The correct order of the spin-only magnetic
moments of the following complexes is :
(I) [Cr(H2O)6]Br2
(II) Na4[Fe(CN)6]
(III) Na3[Fe(C2O4)3] ($\Delta$0 $>$ P)
(IV) (Et4N)2[CoCl4]
Solution
(I) [Cr(H<sub>2</sub>O)<sub>6</sub>]Br<sub>2</sub>
<br><br>H<sub>2</sub>O is weak field ligand so it can not pair up all the electrons.
<br><br>Cr<sup>2+</sup> : [Ar] 4s<sup>0</sup>3d<sup>4</sup> ($t_{2g}^3e{g^1}$)
<br><br>Unpaired e<sup>–</sup> = 4
<br><br>Magnetic moment = $\sqrt {24}$ = 4.89 BM
<br><br>(II) Na<sub>4</sub>[Fe(CN)<sub>6</sub>]
<br><br>CN<sup>-</sup> is strong field ligand so it pair up all the electrons.
<br><br>Fe<sup>2+</sup> = [Ar] 4s<sup>0</sup>3d<sup>6</sup> ($t_{2g}^6e{g^0}$)
<br><br>Unpaired e<sup>–</sup> = 0
<br><br>Magnetic moment = 0 BM
<br><br>(III) Na<sub>3</sub>[Fe(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>]
<br><br>As $\Delta$<sub>0</sub> $>$ P, so pairing of electrons happens.
<br><br> Fe<sup>3+</sup> = [Ar] 4s<sup>0</sup>3d<sup>5</sup> ($t_{2g}^5e{g^0}$)
<br><br>Unpaired e<sup>–</sup> = 1
<br><br>Magnetic moment = $\sqrt 3$ = 1.73 BM
<br><br>(IV) (Et<sub>4</sub>N)<sub>2</sub>[CoCl<sub>4</sub>]
<br><br>Cl<sup>-</sup>is weak field ligand so it can not pair up all the electrons.
<br><br>Co<sup>2+</sup> = [Ar] 4s<sup>0</sup>3d<sup>7</sup> ($e{g^4}t_{2g}^3$)
<br><br>Unpaired e<sup>–</sup> = 3
<br><br>Magnetic moment = $\sqrt {15}$ = 3.87 BM
<br><br>Hence order of magnetic moment is
<br>I > IV > III > II
About this question
Subject: Chemistry · Chapter: Coordination Compounds · Topic: Ligands and Coordination Number
This question is part of PrepWiser's free JEE Main question bank. 173 more solved questions on Coordination Compounds are available — start with the harder ones if your accuracy is >70%.