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

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 (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 (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.