Question

Atomic numbers of Cr and Fe are respectively 25 and 26 , which of the following is paramagnetic with the spin of the electron?

(a) \(\left[\mathrm{Cr}(\mathrm{CO})_{6}\right]\)

(b) \(\left[\mathrm{Fe}(\mathrm{CO})_{5}\right]\)

(c) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\)

(d) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\)

Answer 1

Correct option is (d) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\)

Out of CO, CN^- and NH₃, increasing field strength is NH₃ < CN⁻ <CO

Cr has 24 electrons, Fe → 26 electrons.

Fe²⁺ has 24 electrons and Cr⁺³ has 21 electrons.

The 4 unpaired electrons in Cr and Fe can pair up because of strong field ligand ′CO′. CN^- also strong field ligand. So, 4 electrons in Fe²⁺ also pair up.

Though NH₃ is a strong field ligand, only one electron remains in Cr⁺³. So no case of pairing here and [Cr(NH₃)₆]³⁺ is paramagnetic.