Notes on Cobalamin (2V3N)

In cobalamin, vitamin B12, a cobalt atom is at the center of a corrin ring.  This ring is unlike the porphyrin ring in that the central metal atom bonds to three similar rings as opposed to the porpyrins where four similar rings are involved.  In the corrin ring the metal atom does bond to a fourth ring, but  that has only five atoms, not six as in the other rings.

Three or four clicks on "Connected Neighbors" then try to draw a Lewis structure, this is quite a challenge!