You can Use this formula to calculate Effective atomic number of coordination compounds.

EAN = [(atomic number of central metal) – (the oxidation state of the metal) + (the number of electrons gained by the metal from the ligands through co-ordination)]

EAN= [Z metal – (ox.state of the metal) + 2(coordination number of the metal)].

Now consider the following compound

[Co(NH3)6]3+ →EAN

= [27 – 3 + 2(6)]

= 36

You can read this concept in JD LEE