The paramagnetic resonance method has been applied to investigate the magnetic properties of complexes containing elements of the 4
d
and 5
d
groups. It is shown that the resonance results, and such information as is available from susceptibility measurements, can be accounted for to a good approximation by assuming that there are either covalent
σ
-bonds or very strong electrostatic bonds in the complex (Van Vleck 1935). In the case of
d
5
complexes, however, accurate measurements of the spectroscopic splitting factors show small discrepancies with this theory. Furthermore, an anomalous hyperfine structure is observed for the complex [IrCl
6
]
2-
, which displays lines arising not only from the Ir nucleus, but also from the surrounding Cl nuclei. These effects are ascribed to the presence of weak
π
-bonds, in addition to
σ
-bonds, in the complex (Stevens 1953). In magnetically concentrated salts of the 5
d
group, it is found that there is strong exchange interaction between paramagnetic neighbours.
Hydrodynamic theory of magnets with strong exchange interaction
