18-1
Carbonyl Complexes
233
with four vacant orbitals. These orbitals may be sp
3 hybridized, and used by the
four C-O ligands to form four coordinate bonds, as in structure (10). The metal
then acquires formal negative charges of -4 for Ni, -5 for Co and -6 for Fe.
To remove formal charge from the metal and the oxygen atoms, we delocalize
metal electrons into antibonding C-O orbitals. For
4
Ni
 and
5
Co
 , we can delocalize four and five 3d electrons, to generate “increased-valence” structures (11)
and (12).
In each of these structures, we have obtained an uncharged metal atom, and
some (fractional) double bonding for the Ni-C and Co-C bonds. To generate
neutral iron in
2
4
Fe(CO)
 , we need to delocalize six electrons into antibonding
C-O orbitals. Because Fe
6– in structure (10) has only five lone-pairs of valence
electrons, we must generate “increased-valence” structure (13). The CO
– ligand of
this structure possesses two Pauling “3-electron bonds” with uncompensated
electron spins. Therefore because it can imply paramagnetism, structure (13) is