Novel quasi-aromatic transition metal cluster compounds

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Abstract
From the point of view of chemical reactivities, molecular configurations, and bonding characteristics, a series of novel quasi-aromatic transition metal cluster compounds has been studied in this article. Experimental evidences for the cluster compounds [M3(μ3-X)(μ-y)3]4+ (M = Mo, W; X, Y = O, S, Se, Te) containing a puckered {M3(μ-Y)3}six-membered rings reveal benzene-like structural characteristics and chemical behaviours in a series of ligand substitution, addition, and oxidation reactions. The energy localized CNDO/2 molecular orbital method is applied to elucidate the nature of quasi-aromaticity in this type of cluster compound. The localized molecular orbitals (LMOs) of benzene as an aromatic prototype and a series of planar monocyclic polyene hydrocarbons CnHq n as well as certain typical conjugated six-membered rings are investigated, leading a deep-rooted inherent relationship between their aromaticity and bonding characteristics. The LMO analyses for these cluster compounds with [Mo3(μ3-X)(μ-y)3]4+ show that in both benzene and such cluster compounds there exists a closed, smoothly continuous conjugated φ-electron system, formed from three synergistically connected three-centred two-electron φ bonds, which is a (p-p-p)φ conjugated system in the former, but a (d-p-d)n: one for the latter. The size and electronegativity of the bridging atom (μ-Y) affect significantly the degrees of quasi-aromaticity in these cluster compounds with the core [M3(μ-X)(μ-Y)3]4+ (Y = O, S, Se, Te); on the other hand, the stronger interaction between the metal atom M and the terminal ligands with lone-pair electrons results in a smaller quasi-aromaticity of the puckered [M3Y3] ring. The bonding patterns of the cubane-type [Mo3S4ML2](4+q) and the sandwich-type [Mo3S4MS4Mo3]8+ formed from the quasi-aromatic ligand [Mo3S4]4+ with a metal atom M are discussed in detail. Finally, criteria have been formulated for the formation of a closed, smoothly continuous φ-conjugated system in these [M3Y3] rings.

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