Coordination Chemistry

1.1. Introduction;
Coordination chemistry, precisely, is the chemistry of metal atoms "coordinated" by atoms or molecules. Coordination chemistry has always been a challenge to the inorganic chemist. In the early days of chemistry they seemed unusual (hence the name complexes) and seemed to defy the usual rules of valence. Today they comprise a large body of current inorganic research. A survey of articles in recent issues of journal Inorganic chemistry indicates that perhaps 70% could be considered to deal with coordination compounds. Although the usual bonding theories can be extended to accommodate these compounds, they still provide stimulating problems to be resolved. In synthetic work, they continue to provide a challenge in the laboratory.

In these metalloprotiens, manganese can exist in any of the five oxidation states or in mixed valence states [107]. Potential importance of manganese complexes is evidenced by the realization that the active system in photosystem II (PSII) is a tetranuclear manganese complex[107]. Manganese coordination compounds are also of growing importance as homogeneous catalysts in oxidation reactions. Manganese has a vital role in many enzymatic systems such as peroxide dimutase, peroxidase, dioxygenase and catalase in which mononuclear manganese active sites are present [108].Metal complexes of manganese play important roles ranging from bioinorganic chemistry to solid state physics. Manganese coordination compounds are also of growing importance as homogeneous catalysts in oxidation reactions[109-110]. Manganese complexes are also studied for their magnetic behavior.The chemistry of manganese, in various oxidation states of the metal and in various combination of nitrogen and oxygen donor environment, is presently witnessing intense activity [111-112]. The common oxidation states of manganese are +2, +3, +4, +6 and +7, though oxidation states from +1 to +7 are observed.The most common and stable oxidation state of manganese is +2. Majorityof manganese complexes are high spin paramagnetic d5 systems and are