Mercury metal atom cluster

Metal atom clusters in the 26-hedra could (as with water molecules) contain many metal atoms. An example is the fourteen atom Agg+8Ag+ cluster in Y-irradiated zeolite Ag-A[25]. Saturation is achieved for mercury sorbed into silver-exchanged faujasites and other zeolites [26]. The Ag+ is reduced to Ag atoms and then at an approximate critical pressure of mercury vapour there is nucleation of mercury clusters which fill all the pore volume as the pressure of Hg vapour increases further. Mercury-zeolite systems are the oily ones in which sorption isotherms have been investigated quantitatively. Hcwever other metal atoms introduced into zeolites (by ion exchange and reduction, or as metal carbonyls and their decomposition) all show, on heating, a strong tendency to form clusters by migration of atoms, which can aggregate both within and outside the crystals. 

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. 

Mercury will bond directly to certain metal atoms including Co, Ru, Rh, Fe, Pt, and Mn. Such Hg-metal bonds are usually part of linear M-Hg-M or M-Hg-X linkages, and some clusters containing mercury-metal bonds are also known. These compounds are prepared by the reaction of mercury(II) halides with carbonyhnetallates. A rare example of trigonal prismatic (see Trigonal Prism) coordination in a mercury complex is exhibited by [Hg Pt3(2,6-Me2C6H3NC)6 2], which contains Pt-Hg bonds. These types of compound have been reviewed. ... 

The reactions of vdW molecules and clusters can be divided into intra- and intercluster processes, and further into neutral and ionic cluster reactions. The latter were recently reviewed by Mark and Castleman. Therefore the scope of this contribution will be limited to neutral species only. We distinguish between intra- and intercluster reactions. In intracluster processes reactions are induced inside a cluster, usually by light. Examples of such reactions are the reaction of excited mercury atoms with various molecules attached to them, reactions that follow photodissociation in the cluster, and charge transfers inside a large cluster. In intercluster reactions the cross molecular beam technique is usually applied in order to monitor scattered products and their internal energy. The intercluster reactions may be divided into three major categories recombination processes, vdW exchange reactions, and reactions of clusters with metal atoms. 

The local interactions between the metal atoms and water molecules or ions have been obtained from semiempirical and ab initio quantum chemical cluster calculations (see below). This technique has been used by the groups of Spohr and Heinzinger, and later by Berkowitz and coworkers, for platinum surfaces with (100) and (111) surface geometry [48, 49, 52-54, 76] and also for mercury surfaces [40, 77-81]. 

Then, a fundamental question arises how many metal atoms should be in a cluster, which exhibits a metallic nature Mercury is the most suitable element to examine such a question. Liquid mercury is a typical metal while two mercury atoms is weaJcly bonded by a dispersion force, because a mercury atom has a closed electronic structure, where all the orbitals... 

Tetracarbonylcobaltate(l —) forms ionic complexes with group 1 elements. However, compounds of the type M[Co(CO)4] , where 2 and M = zinc, cadmium, mercury, indium, etc., are covalent, possessing M —Co bonds in which the main group metal has normal coordination number. These compounds are monomeric in the solid state. Ag[Co(CO)4] and Cu[Co(CO)4] are tetrameric clusters in which the metal atoms form planar, eight-membered rings. Each of the distorted [Co(CO)4] tetrahedrons is bonded to two atoms of silver or copper. 

A complete treatment must also include formation of neutral atomic clusters A and negative ion clusters A. These species are stabilized by the presence of an ionized electron. They are the fluid state analogues of the polarons in solids described in Sec. 2.3.3(c). The idea that negative clusters affect the optical, dielectric, and thermoelectric properties of dense metal vapors close to the critical point has been put forward by a number of authors (Khrapak and lakubov, 1970 Hefner and Hensel, 1982 Hernandez, 1982 Hefner et al., 1982). We discuss this in relation to the transport properties of mercury in chapter 4. 

Oxidative Additions. Many covalent compounds (XY) such as hydrogen, halogens, mercury and silver halogenides can react with clusters resulting in an homolytic cleavage of the X-Y bond and in the oxidative addition of the two fragments to a metal-metal bond thus increasing the formal oxidation state of the metal atoms. 

