Account for the following:
i) Of the d^4 species, Cr2+ is strongly reducing while Mn2+ os strongly oxidising.
ii)cobalt(II) is unstable in aqueous solution but in the presence of complexing agents it is easily oxidised.
iii)d^1 config is very unstable in ions.

Question

Account for the following:
i) Of the d^4 species, Cr2+ is strongly reducing while Mn2+ os strongly oxidising.
ii)cobalt(II) is unstable in aqueous solution but in the presence of complexing agents it is easily oxidised.
iii)d^1 config is very unstable in ions.

aaronq · Answer

To explain the observations you pose i can say that $\sf Cr^{2+}$ has 4 d electrons, it can lose them, hence it's an reducing agent, while $\sf Mn^{2+} $ has 5 d electrons, (half-filled) it will gain electrons rather than lose them.

This is a hand-wavy argument, and has no basis other than making some connections. 
It's possible to use slater's rules to calculate $\sf Z_{eff}$ for these electrons and correlate them to the observations.

ii) cobalt(II) is unstable in aqueous solution 

what do you mean by "unstable"?

iii) write the configurations