Corrins cavity

It is of interest to compare the porphyrin and corrin macrocycles, and to consider the reactivities of porphyrin- and corrin-bound metal ions. These may be related to the size of the cavity. The corrin macrocycle provides a smaller cavity, into which cobalt fits tightly in the B12 coenzymes. Thus the cobalt does not move relative to the corrin plane, and remains coplanar with the four nitrogen donor atoms. The cobalt remains low-spin, and reactivity is controlled by the loss of the axial ligands, giving five- and four-coordinate species. 

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