Transition metal compounds are paramagnetic when they have one or more unpaired d electrons. Paramagnetism is likely in transition metals because of the partial filling of the d subshell and the movement associated with the orientations of electrons. In octahedral complexes with between four and seven d electrons both high spin and low spin states are possible.
Tetrahedral transition metal complexes such as [FeCl4]2− are high spin because the crystal field splitting is small so that the energy to be gained by virtue of the electrons being in lower energy orbitals is always less that the energy needed to pair up the spins.
Some compounds are diamagnetic. The transition metals also have complex ionic structures because of the availability of d orbitals for participating in chemical bonding.
Electron paramagnetic resonance (EPR) spectroscopy is an analytical tool often used for the identification and characterization of paramagnetic transition metals. Continuous-wave (cw) spectra permit the description of the chemical environment and binding properties of paramagnetic species.