Cluster compounds halides

PPha, pyridine) organic groups (olefines, aromatic derivatives) and also form other derivatives, e.g. halides, hydrides, sulphides, metal cluster compounds Compounds containing clusters of metal atoms linked together by covalent (or co-ordinate) bands, metaldehyde, (C2H40) ( = 4 or 6). A solid crystalline substance, sublimes without melting at I12 1I5" C stable when pure it is readily formed when elhanal is left in the presence of a catalyst at low temperatures, but has unpredictable stability and will revert to the monomer, ft is used for slug control and as a fuel. 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

Investigations of the interaction between 3d transition metals and octahedral halide or oxide metal clusters led to the preparation of a number of novel cluster compounds such as the series AxByNbgClig (A = Li, K, Rb, Cs B=Ti, V, Mn, Cu) [33], and TizNbgOu [34]. 

A review28 describes compounds of the Pt-group metals including halide, oxide, sulfide, coordination compounds, organometallic compounds, and metal cluster compounds. 

The tetrahedral cluster compounds can be synthesized by three methods (1) reaction of the monohalides EX (X = Cl, Br, I) with alkyllithium or Grignard reagents, (2) reduction of suitable organoelement halides of trivalent aluminum, gallium or indium, and (3) thermolysis of R2E-ER2 compounds. 

X-ray crystallography, 40 20-21 synthetic models, 40 23-48 xanthane oxidase, 40 21-23 chalcogenide halides, 23 370-377, 413 Chevrel phases, 23 376-377 metal-metal bonding, 23 330, 373 structural data, 23 373-376 as superconductors, 23 376 synthesis, 23 371-372 chloride, 46 4-24, 35-44 heterocations of, 9 290, 291 cluster compounds, 44 45-46 octahedral, 44 47-49, 53-63 electronic structure, 44 55-63 molecular structure, 44 53-54 synthesis, 44 47-49 rhomboidal, 44 75-82 solid-state clusters and, 44 66-72, 74-75, 80-82, 85-87 tetrahedral, 44 72-75 triangular, 44 82-87 cofactor, 40 2, 4-12 anaerobic isolation, 40 5 molybdopterin and, 40 4-8 reduced form, 40 12 synthesis, 40 8-12 xanthine oxidase, 45 60-63 complexes... 

NMR spectra heteronuclear gold cluster compounds, 39 345-348 Phalaris canariensis esophageal cancer, 36 144-145 scanning proton microprobe, 36 149 structural motifs of silicas, 36 146 Pharmaceuticals, 18 177 Phase transitions, in chalcogenide halide compounds, 23 332, 408, 412 [PhCHjMejNAlHjlj, 41 225-226 [(PhCH2)jNLi]3 molecular structure, 37 94, 96 in solution, 37 107-108... 

There are several interesting families of inorganic mixed-valence compounds that we have not discussed here (see Yvon, 1979 McCarley, 1982). For example, there are metal-cluster compounds such as the Chevrel phases, M,jMo6X8(X = S or Se) and condensed metal-cluster chain compounds such as TlMojScj, TijTe, NaMo O and M PtjO. TTF halides and TTF-TCNQ complexes (Section 1.9) constitute molecular mixed-valent systems in which the mixed valency is associated with an entire molecule the charge on TTF in such compounds is nonintegral. The structure of TTF-Br(, 79 and... 

Metal cluster compounds can be conveniently grouped into two classes (I) polynuclear carbonyls, nitrosyls. and related compounds and (II) halide and oxide complexes. The former group was included m Chapter 15. The second class will be discussed in this section.146... 

Several compounds with network structures which contain S2 units can be prepared by high-temperature reactions (250-700 °C). An example of a reaction of a metal halide with sulfur and S2C12 is shown in equation (7). The conditions under which these S2 -containing cluster compounds are formed illustrate their high stability. 

To relieve the strain of sterically demanding ligands, a metal often remains coordinatively unsaturated. Copper(I) halides and phosphines form cubane-like metal cluster compounds, Lm(CuX) ,12 With the bulky trimesitylphosphine, a monomeric two-coordinate [CuBr(Pmes)3] is formed, Br—Cu—P = 173.7°.252 The d(Cu—P) of 2.193 A is comparable to that in normal tet-rameric complexes, but d(Cu—Br) at 2.225 A is shorter, no doubt due to the reduced coordination number. Heating crowded complexes can also result in a reduction in coordination number (see equation 65). 

The ionic model is of limited applicability for the heavier transition series (4d and 5d). Halides and oxides in the lower oxidation states tend to disproportionate, chiefly because of the very high atomisation enthalpies of the elemental substances. Many of the lower halides turn out to be cluster compounds, containing metal-metal bonds (see Section 8.5). However, the ionic model does help to rationalise the tendency for high oxidation states to dominate in the 4d and 5d series. As an example, we look at the fluorides MF3 and MF4 of the triad Ti, Zr and Hf. As might be expected, the reaction between fluorine gas and the elemental substances leads to the formation of the tetrafluorides MF4. We now investigate the stabilities of the trifluorides MF3 with respect to the disproportionation ... 

The yields of these reactions are often improved by the addition of thallium(I) or silver(I) salts to act as halide scavengers and so generate the gold phosphine fragment in situ. This modification has been most extensively utilized in the addition of Au(PR3)+ to anionic and neutral cluster compounds, and a number of examples are given below ... 

The gold phosphine fragment has also been abstracted from heteronuclear cluster compounds as Au(PR3)X by reaction with halide ions... 

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