Analogies between Group 13 metal clusters in organometallic and intermetallic structures

Analogous metal fluster units are found in organometallic and intermetallic structures containing the Group 13 metals (icosogens) Al, Ga, In, and TI. The organometallic structures typically contain very sterically hindered al kyl or aryl ligands, whereas the intermetallic structures contain the Group 13 metal and a much more electropositive metal such as an alkali or alkali ne 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 or ganometallics [RGa=GaR](2-) (R = bulky aryl group) and the anion Ga-2(5-) i n the intermetallic Ca5Ga3; (2) metal triangles such as the organometallics R3Ga32- and the anion In-3(5-) 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 o rganometallics R4M4 (M = Al, Ga, In, T1; 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 trigo nal bipyramid in [Li(thf)(4)](+)[{(Me3Si)(3)Si}(3)(Me3Si)(2)Ga4Si](-) with relatively weak Ga-eq Ga-eq interactions; (5) larger metal deltahedra of wh ich the only structurally characterized organometallic examples are the ico sahedral [Al(12)i-Bu-12](2-) obtained as its potassium salt and the pentago nal bipyramidal [{(Me3Si)(3)Si}(4){(mu(2)-Me3Si)(3)Si)(3)SiGa}(2)Ga-7](-) o btained as its solvated lithium salt. The deltahedra in some of the interme tallic structures exhibit an unusual type of Battening at some of the degre e 4 vertices to give polyhedra not yet found in organsmetallic species. (C) 2000 Elsevier Science S.A. All rights reserved.