The influence of the ligand geometry on TlI-TlI and InI-InI bonding has been studied within the extended Hückel framework, looking at the following observed molecular and solid-state structures or structural types (respective metal moiety given in parentheses): {(PhCH2)5C5Tl/In}2 (dimer), {TlOMe}4 (bridged tetramer), Tl2Te22- (bridged dimer), {Me5C5In}6 (octahedral hexamer), {MeC5H4In)∞ (dimers), TlS = TlIT1IIIS2 (TlSe-type, linear chains), In11Mo40O62 (chain segments), InMo4O6 (linear chain), and TlCo2S2 (ThCr2Si2-type, square-planar nets). For TlI and InI complexes with large TlI⋯TlI and InI⋯InI separations the ligand environment (especially the L-M-M angle) is found to be the dominant factor in determining the extent of the bonding interaction between the metals. The metal-metal distances in these compounds range from about 265 to 400 pm. We have interpreted the metal-metal interaction in these systems with the help of model complexes, such as Tl2, Tl2H2, {C5H5Tl/In}2, {TlOMe}2, {HIn}6, {C5H5In}6, Tl/In∞, |TlH4/S4}∞, {InO4}∞, Tlnets, and {TlH4/S4}nets with the ligands in various geometrical arrangements. Starting the analysis with a bare Tl2 dimer or Tl∞ chain, it is shown that a mixing of empty p levels into the filled s combinations is the basis for a bonding interaction. The behavior of the overlap population as a function of the ligand geometry is then studied. A detailed angle variation in Tl2H2 helped us to understand why one passes from an almost nonbonding situation in the linear arrangement, over a region of strong bonding upon trans-bending of the hydrogens, to again reach a nonbonding interaction in the bridging geometry. What happens in the trans-bending in Tl2H2 is closely related to the orbital interactions in the pyramidalization of AH3 systems (e.g. NH3) and the bending of AH2 molecules (e.g. H2O). Calculations on the more realistic examples {TlOMe}2, {C5H5Tl/In}2, and Tl2Te22- confirmed that a trans-bent geometry with a ligand-M-M angle close to 120° gives an optimum overlap population at a relatively long, fixed TlI-TlI or InI-InI separation in molecular systems. A bridging ligand geometry, either in a molecular complex or in an extended structure, generally gave a non-or antibonding metal-metal overlap population. This is in agreement with the apparent general consensus in the literature that the bridged species do not display any Tl-Tl interactions. Dimers where the trans-bent geometry has been observed, despite their long M-M contacts (such as 363 pm as in |(PhCH2)5C5Tl/In}2), can be assigned a definite MI-MI bonding interaction. The bonding in solid-state structures with TlI-TlI or InI-InI contacts was analyzed with the help of band structure diagrams, density of state, and crystal orbital overlap population plots. A prior look at the interaction in a (molecular) Tl2 dimer in a square-pyramidal or cubic ligand field facilitated the interpretation. Extended structures showed an optimum at ligand-M-M angles of 90°, resulting in a square-planar ligand field for metal chains or an on-top/bottom position in metal nets. While a square-planar ligand field increases the s-pz mixing, hence overlap population, with respect to the bare Tl/In chain or net, a cubic ligand environment widens the s-pz energy gap, thereby decreasing mixing and overlap population. © 1990, American Chemical Society. All rights reserved.
