When more than
three 3d electrons are present, the required pair of 3d orbitals for
octahedral hybridisation is not directly available (as a consequence of
Hund’s rule) Thus, for d
4 (Cr
2+, Mn
3+), d
5 (Mn
2+, Fe
3+), d
6 (Fe
2+, Co
3+)
cases, a vacant pair of d orbitals results only by pairing of 3d electrons
which leaves two, one and zero unpaired electrons, respectively The magnetic data agree with maximum spin pairing in many cases,
especially with coordination compounds containing d
6 ions However,
with species containing d
4 and d
5 ions there are complications