Which among the following is a paramagnetic complex?
- [Co(NH3)6]3+
- [Pt(en)Cl2]
- [CoBr4]2-
- Mo(CO)6
Answer
(3) [CoBr4]2-
Solution
(1) [Co(NH3)6]3+: Co3+, 3d6; NH3 pairs the electrons (d2sp3, inner orbital) ⇒ no unpaired electron ⇒ diamagnetic.
(2) [Pt(en)Cl2]: Pt2+, 5d8, square planar (dsp2) ⇒ all paired ⇒ diamagnetic.
(3) [CoBr4]2-: Co2+, 3d7, tetrahedral (sp3) with weak field Br- ⇒ 3 unpaired electrons ⇒ paramagnetic.
(4) Mo(CO)6: Mo(0), 4d6 (after pairing by strong field CO), d2sp3 ⇒ diamagnetic.
Answer: (3)