Metal cluster topology 17. Hypervalent indium vertices in isolated hypoelectronic indium polyhedra in binary and ternary intermetallic phases of indium and alkali metals

Abstract

Analyses of the structures and electron counts for the known isolated indium clusters found in binary and ternary alkalimetal-indium intermetallic phases using methods derived from graph theory suggest the presence of hypervalent indium vertices using filled indium d orbitals as well as sp 3 orbitals for skeletal bonding to relieve the apparent electron poverty in the hypoelectronic polyhedra. The novel shapes of the hypoelectronic indium polyhedra in such clusters are based on capping a triangular or rectangular face of a smaller polyhedron, followed by flattening the pyramid generated by the capping vertex so that it moves closer to the center of the polyhedron. Reasonable electron counts and globally delocalized bonding topologies are then obtained for the hypoelectronic indium clusters if all of the indium vertices at the flattened polyhedral sites are hypervalent using sp 3d five-orbital manifolds and the remaining indium vertices are normal using spa four-orbital manifolds, like typical vertex atoms in boranes, carboranes and post-transition metal ‘Zintl-type’ clusters. For example, the empty triflattened pentacapped trigonal prismatic indium cluster anion In 11 7− in K 8In 11 and K 8In 10Hg becomes a globally delocalized 11-vertex deltahedron with three hypervalent and eight normal indium vertices and the required 2 n+2=24 skeletal electrons. Incorporation of an interstitial atom into the center of an isolated hypoelectronic indium polyhedron splits the single multicenter core bond found in an empty globally delocalized polyhedron into two or more core bonds, corresponding to the coordination number and sp n hybrid orbital orientation of the interstitial atom. Thus, the compound K 8In 10Zn can be regarded as a linear two-coordinate Zn 2+ complex of the bidentate In 10 10− ligand and the compound K 10In 10Ni can be regarded as a tetrahedral four-coordinate Ni 0 complex of the tetradentate In 10 10− ligand. This theory predicts the existence of alkali-metal-indium-coinage metal ternary phases such as K 8In 9Cu, which would correspond to a trigonal Cu + complex of a tridentate In 9 9− ligand. In such structures, the hypoelectronic anionic indium polyhedron can be regarded as a multidentate ligand encapsulating the interstitial atom.