Transition-metal coordination mechanisms energetics

Many metal ions are essential to living cells. They are Na, K, Mg, Ca, Mn, Fe, Co, Cu, Mo, Zn and constitute about 3% of the human body weight. Na(I), K(I), and Ca(II) are particularly important in the 30-called ion pump mechanism where active transport of metabolites and energetic processes are taking place. Transitions metals such as Zn(II) and Co(II) are found in various metalloenzymes where they coordinate with amino acids and enhance catalysis at the active site (214). They act as super acid catalysts having a... 

The reduction of molecular nitrogen to ammonium and water oxidation to molecular oxygen causes six- and four-electron transfer to occur eventually in these reactions, respectively. Such processes obviously cannot occur in a single step. Analysis of toe thermodynamics of plausible intermediates rules out one-and two-electron transfers for both reactions and only four-electron mechanisms are energetically allowed.. Evidently, toe direct transport of four electrons fix>m (or to) a mononuclear or even binuclear transition metal complex appears to be ruled out Practically toe only possible variant of toe four-electron mechanism is toe conversion in toe coordination here of a transition metal polynuclear complex. 

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