193px, The intense color of tris(bipyridine)ruthenium(II) arises from a metal-to-ligand charge-transfer band.
Charge-transfer bands are a characteristic feature of the optical spectra of many compounds. These bands are typically more intense than d–d transitions. They typically exhibit
solvatochromism In chemistry, solvatochromism is the phenomenon observed when the colour due to a solute is different when that solute is dissolved in different solvents.
The solvatochromic effect is the way the spectrum of a substance (the solute) varies when th ...
, consistent with shifts of electron density that would be sensitive to solvation.
CT absorptions bands are intense and often lie in the ultraviolet or visible portion of the spectrum. For
coordination complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. ...
es, charge-transfer bands often exhibit molar absorptivities, ε, of about 50000 L mol
−1 cm
−1. By contrast ε values for d–d transitions are in the range of 20–200 L mol
−1. CT transitions are spin-allowed and
Laporte-allowed. The weaker d–d transitions are potentially spin-allowed but always Laporte-forbidden.
Charge-transfer bands of transition metal complexes result from shift of charge density between
molecular orbitals
In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of findin ...
(MO) that are predominantly metal in character and those that are predominantly ligand in character. If the transfer occurs from the MO with ligand-like character to the metal-like one, the transition is called a ligand-to-metal charge-transfer (LMCT). If the electronic charge shifts from the MO with metal-like character to the ligand-like one, the band is called a metal-to-ligand charge-transfer (MLCT). Thus, a MLCT results in oxidation of the metal center, whereas a LMCT results in the reduction of the metal center.
Case studies
IrBr63−/2-
The optical spectrum of this d
6 octahedral complex exhibits an intense absorption near 250 nm corresponding to a transition from ligand σ MO to the empty e
g MO. In IrBr
62−, which is a d
5 complex, two absorptions, one near 600 nm and another near 270 nm, are observed. These are assigned as two LMCT bands, one to t
2g and another to e
g. The 600 nm band corresponds to transition to the t
2g MO and the 270 nm band to the e
g MO.
Charge transfer bands may also arise from transfer of electrons from nonbonding orbitals of the ligand to the e
g MO.
d0 oxometallates
The tetraoxides of d
0 metal centers are often deeply colored for the first row metals. This coloration is assigned to LMCT, involving transfer of nonbonding electrons on the oxo ligands to empty d-levels on the metal. For heavier metals, these same transitions occur in the UV region, hence no color is observed. Hence perrhenate, tungstate, and molybdate are colorless.
The energies of transitions correlate with the order of the electrochemical series. The metal ions that are most easily reduced correspond to the lowest energy transitions. The above trend is consistent with transfer of electrons from the ligand to the metal, thus resulting in a reduction of metal ions by the ligand.
Polypyridine complexes
Complexes of bipyridine, phenanthroline, and related unsaturated heterocycles often exhibit strong C-T bands. Most famous is
Ru(bipy)32+, which upon irradiation gives excited states described as
−)(bipy)2">u(III)(bipy−)(bipy)2sup>2+. The CT excited state is long-lived, allowing a rich chemistry ensues.
[{{cite book , author=Kalyanasundaram, K. , title=Photochemistry of polypyridine and porphyrin complexes , publisher=Academic Press , location=Boston , year=1992 , isbn=0-12-394992-0 ]
Mixed valence complexes
192 px, is intensely blue owing to an intervalence charge transfer band.">Prussian blue is intensely blue owing to an intervalence charge transfer band.
Intervalence charge transfer
192 px, The intense blue color of Prussian blue is a consequence of an intervalence charge transfer band.
In chemistry, intervalence charge transfer, often abbreviated IVCT or even IT, is a type of charge-transfer band that is associated with mi ...
(IVCT) is a type of charge-transfer band that is associated with
mixed-valence compound
Mixed valence complexes contain an element which is present in more than one oxidation state. Well-known mixed valence compounds include the Creutz–Taube complex, Prussian blue, and molybdenum blue. Many solids are mixed-valency including ...
s. Unlike the usual MLCT or LMCT bands, the IVCT bands are lower in energy, usually in the visible or near-
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
region of the spectrum and is broad. Prussian blue, the blue pigment derived from Fe(III), Fe(II), and cyanide, owes its intense color to IVCT.
References
Chemical compounds