Answer
The oxidation number of iron, Fe, in each of the following:
(a) $Fe(CO)_{5}$,
Oxidation number of Fe = 0
(b) $[Fe(dien)_{2}]^{3+}$,
Oxidation number of Fe = +3
(c) $[Fe(CN)_{6}]^{4-}$
Oxidation number of Fe = +2
Work Step by Step
(a) $Fe(CO)_{5}$,
Oxidation number of Fe = 0
CO is a neutral ligand. The complex has no charge. So, Fe is in zero oxidation state.
(b) $[Fe(dien)_{2}]^{3+}$,
Oxidation number of Fe = +3
dien (diethylene triamine) is a neutral ligand. The charge on the complex is due to oxidation state of Fe. So, Fe is in +3 state.
(c) $[Fe(CN)_{6}]^{4-}$
Oxidation number of Fe = +2
Each CN- has -1 charge. There are 6 CN- in the complex.
Fe +6CN- = -4
Fe +6(-1) = -4
Fe = -4+6 =+2
