Question
[Cr(NH3)6)3+ is paramagnetic while [Ni(CN)4]2 – is diamagnetic. Explain why?
Solution
[Cr(NH3)6]3+: Outer electronic configuration of chromium (Z = 24) in ground state is 3d54s1 and in this complex it is in the +3 oxidation state. Chromium achieves the +3 oxidation state by the loss of one 4s electron and two 3d-electrons. The resulting Cr3+ ion has outer electronic configuration of 3d3. The two vacant 3d-orbitals, one 4s-orbital and three 4p-orbitals hybridise to give six equivalent d2sp3 hybrid orbitals. Six pairs of electrons, one from each NH3 molecule occupy the six vacant hybrid orbitals so produced. The resulting complex ion has one octahedral geometry. Since the complex contains three unpaired electrons, it is paramagnetic.
[Ni(CN)4]2– : Outer electronic configuration of nickel (Z = 28) in ground state is 3d 84s2. Nickel in this complex is in the +2 oxidation states. It achieves +2 oxidation state by the loss of the two 4s- electrons. The resulting of Ni2+ ion has outer electronic configuration of 3d8. The two unpaired 3d- electrons are forced to pair up.
The resulting vacant 3d-orbital along with the 4s-orbital and two 4p-orbitals hybridised to give four equivalent dsp2 hybridised orbitals. Four pairs of electrons, one from each cyanide ion occupy the four vacant hybrid orbitals so produced. Therefore, it is diamagnetic because there is no unpaired electrons.
