Structures and bonding in triatomic compounds

Flowing within one of the major intellectual streams of the conference—nonintegral stoichiometry—James Ibers of Northwestern University reported on the structure and bonding in several triatomic compounds containing an alkali metal (A), an actinide metal (An), and a chalcogen (Q) and having the standard stoichiometry, AAn₂Q₆. Such compounds fall into two general crystal types: designated Cmcm and Immm. In both cases, the crystals are remarkably interesting architecturally, each formed by an alternation of layers in which the actinide’s complexation ability plays a key role.

Ibers first described the synthesis of uranium sulfides, a multistep process that initially forms uranium(IV) chloride, (UCl₄) and subsequently the layered crystal structure of the sulfide, which ultimately serves as the framework for building the larger layered crystals when the alkali metal is later incorporated. The electronic structure of the sulfide is such that, according to Ibers’ data, both 6d and 5f electrons of the uranium participate in bonding within the uranium sulfide crystal.

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