Consider these six-coordinate ionic radii for divalent, first-row transition-metal ions:

r(Ti+) = 0.86 , r(V+) = 0.79 , r(Cr+) = 0.80 , r(Mn+) = 0.83 , r(Fe+) = 0.78 , r(Co+) = 0.745 , (Ni+) = 0.69 . For a fixed oxidation state, the ionic radius normally decreases on moving left to right across the periodic table due to the increasing effective nuclear charge. Why then does the radius increase on moving from V2+ to Cr+ to Mn+?