The only hexanuclear carbonyl whose structure has been
determined is Rh6(CO)16 shown on the left. The six Rh metal
atoms are at the corners of an octahedron. Four bridging
carbonyl groups are present, each sharing a pair of electrons
with three Rh atoms on a face. (Multicenter bond, 2/3 electrons
shared with each Rh atom!). Two more terminal CO groups are
present on each Rh atom (those on two Rh atoms in the
picture are omitted for clarity). Thus the number of electrons
around each Rh appears to be [14(Rh) + 22/3(bridge) +
22(terminal) = 91/3] whereas each Rh requires only 9
electrons for inert gas configuration. Thus there are a total of
61/3 = 2 electrons extra in the molecule, and it appears that
the E.A.N. rule is not followed unless the formula is
[Rh6(CO)16]2+.
Metallic carbonyls containing large numbers of CO groups (many of which were described above) are