Rajasthan BoardEnglish MediumSTD 12 ScienceChemistryCoordination Compounds4 Marks
Question
Explain magnetic property of coordination compound.
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Answer
$\rightarrow$ Metal ions with upto three electrons in the $d$ orbitals, like $\ce{Ti ^{3+}\left( d ^1\right) ; V ^{3+}\left( d ^2\right) ; Cr ^{3+}\left( d ^3\right) ;}$ tuo vacant $d$ orbitals are available for octahed $d^3$ hybridization with $4 s$ and $4 p$ orbitals.
$\rightarrow$ The magnetic behaviour of these free jons and their coordination entities is similar.
$\rightarrow$ When more than three $3d$ electrons are present, the required pair of $3d$ orbitals for octahedral hybridization is not directly available $($as a consequence of Hund's rule$).$
$\rightarrow$ Thus, for $\ce{d ^4\left( Cr ^{2+}, Mn ^{3+}\right), d ^5\left( Mn ^{2+}, Fe ^{3+}\right), d ^6}$
$\ce{\left ( Fe ^{2+}, Co ^{3+}\right)}$ cases, a vacant pair of $d$ orbitals results only by pairing of $3 d$ electrons which leaves two, one and zero unpaired electrons, respectively.
$\rightarrow$ 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.
$\rightarrow \ce{\left[ Mn ( CN )_6\right]^{3-}}$ has magnetic moment of two unpaired electrons while $\ce{\left[ MnCl _6\right]^{3-}}$ has a paramagnetic moment of four unpaired electrons.
$\rightarrow \ce{\left[ Fe ( CN )_6\right]^{3-}}$ has magnetic moment of a single unpaired electron while $\ce{\left[ FeF _6\right]^{3-}}$ has a paramagnetic moment of five unpaired electrons.
$\rightarrow \ce{\left[ CoF _6\right]^{3-}}$ is paramagnetic with four unpaired electrons while $\ce{\left[ Co \left( C _2 O _4\right)_3\right]^{3-}}$ is diamagnetic.
$\rightarrow$ This apparent anomaly is explained by valence bond theory in terms of formation of inner orbital and outer orbital coordination entities.
$\rightarrow \ce{\left[ Mn ( CN )_6\right]^{3-},\left[ Fe ( CN )_6\right]^{3-}}$ and $\ce{\left[ Co \left( C _2 O _4\right)_3\right]^{3-}}$ are inner orbital complexes involving $\ce{d ^2 sp ^3}$ hybridisation, the former two complexes are paramagnetic and the latter diamagnetic.
$\rightarrow$ On the other hand, $\ce{\left[ MnCl _6\right]^{3-},\left[ FeF _6\right]^{3-}}$ and $\ce{\left[ CoF _6\right]^{3-}}$ are outer orbital complexes involving $\ce{sp ^3 d^2}$ hybridisation and are paramagnetic corresponding to four, five and four unpaired electrons.
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