Far-Infrared Magnetospectroscopy (FIRMS): An Emerging Technique for Transition Metal (d Block) Chemistry

Abstract

Vibrational (“IR”) spectroscopy is used for routine characterisation of organic and inorganic/organometallic complexes, chiefly in the mid-IR region (~3500 – 700 cm-1). The far-IR (FIR) region, namely below ~700 cm-1 is rarely considered except for “fingerprinting”, although metal-ligand vibrations of interest can occur in this spectral region. Electronic transitions in transition metal complexes would be considered to be at much higher energy, minimally in the near-IR region (above ~10 000 cm-1, wavelength of ~1000 nm). Yet in many cases of paramagnetic transition metal ion complexes, there are lower energy electronic transitions, between spin sublevels resulting from zero-field splitting (zfs), a phenomenon that is the consequence of spin-orbit coupling (SOC). Such transitions can be observed in the far-IR region and their identity confirmed and refined by application of an external magnetic field, hence far-infrared magnetospectroscopy (FIRMS). Spectroscopy of this type has been known for many years, but only recently has emerged as a crucial technique for understanding paramagnetic transition metal complexes with potential application in molecular magnetism and more broadly in quantum information science. Examples of mononuclear complexes of Ni2+, Co2+, and Fe2+, representing three different spin states (S = 1, 3/2, and 2, respectively), and a special case of S = 1/2 Ni+, are given in this Highlight to demonstrate the value of FIRMS in understanding electronic structure, particularly in cases of unusual geometry such as low coordination number (2- and 3-coordinate).