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Which one of the following is an outer orbital complex and exhibits paramagnetic behaviour?
A
$[Co(NH_3)_6]^{3+}$
B
$[Ni(NH_3)_6]^{2+}$
C
$[Zn(NH_3)_6]^{2+}$
D
$[Cr(NH_3)_6]^{3+}$
Detailed Solution
$Ni^{2+}$: $3d^8$. Two vacant 3d orbitals are not available even after pairing, so the 4s, 4p and two 4d orbitals are used: $sp^3d^2$ (outer orbital). It has 2 unpaired electrons, so it is paramagnetic.
$[Co(NH_3)_6]^{3+}$ ($d^6$) is $d^2sp^3$, inner orbital and diamagnetic.
$[Cr(NH_3)_6]^{3+}$ ($d^3$) is $d^2sp^3$, inner orbital.
$[Zn(NH_3)_6]^{2+}$ ($d^{10}$) is outer orbital but diamagnetic.
So $[Ni(NH_3)_6]^{2+}$ is the outer orbital, paramagnetic complex.
$[Co(NH_3)_6]^{3+}$ ($d^6$) is $d^2sp^3$, inner orbital and diamagnetic.
$[Cr(NH_3)_6]^{3+}$ ($d^3$) is $d^2sp^3$, inner orbital.
$[Zn(NH_3)_6]^{2+}$ ($d^{10}$) is outer orbital but diamagnetic.
So $[Ni(NH_3)_6]^{2+}$ is the outer orbital, paramagnetic complex.