The magnetic behaviour of these
free ions and their coordination entities is similar 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