Vitamin B12, also known as Cobalamin, is an amine ring molecule consisting of four amine rings and a tail (marked in Blue)

It is active because it contains a Cobalt ion in the centre (shown in pink - the Cobalt ion gives Vitamin B21 its pink colour) - the amine rings (with N- atoms at the inside) are slightly negatively charged, and the Cobalt is strongly positively charged, so it is fairly stable in this configuration.

The tail (in blue, at the bottom of the diagram) keeps the Cobalt ion in place from one side, and the molecule is active because of the R-group, or REACTIVE group, on the top side (shown in green).

This reactive group is only lightly held, but the link between the Cobalt ion and the R-group is a very special one, and Vitamin B12's ability to change its R group easlily is absolutely vital to a lot of the body's functions.

Forms of B12

The main natural forms of B12 in mammals are methyl-cobalamin and Adenolsyl-cobalamin.  In each case, the start of the name is the R-group.

The Methyl group is small, uncharged, and easy to replace.  The molecule is fairly stable because the R-group is small (doesn't fall off easily)

The Adenosyl group is a bit larger, which means that adenosyl-cobalamin isn't very stable - can't be stored.  It is the highly energetic form of cobalamin (is used by the body to generate energy wherever it's needed)

The easiest forms of B12 to manufacture outside the body are hydroxy-cobalamin and cyano-cobalamin.  Both are fairly stable - no, cyanocobalamin is very stable and is the form usually sold.

Both R-groups are highly charged and small, so they bind tightly to the Cobalt ion.  This is why they are so stable.  It also means that some people can't get the artificial R-group off the cobalamin and we know that some people can't use cyanocobalamin because it is too stable.