Question

what is the oxidation number of Fe in [Fe(H2O)5NO]SO4

Answer 1

The oxidation number of Fe is +1.
This complex is formed in the 'brown ring' test for the nitrate ion, in which Fe is in +1 state, and NO exists as NO+.

Answer 2

Answer: +2

Explanation: Oxidation number is defined as the charge that the central atom carries as calculated by assigning appropriate charges to the ligands and equating the sum of the charges on the central atom and ligands equal to the charge on the coordination sphere.

Let the oxidation state of Fe is, x

As there are 5 water ligands which are neutral and hence carry no charge. The nitrosyl ligand is also a neutral ligand and carry a charge of 0. The sulphate carries a charge of -2 but as the molecule in whole is a neutral the coordination sphere $[Fe(H_2O)_5NO]$ must carry a charge of +2.

$X+5(0)+1(0)=+2$

So, the oxidation state of Fe is +2.