# 12.2.2: Predicting UV-visible Absorption

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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:

1. 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 cannot 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.
2. 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.