146.

How would you account for the following:
(a) Of the d⁴ species, Cr²⁺ is strongly reducing while manganese(III) is strongly oxidising.

(b) Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidized.

(c) The d¹ configuration is very unstable in ions.

(a) Of d⁴ species, Cr²⁺ has 3d⁴ configuration and tends to loose one electron to acquire d³ configuration as it is highly stable and best metallic specie available for complex formation. Cr³⁺can accommodate six lone pair of electrons from ligands due to sp³d² hybridisation e.g. [Cr(NH₃)₆]³⁺ Mn³⁺ although have d⁴ configuration but tends to become Mn²⁺ stable specie by acquiring one electron to attain d⁵ configuration. It becomes exactly half filled on one hand and more energy is released in gain of electron due to higher nuclear charge.


(b)    Co²⁺ is stable in aqueous solution because it get surrounded and weakly bonded to water molecules. In presence of strong ligands and air it gets oxidised to Co(III) as strong ligands get coordinated more strongly with Co(III). The electronic configuration of Co(II) and Co(III) are:

Co(II) = [Ar]₁₈ 4s⁰3d⁷ and
Co(III) = [Ar]₁₈4s⁰3d⁶

In Co(III) specie, 6 lone pairs of electrons from ligands are accommodated by sp³d²hybridisation which is not possible in Co(II).


(c)    Some species with d¹ configuration are reducing and tends to loose one electron to acquire d⁴ stable configuration. Some other species with d¹ configuration like Cr(V) and Mn(VI) undergo disproportionation.