A series of compounds containing the [Mo(3)-&mgr;(3)S-(&mgr;S(2))(3)-(dtc)(3)](+) complex (dtc = diethyldithiocarbamate) with the anions I(-) (1), I(-) and Br(-) (2), S(2)(-) (3), ClO(4)(-) (4), NO(3)(-) (5), and SO(4)(2)(-) (6) was prepared and characterized by elemental analysis, NMR, IR, and Raman spectroscopy, and FAB mass spectrometry. The previously reported crystal structure of 1 was reinvestigated. The X-ray analysis revealed the incorporation of CH(2)Cl(2) in the crystal having the composition [Mo(3)S(7)(dtc)(3)]I.0.5CH(2)Cl(2) (1a), which was in contradiction to the previous protocol. The corresponding ClO(4)(-) compound (4a) is isotypic. Crystal data: C(15.5)H(31)Cl(2)Mo(3)N(3)O(4)S(13), orthorhombic space group Aba2, a = 25.816(5) Å, b = 17.761(4) Å, c = 16.250(3) Å, Z = 8. For 1a, 4a, 6, and the previously analyzed 2 and 3 the crystal structures revealed characteristic interactions between the anions X and the three axial (out-of-plane) sulfur atoms S(ax) of the disulfido bridges. The Raman data showed a significant decrease of the S(eq)-S(ax) stretch resonance frequency in the order 4, 5, 6 > 1 > 3. This decrease is paralleled with a slight increase of the S(eq)-S(ax) bond length and with a significant shortening of the X.S(ax) distances when compared to the sum of the corresponding van der Waals radii. A comprehensive quantum chemical study, using both density functional theory and semiempirical calculations, revealed that for hard counterions such as NO(3)(-) and ClO(4)(-) the S(ax).X interactions can be understood in terms of an almost entirely electrostatic interaction, whereas for soft nucleophiles such as I(-) and S(2)(-) significant covalency is observed. In addition, the general reaction of [Mo(3)S(7)](4+) complexes with a nucleophile was modeled. With regard to the side-on bonding of the &mgr;-S(2) groups to Mo, the calculations indicated a significantly higher bond energy for the axial (out-of-plane) sulfur atoms, explaining the much higher lability of the sulfur atoms in the equatorial (in-plane) position. Analogous differences for the ligating atoms of the peripheral ligands, having a cis and trans position with respect to &mgr;(3)-S, are less pronounced.
