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How did superhyperfine structure show that some transition-metal compounds have covalent bonding?

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magnetic resonance Figure 1: Precession of a magnetic dipole moment μ in the presence of a constant field H and a rotating field H′ (see text)

In the study of transition-metal compounds, it was historically assumed that ions within a crystal were held together solely by electrostatic attraction, with magnetic electrons localized entirely on the transition metal ion.1

The discovery of superhyperfine structure in electron-spin resonance (ESR) provided conclusive evidence that this model was incomplete. In ESR, the interaction of electronic magnetization with a single nucleus creates a hyperfine structure; however, if the electronic magnetization is spread over more than one atom, it interacts with multiple nuclei. This additional coupling splits the hyperfine lines further into a superhyperfine structure.1

Because this structure indicates that the magnetic electrons are interacting with the nuclei of neighboring ions, it demonstrates that the electrons are not strictly localized on the transition metal ion. This delocalization confirms that there is a degree of covalent bonding between the metal and its neighboring ligands, rather than purely ionic electrostatic attraction.1