Porphyrin rings, Group 2 elements

Coenzyme B12 is the cofactor form of vitamin B 2, which is unique among all the vitamins in that it contains not only a complex organic molecule but an essential trace element, cobalt. The complex corrin ring system of vitamin B12 (colored blue in Fig. 2), to which cobalt (as Co3+) is coordinated, is chemically related to the porphyrin ring system of heme and heme proteins (see Fig. 5-1). A fifth coordination position of cobalt is filled by dimethylbenzimidazole ribonucleotide (shaded yellow), bound covalently by its 3 -phosphate group to a side chain of the corrin ring, through aminoisopropanol. 

Hypso-type spectra look similar to normal spectra but the B and Q bands are blue shifted (i.e. hypsochromically). This type of spectrum is shown by d-block elements with unfilled d-orbitals of the type d -d , which includes all the metals from groups VIII to IB. Here, d-electrons may be donated into the porphyrin s empty iT -orbitals (i.e. metal-to-ring charge transfer), thus raising their energy. This increases the energy of the porphyrin ir-ir transition, with respect to the metal-free porphyrin, leading to a blue... 

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