In any
coordination entity with a particular d
n (1<n<9) configuration
and a particular geometry, if the n electrons are distributed so that
they occupy the lowest possible energy levels, the entity is a low-spin
complex. If some of the higher energy d orbitals are occupied before
all the lower energy ones are completely filled, then the entity is a
high-spin complex.
Source:
PAC, 1997, 69, 1251
(Glossary of terms used in bioinorganic chemistry (IUPAC Recommendations 1997))
on page 1283