The lesser number of
oxidation states at the extreme ends stems from either too few electrons
to lose or share (Sc, Ti) or too many d electrons (hence fewer orbitals
available in which to share electrons with others) for higher valence
(Cu, Zn) Thus, early in the series scandium(II) is virtually unknown
and titanium (IV) is more stable than Ti(III) or Ti(II) At the other end,
the only oxidation state of zinc is +2 (no d electrons are involved) The
maximum oxidation states of reasonable stability correspond in value
to the sum of the s and d electrons upto manganese (Ti
IVO2, V
VO2
+,
Cr
V1O4
2–, Mn
VIIO4
–) followed by a rather abrupt decrease in stability of
higher oxidation states, so that the typical species to follow are Fe
II,III,
Co
II,III, Ni
II, Cu
I,II, Zn
II