Coordination chemistry cations

Halpern J, Riley D P, Chan A S C and Pluth J J 1977 Novel coordination chemistry and catalytic properties of cationic 1,2-bis(diphenylphosphino)ethanerhodium(l) complexes J. Am. Chem. Soo. 99 8055-7... 

The majority of U(V1) coordination chemistry has been explored with the trans-ddo s.o uranyl cation, UO " 2- The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatihty as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned ia the iatroduction have been complexed with U(V1) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples iuclude U02X2L (X = hahde, OR, NO3, RCO2, L = NH3, primary, secondary, and tertiary amines, py n = 2-4), U02(N03)2L (L = en, diamiaobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J j)/H20 (x = 2-5). 

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 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. 

Lamb, J. D., Izatt, R. M., Christensen, J. J., Eatough, D. J. Thermodynamics and Kinetics of Cation-Macrocycle Interaction, in Coordination Chemistry of Macrocyclic Compounds (ed. Melson, G. A.), p. 145-217, New York, Plenum Press 1979... 

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. 

Coordination chemistry of aryldiazonium cations aryldiazcnato (arylazo) complexes of transition metals and the aryldiazenato-nitrosyl analogy. D. Sutton, Chem. Soc. Rev., 1975, 4, 443-470 (89). 

Synthesis and reaction chemistry of stable two-coordinate phosphorus cations (phosphenium ions). A. H. Cowley and R. A. Kemp, Chem. Rev., 1985,85, 367 (100). 

As illustrated in the previous sections, the uninegative poly(pyrazo-lyl)borato ligands provide a versatile system for coordination to cationic centers. However, in their triprotonated form, such ligands offer potential for acting as receptors for anions (Scheme 28). Indeed, since the coordination chemistry of anions (199) is significantly less developed than that of cations, protonated poly(pyrazolyl)borato ligands offer potential for exploring the chemistry of such systems. 

The sections below review the coordination chemistry of the most important classes of extractants used commercially. Particular attention is paid to the importance of secondary bonding between extractant components. This facilitates the assembly of ligating packages which match the coordination requirements of particular metal cations or their complexes and enhances both the selectivity and strength of extraction. Flydrogen bonding between ligands—e.g., esters of phosphorus(V) acids (see Section 9.17.4.3)—is particularly prevalent in the hydrocarbon solvents commonly used in industrial processes. 

Mederos, A. et al. Recent aspects of the coordination chemistry of the very toxic cation beryllium(II) The search for sequestering agents 22... 

Neutralization-reionization mass spectrometry (NRMS) is used to generate neutral species in the gas phase that are difficult to prepare or identify by other methods. During NRMS, both cations and anions may be neutralized, generally by collision, and then reionized to confirm the stability of the neutral species. Two reviews, with particular examples in coordination chemistry, provide good information on this method and offer many examples (76,77). A good example is AuF, which has been predicted to be stable. The [AuF]+ and AuF complexes were both neutralized and reionized and the AuF species was obtained in each recovery signal. It was postulated that the elusiveness of this molecule in the condensed phases was not due to instability but rather to inter-molecular reactions (78). 

Table I gives a selection of bare metal cation reactions with neural molecules. As Eller and Schwarz (9) comprehensively reviewed the area of bare metal ion reactions, most of the reactions discussed in this chapter will be of work later than 1990. Earlier work will be used if specific examples show interesting coordination chemistry or if the early examples differ from later work.

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