A property of a substance with unpaired electrons; a paramagnetic object gets pulled into a magnetic field.
A property of a substance with no unpaired electrons; a diamagnetic object gets pushed out of a magnetic field.
The Magnetism of Complexes
Most complexes are paramagnetic to some degree, with at least one unpaired electron present.
Weak-field ligands and Magnetism
Weak-field ligands will add one electron to each d-orbital before doubling up, and will tend to be strongly paramagnetic as a result.
Strong-field ligands and Magnetism
Strong-field ligands will pair electrons in tg obitals before filling eg orbitals, and will tend to be weakly paramagnetic as a result.
The magnetic properties of a complex depend on the magnitude of the ligand field splitting. Strong-field ligands tend to form low-spin, weakly paramagnetic complexes; weak-field ligands tend to form high-spin, strongly paramagnetic complexes.