explain how following complexes obey EAN rule Fe2(CO)9and Co4(CO)12 in inorganic chemistry?

Answer 1

Each CO is contributing 2e
Fe is 8e
Co 9e

For Fe2(CO)9:
2*8+2*9=16+18=34. In order for it to obey EAN it should have 36e (like el configuration of Kr). The extra 2e come from a Fe-Fe covalent bond.

For Co4(CO)12:
2*9+2*12=18+24=42. It should have 54 e (like el configuration of
Xe). You need 12e which come from 6 bonds between the Co atoms (imagine the four Co atoms on the corners of a tetrahedron and the 6 edges as the bonds)

Dimerization/oligomerization is a common way to get stable complexes when monomers are not obeying EAN

look at

http://everything2.com/index.pl?node_id=1456378