Metal ions lower coordination number species

The concepts rely also on our understanding of preferred coordination numbers and geometries. For example, for many metal ions six-coordination is common, but some stable examples of both five- and seven-coordination are known. Therefore, it isn t too much of a jump to consider a short-lived five- or seven-coordinate species existing for a species stable only in six-coordination - in other words, a transition state of lower or higher coordination number. As an example, six-coordinate octahedral is the overwhelmingly dominant coordination mode for stable cobalt(ni) complexes. Yet, in recent years, rare examples of isolable but usually very reactive compounds with five-coordination and seven-coordination have been prepared. It doesn t take too much of an act of faith to assert that such geometries form as short-lived transition state species in substitution reactions of this whole family of complexes. 

Carbon dioxide, C02, is a fairly small molecule with acidic properties, which has frequently been used as a probe molecule for basic surface sites and as a poison in catalytic reactions. As shown in the following, C02 adsorption onto oxide surfaces leads to a variety of surface species such as bicarbonates and carbonates that coordinate to surface metal ions in various ways. The type of the coordination influences the symmetry of these ligands so that different surface species held by distinct surface sites can be distinguished by means of their infrared absorptions (162). The characteristic infrared (and Raman) bands of C02 and possible surface species are summarized in Table VI. The wave-number range below 1000 cm"1 was usually not accessible in studies on adsorbed C02 because of the strong absorption of the oxides at lower wave numbers. 

Nitriles. Acetonitrile adducts have been reported for several uranyl salts, including the chloride and nitrate. Although lower metal coordination numbers (e.g., five) have been suggested in the formulation of some of these compounds, it is likely that the species contain six- or seven-coordinate metal ions. As illustration of this, the molecular structure of U02Cl2(MeCN)2(H20) " possesses a seven-coordinate uranium center, with mutually traro-acetonitrile and chloride ligands. 

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