Coordination numbers cluster complexes

For trinuclear cluster complexes, open (chain) or closed (cycHc) stmctures are possible. Which cluster depends for the most part on the number of valence electrons, 50 in the former and 48 in the latter. The 48-valence electron complex Os2(CO)22 is observed in the cycHc stmcture (7). The molecule possesses a triangular arrangement of osmium atoms with four terminal CO ligands coordinated in a i j -octahedral array about each osmium atom. The molecule Ru (00) 2 is also cycHc and is isomorphous with the osmium analogue. 

The versatile binding modes of the Cu2+ ion with coordination number from four to six due to Jahn-Teller distortion is one of the important reasons for the diverse structures of the Cu-Ln amino acid complexes. In contrast, other transition metal ions prefer the octahedral mode. For the divalent ions Co2+, Ni2+, and Zn2+, only two distinct structures were observed one is a heptanuclear octahedral [LnM6] cluster compound, and the other is also heptanuclear but with a trigonal-prismatic structure. 

The sections are divided by the coordination number of the reacting ion defined as the number of donor atoms that interact with the metal. The nomenclature used for the ligands is L for neutral molecules that act as ligands and X for anions that act as ligands. Most of the examples in this section will involve cations [ML ]+ or [MX ]+, but there will be a short section on bare metal anions, M . The anions of more complexity than M will be discussed in Section IV on clusters. Many reactions produce an initial product that continues to react resulting in further coordi-native changes and possibly redox changes. Tables I and II will indicate the initial reaction product and other major reaction products. 

Ammonia also forms clusters in the gas phase and the reactions of ammonia clusters with bare metal ions have been studied (61). The ammonia clusters probed by electron impact as [(NH3) H]+ showed a monotonic decrease in intensity with increasing value of n, but the metal complex ions [M(NH3) ]+ showed intensity gaps. Thus for most of the metals the [M(NH3)2]+ ion was much more intense than the [M(NH3) ]+ ions, where n 2, and so the coordination number 2 was reported to be the favored coordination number in the first coordination sphere. The favored ions M(NH3)m]+ were n = 2 for Cr+, Mn+, Fe+, Co+, Ni+, and Cu+, and n = 4 for V+. The non-transition metal Mg+ and Al+ had the favored coordination number of 3. 

In dimeric complexes where the central cluster remains constant, the geometry of the cluster does not change appreciably as the peripheral jr-bonding ligands are changed — even when this changes the coordination number. When... 

This is one of two articles in this volume concerned with the borane-carborane structural pattern. In the other (see Williams, this volume, p. 67) Williams has shown how the pattern reflects the coordination number preferences of the various atoms involved. The purpose of the present article is to note some bonding implications of the pattern, and to show its relevance to a wide range of other compounds, including metal clusters, metal-hydrocarbon n complexes, and various neutral or charged hydrocarbons. 

They based this modification on the known adsorbance of OH on glass and on the common occurrence of transition metal mixed water-ammonia complexes with coordination number of 4. Parallel stractural studies of the deposited CdS showed textured growth, supporting a mechanism whereby alternate Cd and S species were involved, in an ion-by-ion process. Such a growth suggests adsorption of a molecular hydroxy-ammine species rather than a cluster. In fact, the mechanism of Ortega-Borges and Lincot also does not differentiate between a hydroxide cluster and molecule. 

EXAFS has been very useful in the study of catalysts, especially in investigating the nature of metal clusters on surfaces of the supported metal catalysts (Kulkarni et al, 1989 Sinfelt et al, 1984). A variety of systems has been examined already and there is still considerable scope for investigation in this area. Since EXAFS gives bond distances and coordination numbers and is absorber-selective, it is possible to study one metal at a time (Fig. 2.12). Thus, an EXAFS investigation of sulphided Co—Mo— Al20j and related catalysts has shown the nature of the reactive surface species (Kulkarni Rao, 1991). Cu/ZnO catalysts have revealed certain unusual features suggesting the complex nature of the species involved in methanol synthesis (Arunarkavalli et al, 1993). Time-resolved EXAFS is of considerable value for the study of catalysts (Sankar et al, 1992). 

The chemistry of tungsten is varied and complex not only because it covers nine oxidation states (-2 to +6), but also because of its ability to form complexes with different coordination numbers and geometries, and because of its tendency to form clusters and polynuclear complexes with a variety of metal atoms. 

Until several years ago, most of the known compounds of Wn were monomeric complexes, which exhibit mainly coordination number seven, and cluster compounds containing the octahedral W6 nucleus with W—W single bonds. The most important advance in the chemistry of W11 has undoubtedly been the preparation and characterization of dinuclear compounds containing W—W quadruple bonds. 

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