The Coordination Chemistry

Coordination compounds of vanadium are mainly based on six coordination, in which vanadium has a pseudooctahedral stmcture. Coordination number four is typical of many vanadates. Coordination numbers five and eight also are known for vanadium compounds, but numbers less than four have not been reported. The coordination chemistry of vanadium has been extensively reviewed (8—12) (see Coordination compounds). 

The coordination chemistry of hydrogen is still being intensively studied and new developments are continually being reported. 

Exciting developments have occurred in the coordination chemistry of the alkali metals during the last few years that have completely rejuvenated what appeared to be a largely predictable and worked-out area of chemistry. Conventional beliefs had reinforced the predominant impression of very weak coordinating ability, and had rationalized this in terms of the relatively large size and low charge of the cations M+. On this view, stability of coordination complexes should diminish in the sequence Li>Na>K>Rb> Cs, and this is frequently observed, though the reverse sequence is also known for the formation constants of, for example, the weak complexes with sulfate, peroxosulfate, thiosulfate and the hexacyanoferrates in aqueous solutions. 

The coordination chemistry of CO2 is by no means as extensive as that of CO (p. 926) but some exciting developments have recently been published. The first transition metal complexes with CO2 were claimed by... 

The coordination chemistry of Pb with conventional ligands from groups 14-16 and with macrocyclic ligands has recently been reviewed. 

The coordination chemistry of NO is often compared to that of CO but, whereas carbonyls are frequently prepared by reactions involving CO at high pressures and temperatures, this route is less viable for nitrosyls because of the thermodynamic instability of NO and its propensity to disproportionate or decompose under such conditions (p. 446). Nitrosyl complexes can sometimes be made by transformations involving pre-existing NO complexes, e.g. by ligand replacement, oxidative addition, reductive elimination or condensation reactions (reductive, thermal or photolytic). Typical examples are ... 

The coordination chemistry of SO2 has been extensively studied during the past two decades and at least 9 different bonding modes have been established.These are illustrated schematically in Fig. 15.26 and typical examples are given in Table 15.17.1 It is clear that nearly all the transition-metal complexes involve the metals in oxidation state zero or -bl. Moreover, SO2 in the pyramidal >7 -dusters tends to be reversibly bound (being eliminated when... 

The coordination chemistry of complexes in which Se is the donor atom has been... 

The coordination chemistry of Zn" and Cd", although much less extensive than for preceding transition metals, is still appreciable. Neither element forms stable fluoro complexes but, with the other halides, they form the complex anions [MX3] and [MX4] , those of Cd" being moderately stable in aqueous solution. "" By using the large cation [Co(NH3)6] + it is also possible to isolate the trigonal bipyramidal [CdCls] "... 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

The coordination chemistry in this oxidation state is essentially confined to the ions Sm", Eu and Yb . These are the only ones with an aqueous chemistry and their solutions may be prepared by electrolytic reduction of the Ln " solutions or, in the case of Eu", by reduction with amalgamated Zn. These solutions are blood-red for Sm", colourless or pale greenish-yellow for Eu" and yellow for Yb", and presumably contain the aquo ions. All are rapidly oxidized by air, and Sm" and Yb" are also oxidized by water itself although aqueous Eu" is relatively stable, especially in the dark. 

S. Trofimenko, Scorpionates—The Coordination Chemistry of Polypyrazolylborate Ligands, Imperial College Press, London, 1999. 

The fact that tantalum and niobium complexes form in fluoride solutions not only supplements fundamental data on the coordination chemistry of fluoride compounds, but also has a broad practical importance. This type of solution is widely used in the technology of tantalum and niobium compounds in raw material digestion, liquid-liquid extraction, precipitation and re-pulping of hydroxides, and in the crystallization and re-crystallization of K-salts and other complex fluoride compounds. 

Very recently, the coordination chemistry of low valent silicon ligands has been established as an independent, rapidly expanding research area. With the discovery of stable coordination compounds of silylenes [35-38], a major breakthrough was achieved. Within a short time a variety of stable complexes with a surprising diversity of structural elements was realized. Besides neutral coordination compounds (A, B) [35, 36, 38], and cationic compounds (C) [37], also cyclic bissilylene complexes (D) [39,40] exist. A common feature of the above-mentioned compounds is the coordination of an additional stabilizing base (solvent) to the silicon. However, base-free silylene complexes (A) are also accessible as reactive intermediates at low temperatures. 

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