Electronic MgXg clusters

The [MgXg] clusters found in the iodides Nbgln, [33] CsNbglu, [34] NbgIg(NR3)g, [35] and the unusual chain compound NbgLS [36] are rather an exception for the valence electron poor metal Nb. This type of duster however, is common with the (chalcogenide) halides of the more electron rich metals Mo [7, 9, 37, 38] and W as well as the (halide) chalcogenides of Re [7, 9, 39] and Tc. [40] There is a suffident number of d electrons in the dihalides of Mo and W to fill... 

The discussion so far has concentrated on the structural chemistry of the electron rich clusters of the [MgXg] type. To prepare for the next paragraph, interest is now focussed on metals with smaller numbers of valence electrons which then prefer to form [MgX j type clusters. A tremendous amount of work has been performed on [MgX,2] type halide clusters of the metals Nb and Th both in the solid state and in solution. Ousters with 14, 15, and 16 electrons in M-M bonding states are well known. Examples of compounds with this cluster type but formed with divalent anions had been very scarce, being represented by only two which contained the [NbgOi2] cluster. [57, 58] Recently, a rather broad chemistry of reduced oxoniobates based on this cluster has emerged. [16]... 

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