Coordination Number and Stereochemistry

An important application of the crystal field theory is prediction of the geometry corresponding to different coordination numbers based on the magnitude of A and the nature of the bound ligands (Table 3.7). 

The use of the coordination number as a parameter in prediction of metal toxicity has potential significance, although the coordination number is difficult to determine in biological systems. Moreover, two or more coordination numbers are possible for some metal ions. 

Coordination Numbers (CN), Geometries, and Preferred Ligands for Selected Metal Ions in Biological Systems 

Metal Ion Coordination Number Geometry Ligand Atom D  

Source Adapted from J.A. Cowan, Inorganic Biochemistry An Introduction. (New York Wiley-VCH Inc., 1996) and R.M. Roat-Malone, Bioinorganic Chemistry A Short Course. Hoboken, NJ Wiley, 2003). 

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The most common coordination number of titanium is six (recognized for all oxidation states of the metal), although compounds are known in which the coordination number is four, five, seven or eight. The common oxidation states of titanium with the associated coordination numbers and stereochemistries are summarized in Table 3. The properties of these molecules will be discussed in the appropriate sections. In brief, however, titanium compounds in the +III or lower oxidation states are readily oxidized to the +IV state. Furthermore, titanium compounds can usually be hydrolyzed to compounds containing Ti—O linkages. 

Copper(II) has a 3d9 electronic configuration. In principle, pure octahedral and tetrahedral symmetries can never be observed because Jahn-Teller distortions (see Section 3.3.1) remove the orbital degeneracy of the ground state. The separation of the electronic energy levels depends on the coordination number and stereochemistry, as well as on the nature of the ligands. However, the ground state orbital is always well isolated from the excited states, and therefore the electronic relaxation mechanisms are relatively inefficient. Copperfll) complexes have thus relatively sharp EPR signals, and it is often possible to record these spectra at room temperature. 

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. 

Table 19-2 Oxidation States, Coordination Numbers, and Stereochemistry of Lanthanide Ions...

In some of these mixed gold-main group compounds, the coordination numbers and stereochemistry associated with main group atoms such as carbon and nitrogen are very... 

The most common coordination number of titanium is six, although four-, five-, seven-, and eight-coordinate compounds are known (Table 2). Table 3 summarizes the common oxidation states of titanium with the associated coordination numbers and stereochemistries. Zirconium shows a similar range of oxidation states (see Zirconium Hafnium Inorganic Coordination Chemistry), however, Zr and Flfr are much less stable, relative to Zr and Hf, than is the case for titanium. 

Interest in the lanthanide complex species is centered currently largely in coordination number and stereochemistry, in thermodynamic stability and its interpretation, in the effects of the ligand on the properties of the lanthanide ion, and in practical applications. 

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