Bridging oxo groups

The inner-sphere mechanism is restricted to those complexes containing at least one ligand which can bridge between two metal centers. The commonest examples of such ligands are the halides, hydroxy or oxo groups, amido groups, thiocyanate... 

There is evidence that in the oxidized form, an oxo or hydroxo group bridges the Ni and Fe atoms, which should be absent in the enzymatically active reduced form. 

Type B is represented by 4D and 4E which contain only p-oxo groups. In 4D the metal atoms form a tetrahedron and are bridged by two p-oxo, two p-hydroxo and two p-phenoxo groups(lO). The Fe-p-oxo bond lengths, 1.791(3) A, are normal, similar to those found in p-oxo dinuclear complexes. The same type of bonds are 1.829(4) A long for the centrosymmetric 4E which comprises two... 

The two dinuclear units are linked by /13-oxo and /i-carboxylato groups. The third position in the "coordination sphere" of the /13-oxo groups is occupied by an unusual 5 coordinate, trigonal bipyramidal iron atom. This is the only case known so far in oxo-iron aggregates. The /13-oxo bridge is of type B since the surrounding metal atoms form an isosceles triangle. 

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