Localized molecular orbital study on chemical bonding and reactivities of certain Mo-Cu-S cluster compounds

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The localized molecular orbitals (LMOs) and Mulliken bond orders for the cluster anions of [Et4N]2[Mo2S4(edt)2] (1), [(CH3CH2)4N][Mo2CuS4(PPh3)(edt)2] (2) and the cluster molecule [Mo2Cu2S4(PPh3)2(edt)2] (3) have been calculated by means of the Edmiston-Ruedenberg energy localization technique under the CNDO/2 approximation. The results of the LMOs calculations show that there are two pairs of π(S1 → Mo) bonds each pair of the Mo atoms and the terminal S1 atoms in the anion of (1). In the addition reactions of the cluster compound (1) with one or two Cu atoms these π(S1 → Mo) bonds are sequentially opened and form correspondently the cluster compounds (2) and (3) with the three-centered or four-centered bonds among the copper, sulfur, and molybdenum atoms, accounting for the stronger bonding of the Cu atom to the Mo-S cluster compounds such as (1). This also gives the explanation of chemical bonding for the [2+1] and [2+2x1] synthetic reaction schemes of the compounds (2) and (3). The structural features for the cluster compounds studies are briefly discussed as well.