""EAN of Co(CO)₄ is 35. It attains stablity by dimerisation or reduction of Co""

Can u explain dimerisation concept in this question?

EAN rule is that, in a number of metal complexes the metal atom tends to surround itself with sufficient ligands that the resulting effective atomic number is numerically equal to the atomic number of the noble-gas element found in the same period in which the metal is present

Here the nearest inert gas is Kypton(36)

Since EAN of Co(CO)₄ is 35 , So the metals that cannot attain the EAN of the nearest inert gas in mono nuclear carbons will dimerize Co₂(CO)₈ in which oxidation state of metal increases by 1 unit.