Cation size, metal coordination number

Whereas the metal coordination number would appear to be largely determined by cation size, distribution of positive charge is undoubtedly determinative for the detailed structure of the anion. This has been further demonstrated by Hartl and co-workers in studies involving pairs of isomeric cations, e.g., [N(CH3)4]+ and [CH3)3CNH3]+ and[(CH3)3NH]+ and[(CH3)2CNH3]+ (102). All four cations crystallize with iodocuprate(I) anions of stoichiometry [Cu2I3] composed of cop-... 

Moreover, pore and channel size and shape can be tuned by using different alkaline-earth cations (Mg2+, Ca2+, Sr2+, Ba2+) having different sizes and coordination numbers. The heat of water adsorption ranges from —46 to —52 kj mol-1, values similar to those of silica-containing zeolites. As recently reported, the properties of these materials can be further differentiated through the incorporation of transition (i.e. Co2+) or alkaline cations (i.e. K+) into the channels of barium-linked materials composed of metal-assembled cages. 

Compounds of the first group, complex oxides, maybe regarded as assemblies of ions of two (or more) metals and 0 ions. The numbers of oxygen ions surrounding the cations (their oxygen coordination numbers) are related to the sizes of the ions (Chapter 7). These coordination numbers are high (up to 12) for the largest ions, for example, Cs and Ba , and usually vary, within certain limits, in different structures. The crystal structures of complex oxides are described in Chapter 13. 

C-carboxylation reactions in nature use either CO2 or its hydrated form, HCO3 (Scheme 1.3), depending on whether the enzyme active site is hydrophobic or not. Often a metal cation is required as cofactor. The most used metal ions are Mg, Mn, Co and Fe, with some evidence for the involvement of -i-3 cations such as Co, Al and Fe (Table 1.4). The size of the cations, and their coordination number and charge density may play a key role in the stabilization of enzymes and in driving their catalytic activity. Cations with ionic radii in the range 85-110 pm [53] with a coordination number of 6 (octahedral geometry) are frequently encountered as cofactors in phosphoenolpyruvate carboxylases and other enzymes. The most abundant carboxylation enzyme in nature is ribu-lose l,5-bis(phosphate)-carboxylase-oxidase (RuBisCO) [54], which is found in all eukaryotes and the majority of prokaryotes. 

The dominant features which control the stoichiometry of transition-metal complexes relate to the relative sizes of the metal ions and the ligands, rather than the niceties of electronic configuration. You will recall that the structures of simple ionic solids may be predicted with reasonable accuracy on the basis of radius-ratio rules in which the relative ionic sizes of the cations and anions in the lattice determine the structure adopted. Similar effects are important in determining coordination numbers in transition-metal compounds. In short, it is possible to pack more small ligands than large ligands about a metal ion of a given size. 

Attention will be restricted here to fluorine compounds of the three d-transition series elements, in which the metal ion is octahedrally coordinated. Octahedral coordination, however, is found in nearly all these cases, which is quite reasonable considering the sizes of the ions in question. A close-packed octahedron of fluoride ions of radius 1.33 A adapts a size of its octahedral interstice appropriate to a sphere of radius 0.55 A. Cations having this size and larger ones meet the conditions of a contact between cations and anions. Thus stability is predicted for octahedral coordination until such contacts of ions become possible for coordination numbers higher than 6. For a coordination of 8 fluoride ions this is only the case if the radii of the cations are as large as 0.86 A (square antiprism) or 0.97 A (cube) resp. 

Why should the early transition metals form so many polyoxoanions The answer lies in the size of the M5/6+ cations and their -acceptor properties.1,5 The effective ionic radii of V5+ (0.68 A), Mo6+ (0.77 A) and W6+ (0.74 A) are consistent with the observation that these cations adopt four-, five- and six-fold coordination by oxide ion. With very few exceptions V, Mo and W atoms in heteropolyanions are six-coordinate. On the other hand Cr6+ (0.58 A) hap a maximum coordination number of four in oxides and oxoanions. Few isopoly- and heteropoly-chromates are known and they are all based on groups of corner-shared Cr04 tetrahedra [Cr207]2-, [Cr3O10]2-, [Cr4Oi3]2-, [03SOCrO3]2-, [02I0Cr03]-,... 

In the following presentation, solid-state structures documented hitherto for halogenocuprate(I) and halogenoargentate(I) ions are described in order of increasing coordination number of the metal, Possible correlations between the coordination number of copper(I) or silver(I) in the anion and properties of the cation with which it is coprecipitated, such as size, shape, and exposure of the positive charge, are then discussed. 

That there is a relationship between the coordination number of the metal in crystalline halogenocuprates(I) and halogenoargentates(I) and the properties of the cation with which it is coprecipitated would now seem to be well established. The tuning of anionic configurations to cation properties reflects the versatility in coordination requirements not only of the metal but also of the ligands. Cation size would appear to be of prime importance for the determination of a particular metal(I) coordination number, the tendency to attainment of a higher coordina-... 

Scheme I and, in more detail, Table 4 represent the trend of ionic radii of these large cations which prefer formal coordination numbers in the range of 8-12 [77]. For example, considering the effective Ln(III) radii for 9-co-ordination, a discrepancy of 0.164 A allows the steric fine-tuning of the metal center [60]. The structural implications of the lanthanide contraction can be visually illustrated by the well-examined homoleptic cyclopentadienyl derivatives (Fig. 2) [78], Three structure types are observed, depending on the size of the central metal atom A, [( j5—Cp)2Ln(ji— 5 rf — Cp)] x, 1 < % < 2 B Ln(fj5 —Cp)3 C, [fo -CpJjLnCi- 1 ff1—Cp)], these exhibit coordination numbers of 11 (10), 9, and 8, respectively. Also a small change in ligand substitution leads to a change in coordination behavior and number (10), as...

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