Polycation clusters

The interaction of polycations with free chalcogens leads, in some cases, to new cluster compounds.185 Reaction of the aromatic tetraatomic selenium dication with diphenyldiselenide affords Ph2Se62+ - compound 159 with a six-membered selenium ring (Scheme 66).186... 

Prior to any work on heteroatom clusters the notion was expressed (20) that heteroatom placement within the polyatomic clusters would lead to a decrease In delocalization and bonding and thence stability. Although this may lessen stability the substitution clearly does not preclude It. Furthermore, many of the likely polyhedra already have Inequlvalent atom positions, the 5, 7, 9 and 10 atom examples already considered here for example, and mixed species especially with elements from different groups may be quite stable within the discrimination provided by Inequlvalent positions. Even the nominally equivalent atom positions In a tetrahedron can obviously accommodate substantial differences. Additional examples of mixed element polycations are certainly to be expected. An Inadequate foresight was revealed In a review of polycations (20) written for a 1974 award symposium, about one year before the crypt discoveries, by the expectation that polycations should be more stable than polyanions for the metallic elements. In hindsight, metallic behavior Is a property of the dense solid state and has little to do with the stability of small clusters where electronic and geometric factors are far more important. 

Mixed group 15 group 16 polycations are rare and tend to form clusters. For example, the arsenic-chalcogen cations [As3E4]" (E = S, Se) have polycyclic structures similar to those of the well-known isovalent species P4S3 and the bismuth-tellurium cation [Bi4Te4]" " adopts a slightly distorted cubic... 

Table 1 (a) Polycations with Oxidation Numbers Lower Than +1 and (b) Cluster Compounds of Mercury... 

Metal oxo-alkoxides, MO (OR)x 2 > , may be considered intermediates between metal alkoxides M(OR)x and the corresponding metal oxide, and are the counterparts of the polyanions and polycations in aqueous systems. Controlled partial hydrolysis of M(OR)x (Equation (20)) does indeed sometimes result in structurally well-defined metal oxo-alkoxo oligomers (clusters) where //2—0x0 ligands connect the metal polyhedrons from which the clusters are constructed. 

The [Moig03o] cluster and the Iu5 chain with their nearest neighbor O atoms are shown in Figure 5-24. Here, we have the interesting situation that both M-M bonded polycations and polyanions occur simultaneously in InnMo4oOg2- The Ins chain can be formally decomposed into (2 x In ) (3 x In ) and thus... 

Besides the anionic cluster framework, the counter cations in the channels between the clusters also exhibit very interesting features in that they frequently form hitherto unknown polycations. For instance, indium in a -1-1 oxidation state normally has a lone pair configuration which can be either stereochemically active or inactive according to the known salt chemistry of this element. In the structures of these oxomolybdates, however, the rigid matrix of the duster anions forces the In cations into such close proximity that the lone pair configuration is transformed into two In-In bonds for each In atom. The bonds manifest themselves by an even closer approach of the atoms within the In chain (d(In-In) 2 262 pm) than required by the matrix. Hence, these oxomolybdates offer the possibility to stabilize hitherto unkown linear polycations of In with the help of suitable anionic clusters. 

By contrast with the above, corresponding data obtained for another polycation, acrylamide/p-methacryloxyethyltrimethylammonium chloride (Reten, Hercules), at 1% concentration, showed no change in viscosity with added SDS in the preprecipitation zone (80). There is thus little doubt that the clustering of bound SDS that takes place is intra-polymolecular in this case. A similar situation was reported by Abuin and Scaiano (81) for mixtures of polystyrene sulfonate and DTAB in which an observed 10-fold reduction of viscosity was ascribed to coiling of the polymer around small clusters of surfactant molecules that formed. On the other hand, McCormick and Johnson (82) reported that... 

The element bismuth appears to have a special tendency to form polycations. As observed in Table 4.20 at least three cluster species, namely Bis", and Big have been clearly identified. 

Although no further studies of cationic mercury clusters have been reported, evidence for the formation of cadmium and zinc analogues has been presented. Structural and spectroscopic studies of dehydrated Cd-A (LTA) after reaction with cadmium vapour suggest that various clusters such as Cd ( = 2-4) and Cdj may be present [139-141]. Boddenberg and co-workers [142-144] have examined the reaction of cadmiiun and zinc vapour with H-Y (FAU), and concluded that Cd + or Znf+ polycations are produced. 

These are the largest iron polycations known to date, containing 17 and 19 metal cations. It should be noted that the core of the ferric clusters contains /X3—OH bridges only, with a topology close to that of the tetramer involved temporarily in the formation of goethite (see below). 

---