#460 CIF Metal-O=C... bond

[Geoff Hutchison]

From Miguel Quiros Olozabal:
- When an atom as O, N or S is bonded to a metal and also forms a double
bond to a carbon and thus should be represented as M, the
bond is transformed into simple to keep "standard valency" of atom X and
this introduces more inaccuracies, for example is quite frequent to find
something like M, a false trivalent carbon.