How would you account for the following:
(i) Of the d4 species, Cr2+ is strongly reducing while manganese(III) is strongly oxidizing.
(ii) Cobalt(II) is stable in an aqueous solution but in the presence of complexing reagents, it is easily oxidized.
(iii) The d1 configuration is very unstable in ions.

Cr2+ is strongly reducing in nature. It has a dconfiguration. While acting as a

reducing agent, it gets oxidized to Cr3+(electronic configuration, d3the latter having a half-filled t2g level which is a more stable configuration.

In the case of Mn3+ (d4), it acts as an oxidizing agent and gets reduced to Mn2+ (d5). This has an exactly

half-filled d-orbital and is highly stable. Hence, manganese(III) is strongly oxidizing.

Co(II) is stable in aqueous solutions, however, in the presence of strong field complexing agents, it is oxidized to Co(III). Although the 3rd ionization energy for Co is high but the higher amount of crystal field stabilization energy overcomes this ionization energy. 

The ions in d1 
configuration tend to lose one more electron to get into stable d0 configuration. Also, the hydration or lattice energy is more than sufficient to remove the only electron present in the d-orbital of these ions.

Therefore, they act as reducing agents.