Cluster halides

Fig. 5.17 View of the structure of the orthorhombic mixed-halide cluster phase Cs2[ Zr6B)CbT.98l3.02] showing the two types of cluster chains (thermal ellipsoids with 50% probability). Cs cations and inner halides are omitted for clarity.

J.D. Corbett, Structural and bonding principles in metal halide cluster chemistry. In Modem Perspectives in Inorganic Crystal Chemistry (E. Parthe, ed.), p. 27. Kluwer, 1992. 

Fig. 32, Structures of the octahedral metal-halide clusters (MosClg) and (Nb6Cli2)2+ (49, 133, 144, 165, 178).

Early transition metal halide clusters, see Group 5 metal halide clusters Group 6 metal halide clusters... 

Raman earth halide clusters, 46 2-3 Raman laser temperature-jump technique, 32 17-18... 

The electrochemical reduction of TiCU and ZrCl4, in chloroaluminate melts and other molten salt systems, to lower valent halides has been fairly widely studied [4-7]. This has also been extended to studies of centered hexanuclear Zr halide clusters. Thus, ambient temperature AICI3 -1 -ethyl-3-methylimidazolium chloride (ImCl) molten salts, both basic (40/60 mol% AlCls/ImCl) and acidic (60/40 mol% AICI3 /ImCl), were used in an electrochemical investigation of clusters... 

Coordination complexes that have metal halide clusters as their central cations represent a unique branch of coordination chemistry. The [M6X12]" cluster... 

All the hexamethylbenzene metal halide cluster cations exhibit characteristic electronic spectra which can be used for their identification in their green or brown solutions. Various attempts to obtain derivatives of [Nb3(/i-Cl)6(C6Me6)3] under the action of donors or organometallic reagents failed. 

A second feature of metal halide cluster chemistry is that the early transition metals are more prone to form metal -metal bonds than are the later noble metals and coinage metals. Again the polynuclear metal carbonyls differ in this facet of metal-metal bond behavior, and, in fact, metal carbonyl clusters become more common on going from the left to the right of the Periodic Table. 

Lowering the temperature at which cluster-forming solid-state reactions can be carried out is of some interest, particularly as a means of gaining a measure of kinetic control over the process. Several decades ago, it was discovered that the yields for forming tungsten halide cluster phases could be improved by... 

Excision reactions are sometimes accompanied by redox chemistry. For example, dissolution of the 2D solid Na4Zr6BeCli6 in acetonitrile in the presence of an alkylammonium chloride salt results in simultaneous reduction of the cluster cores (144). Here, the oxidation product remains unidentified, but is presumably the solvent itself. As a means of preventing such redox activity, Hughbanks (6) developed the use of some room temperature molten salts as excision media, specifically with application to centered zirconium-halide cluster phases. A number of these solids have been shown to dissolve in l-ethyl-2-methylimidazolium chloride-aluminum chloride ionic liquids, providing an efficient route to molecular clusters with a full compliments of terminal chloride ligands. Such molten salts are also well suited for electrochemical studies. 

Fig. 4.15 Some metal/halide clusters described in 1969 by Cotton, Ref. [122].

T. P. Martin, Alkali-Halide Clusters and Micro-Crystals. Phys. Rev. 1983, 95, 167-199. 

Technetium(II) complexes are paramagnetic with the d5 low-spin configuration. A characteristic feature is the considerable number of mixed-valence halide clusters containing Tc in oxidation states of +1.5 to + 3. This area has been reviewed (42). For convenience, all complexes, except those of [Tc2]6+, are treated together here. EPR spectroscopy is particularly useful in both the detection of species in this oxidation state and the study of exchange reactions in solution. The nuclear spin of "Tc (1 = f) results in spectra of 10 lines with superimposed hyperfine splitting. The d5 low-spin system is treated as a d1 system in the hole formalism (40). 

Both of these elements show a marked tendency to form metal atom cluster compounds in their lower oxidation states. The best known are oxo and halide cluster complexes. 

In the kinetic interpretation of ECL data presented above the closest approach of reactants has been assumed. It seems to be fulfilled in the case of organic ECL systems (because of the Coulombic attraction between oppositely charged ions). In contrast, in ECL systems involving identically charged transition-metal complexes, Coulombic repulsion may lead to an increase in the electron transfer distance. The molybdenum(II) halide cluster ion MoeCliJ [197-199] is the one of the best-understood examples. In electrochemical reactions MogCliJ is reversibly reduced or oxidized to stable MosCl and Mo6Cl["4 respectively ... 

Fig. 31. The excited state efficiency 4es as a function of the reaction exothermicity AGe (data from [198, 199]) in the annihilation reactions of MoeClJj with the tungsten halide cluster acceptor WXgYj and Mo6Cl[ 4 in dichloromethane solutions. AGe values are calculated from the differences in the standard redox potentials corrected for the Coulombic interactions between reactions and product. The shape of the parabolic curve corresponds to the outer reorganization energy An = 1.2eV.

Tungsten halide cluster ions 54 Two-level tunneling 73... 

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