Hund’s rule). Thus, for d (Cr + , Mn + ), d (Mn + , Fe + ), d (Fe + , Co + ) cases, a vacant pair of d orbitals results only by pairing of d 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 d ions. However, with species containing d and d ions there are complications.
[Mn(CN) ] has magnetic moment of two unpaired electrons while [MnCl ] – has a paramagnetic moment of four unpaired electrons. [Fe(CN) ] – has magnetic moment of a single unpaired electron while [FeF ] – has a paramagnetic moment of five unpaired electrons. [CoF ] – is paramagnetic with four unpaired electrons while [Co(C O ) ] – is diamagnetic. This apparent anomaly is explained by valence bond theory in terms of formation of inner orbital and outer orbital coordination entities.
[Mn(CN) ] – , [Fe(CN) ] and [Co(C O ) ] – are inner orbital complexes involving d sp hybridisation, the former two complexes are paramagnetic and the latter diamagnetic. On the other hand, [MnCl ] – , [FeF ] – and [CoF - ] – are outer orbital complexes involving sp hybridisation and are paramagnetic corresponding to four, five and four unpaired electrons. The spin only magnetic moment of [MnBr ] – is . BM.
Predict the geometry of the complex ion ? Since the coordination number of Mn + ion in the complex ion is , it will be either tetrahedral ( sp hybridisation) or square planar ( dsp hybridisation). But the fact that the magnetic moment of the complex ion is . BM, it should be tetrahedral in shape rather than square planar because of the presence of five unpaired electrons in the d orbitals.