Clusters with supporting bridge ligands are rare among Group 5 metals, the only crystallographically established example being [Cp,Nb,(CO),]['] (Cp = q5-C,H,). On the other hand, halo-, 0x0-, and sulfido-bridged clusters of vanadium, niobium and tantalum in oxidation states higher than I are well documented in the literature.L2I Vanadinm(0) compounds are readily accessible,[31 whereas niobium(0) and tantalum(0) complexes mostly have to be prepared by metal vapori~ation.'~' Among the mixed metal clusters [((PPh,)-Au),V(CO),] has been prepared and structurally characterized,l5! while a positive oxidation state for niobium has been foundLb1 in [{(PPh,)Au),Nb(C,H,R),]@.We now wish to report the first structurally characterized carbonyl clusters containing niobium and tantalum in the formal zero oxidation state.The reactions of 1 (dmpe = 1,2-bis(dimethylphosphino)ethane) with silver salts of poorly coordinating anions, namely AgNO, and AgBF,, in THF at room temperature afford compounds of analytical composition 2 [Eq. (a)].The solid state structures of 2 were established by single crystal X-ray diffraction methods.''] Both compounds are isotypic, as shown by Gandolfi photographs of the single crystals used for subsequent intensity measurements.The complexes 2 are structurally related to the osmium" 2] and iron-platinum[' raft-like clusters. The ten non-hydrogen atoms in the asymmetric unit are arranged around the intersection of a crystallographic threefold axis with three twofold axes, i.e. the point symmetry of the resulting molecule is D,, the three silver atoms and the three Group 5 metal atoms lying in the plane containing the twofold axes (Fig. 1).The arrangement of the metal atoms in almost equilateral triangles corresponds to that of a (1 11) plane in a closepacked metal structure, the intermetallic angles being all close to 60". The almost identical Ag-Ag distances of 2.842 A in 2a and 2b are shorter than in silver metal (2.883 In the intermetallic cubic Laves phase NaAg,, which contains a network of corner-connected Ag, tetrahedra, the Ag-Ag distances are 2.810Moreover, a AgAg distance of 2.994(6) A has been in the heterometallic cluster [Pt,Ag,Cl,(C,F,),J20.The distances between Ag and Nb or Ta in 2 a and 2 b are also very similar (2.888 A) and might be compared with the sum of the metallic radii (2.91 A['41).
