Analogies between Group 13 metal clusters in organometallic and intermetallic structures

Analogous metal cluster units are found in organometallic and intermetallic structures containing the Group 13 metals (icosogens) Al, Ga, In, and Tl. The organometallic structures typically contain very sterically hindered alkyl or aryl ligands, whereas the intermetallic structures contain the Group 13 metal and a much more electropositive metal such as an alkali or alkaline earth metal. The Group 13 metal cluster units found in these structures are of the following types: (1) multiply bonded metal dimers such as the organometallics [RGaGaR]2− (R=bulky aryl group) and the anion Ga25− in the intermetallic Ca5Ga3; (2) metal triangles such as the organometallics R3Ga32− and the anion In35− found in the intermetallic Ca5In9Sn6, both of which are isoelectronic with the cyclopropenium ions R3C3+ and thus may be regarded as metalloaromatic species; (3) metal tetrahedra found in the organometallics R4M4 (M=Al, Ga, In, Tl; R=bulky alkyl group), which can be considered to have MMM two-electron three-center bonds in each of the four faces of the tetrahedron and which dissociate readily in the solution and gas phases to the monomeric one-coordinate alkyls RM; (4) a metal trigonal bipyramid in [Li(thf)4]+[{(Me3Si)3Si}3(Me3Si)2Ga4Si]− with relatively weak Gaeq⋯Gaeq interactions; (5) larger metal deltahedra of which the only structurally characterized organometallic examples are the icosahedral [Al12i-Bu12]2− obtained as its potassium salt and the pentagonal bipyramidal [{(Me3Si)3Si}4{(μ2-Me3Si)3SiGa}2Ga7]− obtained as its solvated lithium salt. The deltahedra in some of the intermetallic structures exhibit an unusual type of flattening at some of the degree 4 vertices to give polyhedra not yet found in organometallic species.