Rare binding with anions

Among the clusters derivatized by these methods, [ Mo(tacn) 3(PdCl)(n3-S)4]3+ (tacn = 1,4,7-triazacyclononane) is noteworthy, since this cluster can bind the substrate molecules such as alkene and CO at its tetrahedral Pd site generated by dissociation of the Cl anion. More interestingly, it has turned out that addition of ROH and RCOOH to various alkynes proceeds at this Pd site to give vinyl ethers and vinyl carboxylates catalytically.53 These reactions represent one of the yet rare examples that are catalyzed by well-defined cubane-type clusters with retention of their core structures. 

Ions which do not have a rare gas configuration appear not to fit into this rule. They are much more firmly bound than the alkali and alkaline earth cations. A dependence on radius is not observed and for Hg(I) the binding is stronger with the diphosphate anion than with triphosphate 366). (See, also the binding of cations by highmolccular polyphosphates, Section IV,E,/,d.) For the analysis and determination of triphosphates, see Section VII,B. 

As a consequence, it is not surprising that there appears to be only one isolated structure claimed. This is of the indium(I) and thallium(I) complexes with a single M—C bond from a o--bonded benzene anion that carries two bulky tri-substituted benzene substituents in ortho positions these partially block approach of other potential donors to the metal cation (Figure 4.3). An X-ray crystal structure has been determined, defining the shape. It is a rare observation, as other complexes (such as Mn(I) and Fe(I)) of the same ligand bind another ligand at the open side of the molecule, as expected. In any case, this coordination number is trivial in the sense that it can have only one shape - a linear M—L arrangement. 

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