Gas-phase electron diffraction data of Cp*Al = (C&fe5)Al, Me = CH3, recorded with reservoir and nozzle temperatures of 139 f 4 "C, show that the gas consists of monomeric (q5-Cp*)Al units. Least-squares refinements of a molecular model of C5, symmetry yield the bond distances AI-C = 238.8(7), C-C(endocyc1ic) = 141.4(5), and C-C(exocyc1ic) = 152.9(5) p m and a perpendicular metal-to-ring distance of 206.3(9) p m .

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The gallium(I) compound Ga4[C(SiMe3)3]4 (1) with a tetrahedral Ga4 core in the solid state gives on evaporation the monomeric Ga(I) alkyl Ga[C(SiMe3)3] with the unique structural situation of a Ga atom solely coordinated by one singly bonded carbon ligand. Its molecular structure was determined now by gas-phase electron diffraction yielding a Ga−C bond length of 206.4(17) pm, similar to that found in the solid state for the tetramer and much longer than in compounds with gallium in the normal oxidation state of +3. As shown by cyclic voltammetry, 1 and the analogous cluster In4[C(SiMe3)3]4 (2) could both be reversibly reduced at about −2 V vs Fc+/0. Whereas the reduced organoindium tetramer did not exhibit any detectable EPR signals between 4 and 300 K, the gallium analogue showed an EPR spectrum compatible with the formulation [(RGa0.75) 4]•-; i.e. the unpaired electron was found to be delocalized over all four metal centers (a(69Ga) = 1.93 mT, a(71Ga) = 2.45 mT, 4 Ga).

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Gas electron diffraction data for monomeric GaCI,, monomeric InCI, and PbCI, have been recorded with nozzle temperatures of about 380,480 and 20 "C respectively. The data for GaCI, and InCI, are consistent with equilibrium structures of D,, symmetry and bond distances r, = 210.8(3) and 228.9(5) p m respectively. The data for PbCI, are consistent with an equilibrium structure of T, symmetry and r, = 237.3(3) pm. Bond energies and distances from the literature show that the M-CI bonds in MCI(g) are stronger, but longer, than in MCl,(g) for M = Al, Ga or In, and that M-CI bonds in MCl,(g) are stronger, but longer, than in MCI,(g) for M = Ge, Sn or Pb. It is suggested that the relative weakness of the bonds in group-valent chlorides is due to the energy required t o promote the Group 1 3 metal atoms from the 'P(s2p) ground states to 4P(sp2) valence states, or t o promote the Group 1 4 metal atoms from ,P(s2p2) ground states to 5S(sp3) valence states. Further that the decreasing stability of the group-valent relative t o subvalent chlorides as the Groups are descended is due both t o increasing promotional energies and t o decreasing M-CI bond strengths.Several years ago we reviewed the structures of chlorides of main-group elements (Groups 1 and 2 and 12-17) and discussed the variation of element-to-chlorine bond distances across the Periodic Table .' Since then we wished to determine the structures of the remaining homoleptic chlorides of main-group elements which are stable under the conditions of gas electron diffraction (GED), and have published the structures of SeCl,,' TeCl,,, BiCl, and (monomeric) CdC12.5 The gas-phase structure of ZnC1, has been determined by the Hungarian GED group.6 An up-to-date survey of the structures of gaseous metal halides has recently been published by Hargitta.7 In this article we report the gas-phase structures of monomeric GaCl,, monomeric InCl, and PbCl, and discuss the structure and bonding in group-valent and sub-valent chlorides of the Group 13 and 14 elements based on bond energies, bond distances and bond-stretch force constants.