The E
o (M
2+/M) values are not regular which can be explained from
the irregular variation of ionisation enthalpies (
i
i

1 
2
H
H ) and also
the sublimation enthalpies which are relatively much less for
manganese and vanadium Why is the E
o value for the Mn
3+/Mn
2+ couple much more positive
than that for Cr
3+/Cr
2+ or Fe
3+/Fe
2+ Explain Much larger third ionisation energy of Mn (where the required change
is d
5 to d
4) is mainly responsible for this