The
way that the outer electrons in a transition-metal ion fill these
two levels depends on the energy difference between the t
and e levels, which is called the crystal-field splitting
energy, A. The
ion has three outer electrons, and in the ground state of the ion
each electron occupies one of the three ,
and
orbitals in the t level (right).
has five outer electrons, and if the ligands around the iron produce
only a small crystal-field splitting, A, then the electrostatic
repulsion between electrons paired in the same orbital will ensure
that one electron goes into each d orbital: three in the
t level and two in the e. This is called a high-spin
complex because the electron spins are not paired in the orbitals.
These spins can be observed by magnetic measurements. If the ligands
produce a large energy splitting, it may take less energy to pair
electrons in the t orbitals than to place two electrons in
the higher-energy e orbitals (
and ) where they
must come close to the electrons of the ligands. In such a case
four electrons are paired in the lower orbitals, thus leaving only
one electron unpaired and creating a low-spin complex, as
shown at the right.