SYSTEMATIC EXPERIMENTAL AND QUANTUM-CHEMICAL INVESTIGATION INTO THE STRUCTURES, THE STABILITY, AND THE SPECTROSCOPIC PROPERTIES OF ALKYLINDIUM(I) COMPOUNDS - TETRAMERIC IN-4[C(SIMERR')(3)](4) VERSUS MONOMERIC INC(SIMERR')(3) DERIVATIVES

The reaction of indium monobromide (InBr) with LiC(SiMeRR')(3). xTHF g ives in a high yield alkylindium(I) derivatives with the indium atoms in the unusual low oxidation state of +1. The properties of these prod ucts are investigated by the systematic variation of the steric demand of the C(SiMeRR')(3) substitutents (R = Me, Et; R' = Me, Et, Bu-n, Pr -i, Ph). Tetrahedral In-4 clusters are observed in the solid state and in solution for smaller substituents such as C(SiMe3)(3), C(SiMe2Et)( 3) (1), and C((SiMe2Bu)-Bu-n)(3) (2). As shown by two crystal structur e determinations, those clusters exhibit undistorted tetrahedra of fou r indium atoms and short In-In distances of 3.00 Angstrom on average. In contrast, owing to the larger steric stress, complete dissociation into the monomeric formula units InR is observed in solution with the more voluminous C((SiMe2Pr)-Pr-i)(3) group (3), while in the solid sta te once more an almost undistorted tetrahedron of the tetramer is foun d, showing, however, much elongated In-In distances of 3.155 Angstrom on average. Single crystals could not be obtained for the compounds wi th C(SiMe2Ph)(3) (4) and C(SiEt2Me)(3) groups (5); 4 is monomeric in b enzene, while 5 gives the formula mass of the dimer, and complete diss ociation into the monomer is observed only in very dilute solutions. A ll compounds show unusual downfield shifts of the resonances of the ca rbon atoms bound to indium up to delta = 76 ppm, which are very charac teristic of the alkylindium(I) compounds. Quantum chemical NMR chemica l shift calculations using density functional theory indicate that the large shifts are related to the presence of low-lying magnetically al lowed excited states, and they are further enhanced by unusually large spin-orbit effects. Optimized structure parameters for InCH3, In-4(CH 3)(4), InH, and In4H4 are compared with the experimental results. Our best estimate for the tetramerization energy of InCH3 is cal 290 kJ/mo l at the MP2 level.