Review of Complexation and Complex Species

A complex or coordination compound generally consists of one or more groups (molecules or anions) surrounding and bonded to a central metal (cation or neutral atom). Depending upon the charges on the central metal and bonded groups, the complex can be cationic, anionic, or neutral. Examples are  

The stability (or tendency of the donor atoms to bond to the central metal) of a complex is expressed in terms of a formation constant, ( vhere n is the number of ligands attached to the central metal). Formation constants are equilibrium constants, K, written for the formation of a complex from the fi-ee ligands and central metal. Consider the following example of the formation of several Ni complexes with ammonia  

It should be noted that the larger the formation constant the more stable the complex. Consider the following example of Ni+ complexes with cyanide and ammonia  

Cyanide forms a much more stable complex with Ni+ than ammonia. 

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