Finally, the stereodynamics of two mercury-containing metal clusters have been reported. The mercury-bridged osmium cluster [ (ju-H)(/i3-S) (C0)90s3 2(/i4-Hg)] exhibits variable-temperature NMR spectra which are attributed to movements of both fi-Hg and /i-H moieties at comparable rates. Mercury halides form adamantane-like anions with chalcogen bridges. The resulting structures [( i-ER)6(HgX)4] (E = S, Se, Te X = C1, Br, I) have been examined in solution by Se, Te, and Hg NMR spectroscopy. The species tend to dissociate in solution. Pyramidal inversions at Se and Te atoms are slow on the NMR time scale at reduced temperatures, but tend to become rapid at ambient temperatures. ... 

One of the important electrochemical interfaces is that between water and liquid mercury. The potential energy functions for modeling liquid metals are, in general, more complex than those suitable for modeling sohds or simple molecular liquids, because the electronic structure of the metal plays an important role in the determination of its structure." However, based on the X-ray structure of liquid mercury, which shows a similarity with the solid a-mercury structure, Heinzinger and co-workers presented a water/Hg potential that is similar in form to the water/Pt potential described earlier. This potential was based on quantum mechanical calculations of the adsorption of a water molecule on a cluster of mercury atoms. ... 

In recent publications [120, 121, 122,123] it has been shown that both the ionization potentials and the optical properties of bare and uncharged mercury clusters in a molecular beam experiment demonstrate a gradual size dependent evolution of metallic properties, starting at about 13 atoms and already bulklike at about 70 atoms. It has been predicted theoretically [124] that plasmons should begin to develop for such mercury clusters at about Hgi5. We should keep this in mind in the discussion of the electronic properties of AU55. 

The reaction of the preformed 16-electron clusters [ (Ph3P)Au 8M](N03)2 (M = Pt, Pd) with metallic Hg resulted in isolation of the isomorphous 20-electron clusters [ (Ph3P) Au 8M (Hg)2] (N03)2 in which two mercury atoms cap in a (i4-fashion the gold atoms of two opposite square faces in an M-centered square-antiprism [Au-Hg 2.915... 

There are syntheses where the new metallic species enhances a different type of reactivity. This has been observed in the reactions between alkyne-substituted ruthenium clusters and compounds of mercury (145, 146). In most of the characterized products a mercury atom or an HgX2 (X = halogen) fragment serves as a bridge between two ruthenium cluster frameworks which retain the coordinated alkyne. 

The Hg atom has a 6s closed electronic shell. It is isoelec-tronic with helium, and is therefore van der Waals bound in the diatomic molecule and in small clusters. For intermediate sized clusters the bands derived from the atomic 6s and 6p orbitals broaden as indicated in fig. 1, but a finite gap A remains until the full 6s band overlaps with the empty 6p band, giving bulk Hg its metallic character. This change in chemical binding has a strong influence, not only on the physical properties of mercury clusters, but also on the properties of expanded Hg, and on Hg layers on solid and liquid surfaces. For a rigid cluster the electronic states are discreet and not continuous as in fig. 1. Also the term band for a bundle of electronic states will be used repeatedly in this paper, although incipient band might be better. As the clusters discussed here are relatively hot, possibly liquid, any discreet structure will be broadened into some form of structured band . 

Fig. 1. The Hg atom has a 6s closed electronic shell. The atomic lines broaden into bands for the cluster. The gap A(n) decreases as a function of the cluster size. The two bands overlap in the solid, giving mercury its metallic character. For large A the binding is of the van der Waals type, for intermediate it is covalent, while for vanishing A it is metallic. From this experiment it is deduced that the gap closes rather abruptly at ca. n m 100 atoms per cluster. This value is a factor of 2 to 7.7 higher than determined in ref (I), (6), (28) and (29).

In summary, ionisation potentials, dissociation and cohesive energies for mercury clusters have been determined. The mass spectrum of negatively charged Hg clusters is reported. The influence of the transition from van der Waals (n < 13), to covalent (30 < n < 70) to metallic bonding (n > 100) is discussed. A cluster is defined to be metallic , if the ionisation potential behaves like that calculated for a metal sphere. The difference between the measured ionisation potential and that expected for a metallic cluster vanishes rather suddenly around n 100 Hg atoms per cluster. Two possible interpretations are discussed, a rapid decrease of the nearest-neighbour distance and/or the analogue of a Mott transition in a finite system. Electronic correlation effects are strong they make the experimentally observed transitions van der Waals/covalent and covalent/metallic more pronounced than calculated in an independent electron theory. 

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