Mercury cationic clusters

Up till now anionic mercury clusters have only existed as clearly separable structural units in alloys obtained by highly exothermic reactions between electropositive metals (preferably alkali and alkaline earth metals) and mercury. There is, however, weak evidence that some of the clusters might exist as intermediate species in liquid ammonia [13]. Cationic mercury clusters on the other hand are exclusively synthesized and crystallized by solvent reactions. Figure 2.4-2 gives an overview of the shapes of small monomeric and oligomeric anionic mercury clusters found in alkali and alkaline earth amalgams in comparison with a selection of cationic clusters. For isolated single mercury anions and extended network structures of mercury see Section 2.4.2.4. 

A novel mixed-metal Ir-Hg cationic cluster [Ir6Hg8(C0)6Cp6](CF3C02)6 (21) has been prepared from reaction between [Ir(CO)2Cp] and Hg(02CCF3)2. The cluster contains mercury atoms in three different environments (six- and four-coordinate) and five-coordinate iridium atoms arranged in an unprecedented distorted square-pyramidal geometry. ... 

It is known from a variety of crystal structure determinations that the typical interatomic distances d(Hg-Hg) in cationic mercury clusters are significantly smaller ( 250 pm) than in neutral ( 330 pm) and anionic ones ( 300 pm). In a first approximation this is due to a preferred covalent Waals bonding in the neutral (weak) and a preferred Op bonding (medium) in the anionic forms. 

Fig. 2.4-2. A comparison between (a) cationic and (b) small anionic mercury clusters in alkali and alkaline earth amalgams (distances in pm).

Au 8M](N03)2 [43]. The structure of [ (Ph3P)Au 8Pt(Hg)](N03)2 14 was determined and the cation was found to be similar [Au-Hg 2.979(5)-3.079(5) A] to that observed in cluster 12, one of the capping Hg atoms being removed (Figure 4.6d). In compounds 12-14 the mercury atoms also interact with the central metal atom (Pd or Pt) while, in contrast to compounds 10 and 11, the capping Hg atoms have no close contacts with N03- anions. 

The electrodeposition of mercury, Hg, has been studied in both basic and acidic EMICI-AICI3 ionic liquids [73]. Mercury dichloride, HgCl2, is soluble in the acidic ionic liquid to form a divalent mercury species, Hg " ", which can be reduced irreversibly to a cluster cation, Hg ... 

Specific cluster-counterion and/or inter-cluster interactions are evident in many solid structures containing formally naked clusters. With the exception of the mercury clusters, these interactions are generally weak enough to validate the notion of the cluster as naked . However, they are nevertheless interesting since in some cases they influence the cluster structures. In addition, the cation-Zintl ion and inter-cluster interactions are of interest since they represent bridges to the structures found in Zintl phases and other intermetallics. ... 

A natural development of the ideas outlined above is to regard the dehydrated zeolite as a polar solid solvent and reactions (1) and (2) as dissolution processes. When Barrer and Whiteman [4] studied the reaction of mercury metal with a number of different ion-exchanged zeolites, they found that mercury uptake was limited in sodium-, calcium- and lead-exchanged forms and copious in silver- and mercury-exchanged forms, where reduction of the exchangeable cations Ag+ and Hg + (to Ag° or Hgl" ) by Hg° would be possible. The implication of these observations is that the ionization process outlined in reaction (1) is necessary for the reaction of elemental metals with zeolites - one of the main synthetic routes to ionic clusters in zeolites - to occur. Reactions (1) and (2), in fact, can usefully be regarded as models for the formation of ionic clusters in zeolites, and detailed calculations of solvation energies [1] can help... 

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. 

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