge established, classical ideas of bonding and electronic structure. This discussion focuses on recent advances in the structure and chemistry of molecular subvalent compounds, with an emphasis on the groups 13 and 14 elements. In addition to mononuclear and simple homonuclear single‐bonded compounds, several classes of cluster compounds are examined, including electron‐precise, electron‐rich, and electron‐deficient clusters. The latter include the Zintl ions. A very new class of aluminum and gallium metalloid clusters containing metal–metal bonds unsupported by ligands represents species on the pathway to formation of the bulk metal. An area in which exciting developments have taken place in the last decade encompasses subvalent compounds that are heavier main group element analogs of alkenes and alkynes, and significant structural features and new bonding models for these are discussed.