A note from Dr. Haas: The Inorganic Spectroscopy Tutorial from the University of Alberta is an excellent resource for brushing up on Inorganic Spectroscopies. Conveniently, it has a tutorial on UV-visible spectroscopy for Inorganic complexes . Please review this tutorial before reading the page below. Types of transitions related to the metal ion: d-d transitions : d-d transitions are electronic transitions that occur between the molecular orbitals (MOs) that are mostly metal in character; specifically the orbitals that we think of as the d-orbitals of a transition metal complex. These transitions are useful in determining the energy of splitting and can be used to indicate coordination chemistry (geometry and ligand sets). In octahedral complexes, d-d transitions occur between the \(t_{2g}\) and \(e_g\) orbitals (across \(\Delta\)). These transitions can not occur in metal complexes where the d-orbital is completely empty (\(d^0\)) or completely full (\(d^{10}\)). In other words, a d-d transition is only possible in \(d^1 - d^9\) metal ions. In a UV-visible absorption spectrum, d-d transitions appear as relatively weak absorption with extinction coefficients (\(\varepsilon\)) less than 1,000. Charge transfer (CT) transitions : Charge transfer transitions occur between MOs that are mostly metal in character and those that are mostly ligand in character. These transitions depend on the type of ligand: they occur only when the metal is bound to ligands that are \(\pi\)-donors or \(\pi\)-acceptors. And there are two types of CT transitions. If the metal is bound to a \(\pi\)-donor ligand, electrons from lower-energy MO's that are mostly ligand in character can become excited to MO's that are mostly metal in character. These are ligand to metal charge transfers (LMCT) transitions (Figure \(\PageIndex{1}\), left diagram). If the the metal is bound to ligands that are \(\pi\)-acceptors, electrons from the MO's that are mostly metal in character can become excited to higher-energy orbitals that are mostly ligand in character. These are metal to ligand charge transfer (MLCT) transitions (Figure \(\PageIndex{1}\), right diagram). In a UV-visible absorption spectrum, CT transitions appear as relatively intense absorptions with extinction coefficients (\(\varepsilon\)) much greater than 1,000. Attribution Curated or created by Kathryn Haas