Cluster Compound

Cluster compounds have been included iu previous sections of this chapter and earlier chapters. Transition-metal cluster chemistry has developed rapidly since the 1980s. Beginning with simple dimeric molecules, such as Co2(CO)g and Fe2(CO)9, chemists have developed syntheses of far more complex clusters, some with interesting and unusual structures and chemical properties. Large clusters have been studied with the objective of developing catalysts that may improve on the properties of heterogeneous catalysts the surface of a cluster may mimic the behavior of the surface of a solid catalyst. 

Molecular Metal-Metal Bonds Compounds, Synthesis, Properties, First Edition. 

A number of examples, where two or more isomers of metal carbonyl clusters with identical metal core geometries and different ligand dispositions have been struc- 

For mechanistic studies, special force fields for transition states have been developed [110, 307], but again the reference data are usually based (at least partly) on DFT calculations. 

The ratio of the first three force constants is about 6 3 1. Although the NbNb stretching constant was estimated to be 1 mdyn/A, this value does not represent the strength of this bond, since such a value can be obtained without any M—M interaction. Rather, these workers suggest the absence of Nb—Nb interactions for Jq [Nb(V)] ions because the M—M bond breathing mode, which normally appears strongly in Raman spectra, is weak. Mattes et al. [1463] also carried out normal coordinate analysis on the same system and obtained a ratio of 8 4 1 for the three M—O stretching force constants mentioned above. 

The Raman spectra of the [Ta60i9] ion [1464] and derivatives of the [Mo60i9] ion such as (Mo60i8](NPh)] [1465] have been reported. 

---

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. 

Elemental composition, ionic charge, and oxidation state are the dominant considerations in inorganic nomenclature. Coimectivity, ie, which atoms are linked by bonds to which other atoms, has not generally been considered to be important, and indeed, in some types of compounds, such as cluster compounds, it caimot be appHed unambiguously. However, when it is necessary to indicate coimectivity, itaUcized symbols for the connected atoms are used, as in trioxodinitrate(A/,A/), O2N—NO . The nomenclature that has been presented appHes to isolated molecules (or ions). Eor substances in the soHd state, which may have more than one crystal stmcture, with individual connectivities, two devices are used. The name of a mineral that exemplifies a particular crystal stmcture, eg, mtile or perovskite, may be appended. Alternatively, the crystal stmcture symmetry, eg, rhombic or triclinic, may be cited, or the stmcture may be stated in a phrase, eg, face-centered cubic. 

A particularly significant part of rhenium chemistry involves cluster compounds in which there is metal—metal bonding. This chemistry centers largely around the +3 oxidation state. 

Tungsten dichlofide [13470-12-7], WCI2, is an amorphous powder. It is a cluster compound and maybe prepared by the reduction of the hexachloride with aluminum in a sodium tetrachloroalurninate melt (11). 

The nomenclature of boron hydride derivatives has been somewhat confusing and many inconsistencies exist in the Hterature. The stmctures of some reported boron hydride clusters are so compHcated that only a stmctural drawing or graph, often accompanied by explanatory text, is used to describe them. Traditional nomenclature systems often can be used to describe compounds unambiguously, but the resulting descriptions may be so long and unwieldy that they are of Htde use. The lUPAC (7) and the Chemical Abstract Service (8) have made recommendations, and nomenclature methods have now been developed that can adequately handle nearly all clusters compounds however, these methods have yet to be widely adopted. Eor the most part, nomenclature used in the original Hterature is retained herein. 

J. B. Casey, W. J. Evans, W. H. PoweU, and T. E. Sloan, "A Stmctural Definitive Descriptor and Numbering System for Cluster Compound Nomenclature," Chemical Abstract Service, presented at the 198th National Meeting of the American Chemical Society, Miami Beach, FI., Sept. 1989. 

Piezochromism has been observed in a wide variety of materials. Three classes which illustrate well some of the generalizations that have been developed are organic molecules in crystals and polymer films, metal cluster compounds, and organometaUic complexes of Cu(II). 

Many novel cluster compounds have now been prepared in this way, including mixed metal clusters. Further routes involve the oxidative fusion of dicarbon metallacarborane anions to give dimetal tetracarbon clusters such as (103) and (104) O (jjg insertion of isonitriles into inetallaborane clusters to give monocarbon meiallacarboranes such as (105) and the reaction of small ii/t/o-carboranes with alane adducts such as Et3NAlH3 to give the commo species (106) ... 

Phosphaborane cluster compounds have also been synthesized. Eor example, thermolysis of a 1 2 mixture of (Pr 2N)BCl and (Pr2N)B(Cl)-(SiMe3)2 at 160°C results in the smooth elimination of Me3SiCl to give colourless crystals of [c/oso-l,5-P2(BNPr )3] (127) in high yield ( 33)... 

In all the cluster compounds discussed above there are sufficient electrons to form 2-centre 2-electron bonds between each pair of adjacent atoms. Such is not the case, however, for the cationic bismuth species now to be discussed and these must be considered as electron deficient . The unparalleled ability of Bi/BiCb to form numerous low oxidation-state compounds in the presence of suitable complex anions has already been mentioned (p. 564) and the cationic species shown in Table 13.12 have been unequivocally identified. 

These and other classes of cluster compounds will be dealt with more fully in later chapters devoted to the chemistry of the metals involved. 

Niobium and tantalum provide no counterpart to the cationic chemistry of vanadium in the -t-3 and -t-2 oxidation states. Instead, they form a series of cluster compounds based... 

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. 

However, like the mp, bp and enthalpy of atomization, it also reflects the weaker cohesive forces in the metallic lattice since for Tc and Re, which have much stronger metallic bonding, the -t-2 state is of little importance and the occurrence of cluster compounds with M-M bonds is a dominant feature of rhenium(III) chemistry. The almost uniform slope of the plot for Tc presages the facile interconversion between oxidation states, observed for this element. 

A whole series of alkyl cluster compounds ResClsRe has been prepared by reacting ResClg with a large excess of RMgCl in... 

---