Question

Which among the following is a paramagnetic complex :

• A. $[Co(NH_3)_6]^{3+}$
• B. $[Pt(en)Cl_2]$
• C. $[CoBr_4]^{2-}$
• D. $Mo(CO)_6$
  (At. No. Mo = 42, Pt = 78)

Answer 1

Answer: (C)

$Co ^{2+}=\left(3 d ^{5}\right)$
Bromine is a weak ligand but it is known that all tetrahedral complexes are high-spin regardless of the splitting power of the ligand.
The low spin arrangement has five unpaired electrons in the d-orbital.
So it is paramagnetic in nature.