Coordination number of metal ion

Solvent coordination numbers of metal ions in solution. S. F. Lincoln, Coord. Chem. Rev., 1971, 6, 309-329 (133). 

The coordination numbers of metal ions range from I, as in ion pairs such as Na CI- in the vapor phase, to 12 in some mixed metal oxides. The lower limit, I. is barely within the realm of coordination chemistry, since the Na+CI km pair would not normally be considered a coordination compound, and there are few other examples. Likewise, the upper limit of 12 is not particularly important since it is rarely encountered in discrete molecules, and the treatment of solid crystal lattices such as hexagonal BaTiOj and perovskite1 as coordination compounds is not done frequently. The lowest and highest coordination numbers found in typical coordination compounds are 2 and 9 with the intermediate number 6 being the most important. 

Table 3 The Coordination Numbers of Metal Ions in edta Complexes from X-Ray Structure Analyses...

There are some correlations between coordination number of metal ions (as well as geometry of the derived complexes) and metal-binding site in nucleic acid bases. Unhindered donor atoms (e.g. N-7 of purines) can be involved in octahedral complexes, while atoms that are in sterically hindered environments (e.g. N-3 of C, which is adjacent to the bulky N" H2 and groups, or N-1 of A, which is influenced sterically by the adjacent N H2 substituent) prefer to form complexes in which the metal ion has coordination numbers of four or two. ... 

Complex Coordination number of metal ion Ionic radius (A) Macrocycle fold angle (deg) Ref. ... 

Similarly determined EAN values for other metal complexes in many cases equal the atomic numbers of noble gases. There are, however, many exceptions to this rule examples are [Ag(NH3)2] and [Ni(en)3 with EAN values of 50 and 38, respectively. This is unfortunate for if the EAN of the central metal always exactly equaled the atomic number of a noble gas, then it would be possible to estimate the coordination number of metal ions. 

Extractive separation of metals is usually based on complex formation with inorganic and organic ligands. Therefore the use of the ideas, approaches, and methods of coordination chemistry has always been a most fruitful approach to the extraction of the elements. History shows that many problems of selectivity of separation or enhanced isolation have been successively solved by the rational application of coordination chemistry, e.g., the concept of hard and soft acids and bases. The efficiency of extraction depends on, inter alia, the ratio of charge and coordination number of metal ion. Study of this effect permitted the development of ways to improve separation due to changes in hydration of the species to be extracted. 

Ammine complexes. For ammine complexes, situations are more complicate than those in the case of aqua complexes, because the coordination numbers of metal ions sometimes change depending on the number of ammonia molecules coordinated. 

Thus, this is an alternative method for the estimation of the mean ion radius in mixed-valence oxides or in non-stoichiometric oxides, using values of metal ion radii in various oxides of the same metal. The above method allows for the prediction of a mean ionic radius in oxides having so much deformed structure that it is difficult to determine the coordination number of metal ions or this number changes with the oxidation state. 

TABLE 24.1 Some Common Coordination Numbers of Metal Ions ... 

This procedure has been used successfully to determine the composition of many complexes in solution. It is possible to extend this method to cases where more than one complex is formed but the application is quite difficult. Like the logarithmic method, Job s method can be applied to other cases of molecular interaction and is not limited to the formation of coordination compounds. Both methods are based on the assumption that one complex is dominant in the equilibrium mixture. Numerous other methods for determining the number of metal ions and ligands in complexes have been devised, but they are beyond the introduction to the topic presented here. 

One cannot help but be impressed by the explorations in the past decade of the rates of rapid changes in conformation and in coordination number of metal complexes (36-42). For example, equilibria between inclusive (metal ion completely enclosed) and exclusive (metal ion only partly surrounded by organic ligand)... 

The technique of vertical dipping for construction of multilayer LB films containing metal ions is depicted in Figure 3.5.2. Centrosymmetric bilayers, with metal ions sandwiched between the polar head groups in the film, are deposited onto vertically mounted substrates passed down and then up through the metal/surfactant monolayer at the air/water interface. The structure of the film is represented by the bis chelate coordination of the carboxylate function of the fatty acid around a divalent metal ion. This type of structure has been determined for a number of metal ions including Cd, Co, Mn, Pb and Zn (24 and ref. within). The structure of CoSt (25), inferred from infrared (IR) and XRD studies, is depicted in Figure 3.5.3a (25). Centrosymmetric films, where the area of the film transferred to the substrate... 

A number of metal ions of variable valence, for example FeK, Ti111 and V11, can reduce silver. Among the more significant coordination compounds are those of iron and titanium. Iron(II) ion, in the presence of fluoride or pyrophosphate to sequester the Fem formed, is a developer. The bis(oxalato)ferrate(II) ion, [Fe(C204)2p, is also a developer in the presence of excess oxalate. 

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