1,2,3-Dithiazolyl and 1,2,35-Dithiadiazolyl Radicals as Spin-Bearing ...

1.4.2 Magnetic Properties of Organic Radical Coordination Complexes
The nature of the coupling interactions between a metal center and an organic,
paramagnetic ligand is slightly different than coupling between two metal ions as the d-
orbitals of the metal ions must be considered. If we consider regular octahedral
2
coordination geometry, then the d-orbitals are split into two degenerate sets: the d 2 x -y and
d 2 z orbitals make up the higher energy, doubly degenerate e g set of orbitals while the d xz ,
d yz and d xy orbitals make up the lower energy, triply degenerate t 2g set of orbitals. In
section 1.2 it was shown that the SOMO for most of the radicals we considered were of
π* symmetry and so a p-orbital can be used to represent the ligand contribution for this
approach.
Figure 1-16 shows the various ways that the p z orbital can interact with the d-orbitals
of the metal in a π-type fashion. As the x and y directions have been assigned to the
horizontal plane, they are not considered here. In 1-17a, the p z orbital from the π system
of the ligand is shown interacting with the d xz orbital from the metal center and b shows
the p z and d yz orbitals. The lobes are oriented with respect to one another such that there
is a non-zero overlap integral, here they are shown interacting in an anti-bonding type
fashion.
Under these circumstances the Pauli Exclusion Principle states that two
electrons in non-orthogonal, adjacent orbitals are in their lowest energy state when their
spins are anti-aligned. This can be thought of along the lines of the formation of a bond
such as is shown in Figure 1-5 where the two electrons must have different m s quantum
values (+½ or –½) to exist in the same orbital.
38