Using EAN $= Z - (oxidation state) + 2 \times (number of monodentate ligands)$ , and the nearest noble gas being krypton ( $Z = 36$ ), which complex ion has an effective atomic number equal to 36? ( $Z$ : Cr 24, Fe 26, Co 27, Ni 28)

Answer: A

EAN for

[Co (NH X_{3}) X_{6}] X^{3 +}

27 - 3 + 12 = 36

. For the others:

[Fe (CN) X_{6}] X^{3 -}

26 - 3 + 12 = 35

[Cr (NH X_{3}) X_{6}] X^{3 +}

24 - 3 + 12 = 33

and

[Ni (NH X_{3}) X_{6}] X^{2 +}

28 - 2 + 12 = 38