6. Magnetic Properties of Coordination Compounds

Magnetic Properties of Coordination Compounds :

Additional information for understanding the nature of coordination entities is provided by magnetic
susceptibility measurements. We have noted that coordination compounds generally have partially filled d orbitals and as such they are expected to show characteristic magnetic properties depending upon the oxidation state, electron configuration, coordination number of the central metal and the nature of the ligand field. It is experimentally possible to determine the magnetic moments of coordination compounds which can be utilized for understanding the structures of these compounds.

The number of unpaired electrons in any complex can be easily calculated from the configuration of the metal ion, its coordination number and the nature of the ligands involved( strong or weak from the spectrochemical series) and after that the magnetic moment of the complexes can be easily calculated using

Magnetic Moment =  Bohr Magneton                 

For metal ions with upto three electrons in the d-orbitals like Ti³⁺, (d1) ; V³⁺ (d²) ; Cr³⁺ (d³) ; two vacant d-orbitals are easily available for octahedral hybridisation. The magnetic behaviour of these free ions and their coordination entities is similar. When more than three 3d electrons are present, like in Cr²⁺ and Mn³⁺ (d⁴) ; Mn²⁺ and Fe³⁺ (d⁵) ; Fe²⁺ and Co³⁺ (d⁶) ; the required two vacant orbitals for hybridisation is not directly available (as a consequence of Hund’s rules). Thus, for d⁴, d⁵ and d⁶ cases, two vacant d-orbitals are  only available for hybridisation as a result of pairing of 3d electrons 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 d6 ion. However, there are complications with the coordination compounds / species having d⁴ and d⁵ ions.

[Mn(CN)₆]^3– has a magnetic moment equal to two unpaired electrons while [MnCl₆]^3– has a magnetic moment equal to four unpaired electrons.

Similarly [Fe(CN)₆]^3– has magnetic moment of a single unpaired electron while [FeF₆]^3– has a magnetic moment of  five unpaired electrons. [CoF₆]^3– is paramagnetic with four unpaired electrons while [Co(C₂O₄)]^3– is diamagnetic.

This anomalous behaviour is explained by valence bond theory in terms of formation of inner orbitals and outer orbitals complexes.

[Mn(CN)₆]^3– , [Fe(CN)₆]^3– and [Co(C₂O₄)₂]^3– are inner orbital complexes involving d²sp³ hybridisation, the former two are paramagnetic and the latter diamagnetic. [MnCl₆]^3– , [FeF₆]^3– and [CoF₆]^3– are outer orbital complexes involving sp³d² hybridisation and are paramagnetic having four, five and four electrons respectively.