Comparison of Gallium Cluster Polyhedra with Those of the Most Spherical Deltahedra: Effects of Bulky External Groups and Local Surface Curvature

Experimentally accessible stable polyhedral clusters of gallium differ from clusters of its lighter homologue boron by having sterically demanding external groups such as tBu3Si, (Me3Si)3Si, (Me3Si)3C, (Me3Si)2N, and fluorenyl. This restricts closo deltahedral [GanRn]z gallane chemistry to octahedral derivatives such as Ga6R62− (R = SiBuPh2, SiBut3) and Ga6R6 (R = SiMe(SiMe3)2) in which the bulky external silyl groups are spread out because of the relatively high local curvature of the degree 4 vertices of the central Ga6 octahedron. The structures of larger gallium clusters are based on alternatives to closo deltahedra having exclusively high local curvature degree 4 vertices such as the 8-vertex square antiprismatic Ga8R8 (R = fluorenyl) or, more commonly, larger polyhedra or fused polyhedra with some bare gallium vertices. Some of the larger gallium clusters can be considered to be spherically aromatic systems with closed shells according to the jellium model. Examples include [Ga13(SitBu3)6]− with 8 cluster electrons corresponding to a filled 1S21P6 shell as well as [Ga@Ga18{C(SiMe3)3}6]− with 52 cluster electrons, Ga22{E(SiMe3)3]8, (E = C, Si) and Ga@Ga11{GaN(SiMe3)2}11 with 58 skeletal electrons, and [Ga26{Si(SiMe3)3}8]2− with 72 cluster electrons. The 10-vertex species Ga10{Si(SiMe3)3}6, [Ga10(SitBu3)6]−, and Ga10(SitBu3)6 have structures consisting of a fusion of two polyhedra sharing bare gallium vertices with skeletal electron counts, suggesting a multicenter core bond in each of the two polyhedral cavities.