[M0,S(S,)~(dtc),]1, [Mo,S(SeS),(dtc),](dtc), and [Mo$e(Se,),(dtc),](dtc) (dtc = NJ"diethy1dithiocarbamate) were investigated by liquid SIMS-FTMS. The fragmentation pathways were essentially the same for the three compounds and can be explained by two types of fragmentation processes: stepwise abstraction of S/Se atoms as exemplified by the series [Mo,X,(dtc)J+ (4 < z < 7, X = S, Se), and ligand dissociation, as indicated by the generation of [Mo,X,(dtc)J+ (5 < z < 7, X = S, Se). The exclusive elimination of the Se-atoms from [Mo,S(S,,-Se,,),(dtc)3]+ confirmed the inequivalent reactivity of the bridging atoms in axial and equatorial position as observed in previous studies.(3), and [Mo,Se,(dtc),]' (4) revealed distinctly different fragmentation reactions for the SIMS and CID mode. CID of l, 3, and 4 resulted in a two-step reaction with the exclusive elimination of diatomic molecules XY (X,Y = S/Se). In the case of 3, the selective elimination of Se, indicated the abstraction of two Se-atoms located in equatorial positions of two different bridging groups. This result is discussed in terms of mechanisms, based on labile M-X,, and inert M-X,, bonds with an intramolecular formation of a X, fragment prior to the elimination of X,.Introduction. -The recent advance in developing selective preparation methods for complexes containing the cluster cores [ M o~S ( S~)~]~+ [Mo,S(SSe>,J4' [3] [6], and [Mo, in the synthesis of a variety of novel compounds. The characterization of these complexes by X-ray analysis revealed an isostructural geometry for all three cluster cores (Fig. I ) , containing a triangle of three Mo-atoms, capped by a single S-or Se-atom. Additional bridging is provided by three entities of S-S, Se-S, or Se-Se, respectively. The two atoms of these bridging groups are inequivalent and occupy an equatorial (in plane) and an axial (out of plane) position with respect to the Mo, triangle. Some characteristic reactivities of these cores have been established in solution: i ) substitution of the peripheral ligands under complete preservation of the Mo,X, core [1][2][3][4][5] [8] [9]; ii) elimination of the three atoms in equatorial position by suitable nucleophilic agents, resulting in complexes containing the [Mo3X4I4+ core [I] [9] [ll-141; iii) anion binding by the three atoms in axial position [4] [5] [8-lo]. However, no mechanistic details about these reactions are known so far.