Atomic number of Cr and Fe are respectively 24 and 26 , which of...

Odd electrons, ions and molecules are paramagnetic. In (\mathrm{Cr}(\mathrm{CO})_6) molecule 12 electrons are contributed by CO group and it contains no odd electron. (\mathrm{Cr} \rightarrow 3 d^5 4 s^1) (\mathrm{Fe}(\mathrm{CO})_5) molecule also does not contain odd electron. ($$\begin{aligned} & \mathrm{Fe} \rightarrow 3 d^6 4 s^2 \\ & \mathrm{In}\left[\mathrm{Fe}(\mathrm{CN})_6\right]^4 \text{ ion } \mathrm{Fe}(+\mathrm{II}) \rightarrow 3 d^6 4 s^0 \end{aligned}$$) (\therefore) No odd electrons. (\text{In }\left[\mathrm{Cr}\left(\mathrm{NH}_3\right)_6\right]^{3+} \text{ ion } \mathrm{Cr}(+$\mathrm{III}$) \rightarrow 3 d^3 4 s^0) This ion contains odd electron so it is paramagnetic.