Which of the following statements is correct ?

Options

A. `[Mn(CN)_(6)]^(3-)` has magnetic moment of two unpaired electrons while `[Mn(CI)_(6)]^(3-)` has a paramagnetic moment of four unpaired electrons.
B. `[Fe(CN)_(6)]^(3-)` has mahnetic moment of a single unpaired electron while `[FeF_(6)]^(3-)` has a paramagnetic moment of five unaired electrons
C. `[CoF_(6)]^(3-)` is paramagnetic with four unpaired d electrons while `[Co(C_(2)O_(4))_(3)]^(3-)` is diamagnetic
D. All of these

Answer

D

Explanation

The magnetic moment of coordination compounds can be measured by the magnetic susceptibility experiments. The results can be used to obtain information about the structures adopted by metal complexes. A critical study of the megnetic data of coordination compounds of metals of the first transition series reveals some complications. For metal ions with upto three electrons in the 3d subshell, like `Ti^(3+) (d^(1)), V^(3+) (d^(2)) Cr^(3+) (d^(3))` , two vacant 3d orbitals are available for octahedral hybridization `(d^(2)sp^(3))` with 4s and 4p orbitals. The magnetic behaviour of these free ions and their coordination entities are similar. When more then three 3d electrons are present, the required pair of 3d orbitals for octahedral hybridization `(d^(2)sp^(3))` is not directly available (as a consequence of Hand's rule). Thus, for `d^(4)(Cr^(2+) , Mn^(3-)) d^(5) (Mn^(2+) , Fe^(3+)) , d^(6) (Fe^(2+) , Co^(3+))` cases, a vacant pair of 3d orbitals results only by pairing of of 3d electrons (possible with strong field ligands) which leaves two, one and zero unpaired electrons, respectively. The magnetic data agree with maximum spin pairing in many cases, especially with coordination compounds containing `d^(6)` ions. However, with species containing `d^(4)` and `d^(5)` ions there are complications as illustrated by the options (1), (2), and (3). This apparent anomaly is explaned by VBT in terms of formation of orbital and outer orbital coordination entites. `[Mn(CN)_(6)]^(3-) , [Fe(CN)_(6)]^(3-)` and `[Co(C_(2)O_(4))_(3)]^(3-)` are inner orbital complexes (`d^(2) sp^(3)` hybridization), the former two complexes are paramagnetic while the latter diamagnetic. On the other hand, `[MnC1_(6)]^(3-),[FeF_(6)]^(3-)` and `[CoF_(6)]^(3-)` are outer orbital complexes involving to four, five and four unpaired electrons.