Discuss the magnetic properties of coordination compounds.

(N/A) The magnetic moment of coordination compounds can be measured by magnetic susceptibility experiments. The results provide information about the number of unpaired electrons and the structures adopted by metal complexes.
For metal ions like $Ti^{3+}$ $(d^{1})$,$V^{3+}$ $(d^{2})$,and $Cr^{3+}$ $(d^{3})$,two vacant $d$-orbitals are available for octahedral hybridization with $4s$ and $4p$ orbitals. The magnetic behavior of these free ions and their coordination entities is similar.
When more than three $3d$ electrons are present,the required pair of $3d$ orbitals for octahedral hybridization is not directly available according to Hund's rule. Thus,for $d^{4}$ $(Cr^{2+}, Mn^{3+})$,$d^{5}$ $(Mn^{2+}, Fe^{3+})$,and $d^{6}$ $(Fe^{2+}, Co^{3+})$,a vacant pair of $d$-orbitals results only by the pairing of $3d$ electrons,which leaves two,one,and zero unpaired electrons,respectively.
There are complications with species having $d^{4}$ and $d^{5}$ configurations. For example:
$(i)$ $[Mn(CN)_{6}]^{3-}$ has a magnetic moment corresponding to two unpaired electrons,while $[MnCl_{6}]^{3-}$ has a paramagnetic moment corresponding to four unpaired electrons.
$(ii)$ $[Fe(CN)_{6}]^{3-}$ has a magnetic moment corresponding to a single unpaired electron,while $[FeF_{6}]^{3-}$ has a paramagnetic moment corresponding to five unpaired electrons.
$(iii)$ $[CoF_{6}]^{3-}$ is paramagnetic with four unpaired electrons,while $[Co(C_{2}O_{4})_{3}]^{3-}$ is diamagnetic.
The above behavior is explained by Valence Bond Theory in terms of inner orbital and outer orbital coordination entities. The complexes $[Mn(CN)_{6}]^{3-}$,$[Fe(CN)_{6}]^{3-}$,and $[Co(C_{2}O_{4})_{3}]^{3-}$ are inner orbital complexes ($d^{2}sp^{3}$ hybridization).
The complexes $[MnCl_{6}]^{3-}$,$[FeF_{6}]^{3-}$,and $[CoF_{6}]^{3-}$ are outer orbital complexes ($sp^{3}d^{2}$ hybridization) and are paramagnetic,corresponding to four,five,and four unpaired electrons,respectively. With $d^{6}$ configuration,the magnetic data agree with maximum spin pairing in many cases.