ESR spectroscopy

   Further proof of electron sharing comes from electron spin resonance (ESR) spectroscopy.  Unpaired electrons behave as magnets, and may align themselves parallel or antiparallel to a magnetic field. The former configuration has a slightly lower energy than the latter, and transitions from one configuration to the other can be observed by applying radiofrequency radiation. If the metal ion carrying the unpaired electron is linked to a ligand containing nuclei with the nuclear spin quantum number, I ≠ 0, hyperfine splitting of the ESR signal is observed showing that the orbital occupied by the electron has both metal and ligand character, i.e. there is metal–ligand covalent bonding. An example is the ESR spectrum of Na₂[IrCl₆] (paramagnetic low-spin d⁵)  ecorded for a solid solution in Na₂[PtCl₆] (diamagnetic low-spin d 6); this was a classic ESR experiment, reported in 1953