The Various Coordination Numbers

The term secondary valency has been superseded by the term coordination number . This may be defined as the number of donor atoms associated with the central metal atom or ion. For many years a distinction was made between coordination number in this sense and in 

Wilkinson, R. D. Gillard and J. A. McCleverty (eds.), Comprehensive Coordination Chemistry, Pergamon Pre.ss, Oxford, Vol. 1, 1987, 613 pp. D, L, Kepert, Inorganic Stereochemistry, Springer-Verlag, Berlin, 1982, 227 pp, J. A. Davies, C. M. Hockensmith, V. Yu, Kukushkin and Yu. N. Kukushkin, Synthetic Coordination Chemistry Principles and Practise, World Scientific Publ., Singapore, 1996, 452 pp. 

The most commonly occurring coordination numbers for transition elements are 4 and 6, but all values from 2 to 9 are known and a few examples of even higher ones have been established. The more important factors determining the most favourable coordination number for a particular metal and ligand are summarized below. However it is important to realize that, with so many factors involved, it is not difficult to provide facile explanations of varying degrees of plausibility for most experimental observations, and it is therefore prudent to treat such explanations with caution. 

Representative examples of the stereochemistries associated with each of the various coordination numbers will now be discussed. 

Examples of this coordination number are virtually confined to linear Dock complexes of Cu Ag Au and Hg of which a well-known instance is the ammine formed when ammonia is added to an aqueous solution of Ag+ [H3N-Ag-NH3]+ 

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The number of coordination compounds containing an azole-type ligand for which a crystal structure determination is available is very large. This allows one to make a comparison of the various coordination numbers and valence (or spin) states of the different metal ions. Such a comparison of the M—N distances is shown in Table 2. 

Examples of the various coordination numbers and stereochemistries are presented in Table 19-2. The most common coordination numbers are 8 and 9. Many seeming examples of coordination number 6 are invalid because coordinated solvent molecules are present and raise the actual coordination number to 7, 8, or 9. 

A complexity, yet at the same time a source of fascinating chemistry, of systems based on phosphorus is that the phosphorus may appear in functional groups where this atom is attached to anywhere from one to six other atoms. This has led to a description of the group by its coordination number (C.N.), which refers to this number of attached atoms. Thus simple phospholes are designated as having C.N.-3, but the coordination number may range from two to six. However, some of these coordination numbers are rather rare, and most of phosphole chemistry is based on C.N.-3 and C.N.-4, which includes especially the oxides, sulfides, and salts of the phosphines. Structures for the various coordination numbers are given below. 

Verify the cation/anion radius for the various coordination numbers shown in Table 5.1. 

Efficient Methods for Preparing Silicon Compounds is a unique and valuable handbook for chemists and students involved in advanced studies of preparative chemistry in academia as well as industry. Organized by the various coordination numbers (from two to six) of the central silicon atom of the reported compounds, this book provides researchers with a handy and immediate reference for any compound or properties needed in the area. 

A selection of phosphorus-containing compounds, which illustrate the various coordination numbers that phosphorus can adopt in its compounds. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

Analysis of the 1H and 13C NMR spectra of the tin complexes, being derivatives of various di-Schiff bases, [46] has revealed the dependence of the position of tin centre on the type of linker between the two imine units and the metal coordination number.110... 

References to the so-defined AET and the corresponding coordination number, indicated as CNE, are reported for various prototype structures listed in Chapter 7. 

The structures, relative stabilities, and relative Lowry-Bronsted acidities of carboranes and boranes as well as related anions, Lewis base adducts, and heteroelement analogs are rationalized primarily on the basis of rudimentary coordination numbers. The principal factors, in decreasing order of importance, are (a) the various deltahedra and deltahedral fragments, (b) the placement of bridge and endohydrogens, (c) the placement of carbon and other heteroelements, and d) the resulting coordination number of boron. 

Coluccia et al. (5) proposed a model of the MgO surface that shows Mg-O ion pairs of various coordination numbers (Fig. 1). MgO has a highly defective surface structure showing steps, edges, corners, kinks, etc., which provide sites of low... 

Fig. 4. Radius ratios for various coordination numbers. Also shown are the types of coordination polyhedra, and cation radii that fit these criteria. Note that most metal ions of biological interest have coordination numbers of 6 or more.

Fig. 6.3. Ideal (average observed) coordination number versus predicted coordination number for various cations. The predicted coordination numbers are calculated using the ratios of the Pauling (1960) six-coordinate radii by interpolating coordination numbers between the values given in Table 6.1. Most cations are represented by points, selected cations are shown by their chemical symbol and, where necessary, oxidation state.

The previous chapter described the bonding principles responsible for the energetics and structure of coordination compounds. In this chapter the resulting structures will be examined in more detail with particular regard to (he existence of various coordination numbers and molecular structures, and the effect of these structures on their chemical and physical properties. 

A number of representative nickel(II) complexes prepared with Schiff bases derived from pyridine-2-carbaldehyde, pyridine-2,6-dicarbaldehyde and related species are summarized in Table 98, together with some of their distinctive physicochemical properties and preparative routes. All of these complexes involve N and either O or S as donor atoms and exhibit various coordination numbers and geometries depending on the denticity of the ligands and on their steric and electronic requirements. 

Since Dickinson s first determinations, crystal structures of many other complexes of various coordination numbers have been determined. All these investigations and others have provided a complete and direct confirmation of Werner s views to support his indirect configurational proofs obtained during the previous decades by preparation of isomers and resolution of optically active compounds (see Section 1.1.4), and today the terminology and concepts of coordination theory are routinely used in crystallography. 

While it is obvious that the principal quantum number n must be a positive integer (it is impossible to have less than zero nodes in a wavefunction), the values of the other quantum numbers can be negative when they are defined in polar coordinates. The allowed values of the various quantum numbers are... 

It may be concluded that the steric bulk of the alkyl substituents (R) is an extremely important factor in stabilizing the various coordination modes and thus steers the course of the reactions. However, the tendency to form metal-metal bonds and the electronic influence of R may also be important. In this respect it is of interest that in the case of the analogous iron systems the number of products is much smaller. For example Fe2(CO)9 reacts with R-DAB according to equations (8) and (9). 

While low coordination number sites, steps, and kinks, are the active sites for bond breaking in platinum, the atomic terrace sites with larger coordination numbers may also become active sites with unique chemistry for other elements. It will perhaps become possible to identify the bond-breaking ability of various coordination number sites of a given metal in breaking H—H, C—H, C —C, 0=0, N=N, etc., chemical bonds. By varying the atomic surface structure, which would change the relative concentrations of the different coordination number surface sites, the product distribution in surface chemical reactions may be markedly varied. 

Limitations of space preclude a comprehensive survey of the coordination chemistry of minerals. The reader is referred to the general references in this section for more information. Naturally, many other examples might have been chosen to exemplify various coordination numbers, geometries and complexes found in minerals. Nevertheless, the various species mentioned above do indicate the wide range of coordination chemistry in the solid state found in the mineral kingdom and the complexity of coordination compounds in the natural environment. 

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