Spin only magnetic moment in BM of Fe CO 4 C 2 O 4 + is :

In ${\left[\mathrm{Fe}(\mathrm{CO}{)}_4\left({\mathrm{C}}_2{\mathrm{O}}_4\right)\right]}^{+}$

Oxidation no. of $\mathrm{Fe}=+3$ 

${}_{26}{\mathrm{Fe}}^{+3}=\left[{\mathrm{Ar}}_{18}\right]3{ \mathrm{d}}^{5}4{ \mathrm{s}}^0$ 

$\mathrm{CO}$ is a Strong field ligand, so paring occurs against the Hund's rule. 

${\mathrm{Fe}}^{+3}={{\mathrm{t}}_{2\mathrm{g}}}^{2,2,1} {\mathrm{eg}}^{0,0}$ 

Number of unpaired electron $=1$ 

so magnetic moment $\left(\mathrm{\mu }\right)=\sqrt{\mathrm{n}\left(\mathrm{n}+2\right)}$ B.M. $=\sqrt{1\left(1+2\right)}=\sqrt{3}=1.73 \mathrm{BM}$