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 , consistent with shifts of electron density that would be sensitive to solvation.
9-478: In chemistry, intervalence charge transfer , often abbreviated IVCT or even IT , is a type of charge-transfer band that is associated with mixed valence compounds . It is most common for systems with two metal sites differing only in oxidation state. Quite often such electron transfer reverses the oxidation states of the sites. The term is frequently extended to the case of metal-to-metal charge transfer between non-equivalent metal centres. The transition produces
18-493: A characteristically intense absorption in the electromagnetic spectrum . The band is usually found in the visible or near infrared region of the spectrum and is broad. The process can be described as follows: where L is a bridging ligand . Since the energy states of valence tautomers affect the IVCT band, the strength of electronic interaction between the sites, known as α (the mixing coefficient), can be determined by analysis of
27-451: A transition from ligand σ MO to the empty e g MO. In IrBr 6 , which is a d 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
36-429: Is Ru(bipy) 3 , which upon irradiation gives excited states described as [Ru(III)(bipy )(bipy) 2 ] . The CT excited state is long-lived, allowing a rich chemistry ensues. Intervalence charge transfer (IVCT) is a type of charge-transfer band that is associated with mixed-valence compounds . Unlike the usual MLCT or LMCT bands, the IVCT bands are lower in energy, usually in the visible or near- infrared region of
45-403: The IVCT band. Depending on the value of α, mixed valence complexes are classified into three groups: Charge-transfer band CT absorptions bands are intense and often lie in the ultraviolet or visible portion of the spectrum. For coordination complexes , charge-transfer bands often exhibit molar absorptivities, ε, of about 50000 L mol cm . By contrast ε values for d–d transitions are in
54-559: 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. The optical spectrum of this d octahedral complex exhibits an intense absorption near 250 nm corresponding to
63-494: The ligand to the e g MO. The tetraoxides of d 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
72-399: 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. Complexes of bipyridine, phenanthroline, and related unsaturated heterocycles often exhibit strong C-T bands. Most famous
81-418: The range of 20–200 L mol . 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 (MO) that are predominantly metal in character and those that are predominantly ligand in character. If the transfer occurs from
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