Clusters mercury

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. 

Tab. 2.4-1. Summary of alkali metal (M) amalgams containing anionic" mercury clusters, extended mercury partial structures and/or high coordination number polyhedra.

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

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. 

Fig. 2.4-11. The crystal structure of Nal Hg (a) view of the structure emphasizing the centered icosahedral (dark grey) and hexagonal antiprismatic mercury clusters, K atoms outside the clusters are neglected, (b) icosahedral NaHgi2, (c) hexagonal antiprismatic KHg12.

Perrhenate and related building blocks are constituents of several cluster compounds where they act as terminal groups in organometallic rhenium oxides such as in [(cp Re)3(//2-0)3(/U3-0)3Re03]+ (49)21 Qj. jjj heterometallic clusters such as the structurally related [(Re)3(//f dppm)3(/u -0)3Re03]+ (dppm = bis(diphenylphosphino)methane) and Pt4 P(C6H 11)3)4 (//-C0)2(Re04)2]. A series of platinum-rhenium and platinum-rhenium-mercury clusters with Pt-Re multiple bonds has been isolated from reactions of Pt3 precursors with Rc207 or perrhenate. " ... 

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. 

We have already referred, in Sect. 4.1, to the development of a plasma resonance in mercury clusters [124]. 

The bromo analog of the triruthenium-mercury cluster can be synthesized by following the identical procedure outlined in Section (B.1), by substituting mercury(II) bromide for mercury(II) iodide. Thin layer chromatography (TLC) must be used in the work-up of the bromo derivative to separate the desired product from [Ru3(CO)9(C2-f-Bu)]2Hg, which is formed to some extent. 

The symmetrical hexaruthenium-mercury cluster is synthesized using the procedure presented in Section B.1, varying only the amount of mercury(II) iodide added. 

U. Even Prof. Gerber, could you follow the metal-nonmetal transition in mercury clusters ... 

Mercury clusters have also been studied with EA methods [96], using an empirical potential as a guiding function for finding global minima on a HF-plus-dispersion potential, for <15. This study challenges the usual interpretation of experimental data that locate a transition in bonding type from van der Waals to covalent at =13 and positions it at n=11 instead. 

Commentary on Experimental Study of the Transition from van der Waals, over Covalent to Metallic Bonding in Mercury Clusters, H. Haberland, H. Kommeier, H. Langosch, M. Oschwald and G. Tanner, J. Chem. Soc., Faraday Trans., 1990, 86, 2473. 

The results of Haberland et al. are underpinned by several earlier pieces of work. In particular, Rademann et al. measured the ionisation energies of a more limited range of mercury clusters at discrete photon energies. The overall trend in their data is similar to that seen by Haberland et al, but they did not distinguish the van der Waals and covalent contributions to bonding in the smaller mercury clusters. In a slightly different experiment, Brechignac et aU used synchrotron radiation to promote core electrons to valence states in small mercury clusters. The positions of the valence states... 

Experimental Study of the Transition from van der Waals, over Covalent to Metallic Bonding in Mercury Clusters... 

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. 6. Mass spectrum of negatively charged mercury clusters. Only above n = 3 are the Hg clusters observed. The intensity rises exponentially at small n, a minimum is always observed at n = 11 or 12.

Fig. 9. Experimentally determined dissociation energies of mercury clusters ions (open circles) and calculated dissociation energies of neutral van der Waals clusters, scaled to the Hg, dissociation energy.

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. 

Figure 3.1 Size dependence of cohesive energies per atom (CE/n) of mercury clusters Hgn from calculations using a large-core EC-PP and CPP for Hg. Valence correlation is accounted for either within die hybrid model approach (HM) by a pair-potential adjusted for Hg2 or by pure-diffusion quantum Monte Carlo (PDMC) calculations (Wang etal 2000).

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. 

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. ... 

Figure 6. The variation in the measured ionization potential of mercury clusters as a function of cluster size. The work function for bulk Hg (4.49 eV) is indicated. The dashed line is a plot of the ionization potential calculated for the classical (liquid drop) electrostatic model for a metalUc sphere of diameter d. Region III contains clusters which are classified as insulating. Region II denotes the size-induced metal-insulator transition, in which overlap of the 6s and 6p states sets in at around Hgn. The larger clusters, located in Region I, have valence electronic structures that closely resemble the band structures of liquid and crystalline mercury. Adapted from Rademann. i...

The closed sub-shell electronic configuration, 6s, of the free Hg atom causes small mercury clusters to be non-metallic and held together by relatively weak van der Waals dispersion forces (as found, for example, in noble gas clusters). As the cluster grows, the atomic 6s and 6p levels broaden into electronic energy bands (Fig. 7). A metal-insulator transition within the cluster is presumed to occur at a critical nuclearity (Nc) because of 6s-6p band overlap (as shown in Fig. 7), al-... 

Figure 7. The transition from atomic to metallic (bulk) mercury through the intermediary of mercury clusters. The occupied bands are indicted by shading. Here we approximate the energy separation between occupied and unoccupied bands by 4p, equivalent to the Kubo gap.

Some mercury cluster compounds have been found to be luminescent [82]. For bonding interactions in these clusters it is sufficient to consider only the Hg 6s orbitals in a first approximation. Three mercury atoms may be combined to linear or trigonal structures. The stability of both arrangements depends on the number of valence electrons. In the case of Hg + four electrons are available which stabilize a linear structure while for Hg3 + two electrons favor a trigonal geometry. Both clusters are kept together by one bond [82]. While Hgf+ exist as... 

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