Inorganic charge-transfer complexes Charge-transfer complex

1 inorganic charge-transfer complexes

1.1 ligand-to-metal charge transfer

1.1.1 trend of lmct energies


1.2 metal-to-ligand charge transfer

1.2.1 photoreactivity of mlct excited states


1.3 color of charge-transfer complexes

inorganic charge-transfer complexes

charge-transfer occurs in inorganic ligand chemistry involving metals. depending on direction of charge transfer classified either ligand-to-metal (lmct) or metal-to-ligand (mlct) charge transfer.

ligand-to-metal charge transfer

lmct complexes arise transfer of electrons mo ligand-like character metal-like character. type of transfer predominant if complexes have ligands relatively high-energy lone pairs (example s or se) or if metal has low-lying empty orbitals. many such complexes have metals in high oxidation states (even d). these conditions imply acceptor level available , low in energy.

consider d octahedral complex, such irbr6, t2g levels filled. consequence, intense absorption observed around 250 nm corresponding transition ligand σ mo empty eg mo. however, in irbr6 d complex 2 absorptions, 1 near 600 nm , near 270 nm, observed. because 2 transitions possible, 1 t2g (that can accommodate 1 more electron) , eg. 600 nm band corresponds transition t2g mo , 270 nm band eg mo.

charge transfer bands may arise transfer of electrons nonbonding orbitals of ligand eg mo.

trend of lmct energies

oxidation number
+7 mno4 < tco4 < reo4
+6 cro4 < moo4 < wo4
+5 vo4 < nbo4 < tao4

the energies of transitions correlate order of electrochemical series. metal ions reduced correspond lowest energy transitions. above trend consistent transfer of electrons ligand metal, resulting in reduction of metal ions ligand.

examples include:

metal-to-ligand charge transfer

metal-to-ligand charge-transfer (mlct) complexes arise transfer of electrons mo metal-like character ligand-like character. commonly observed in complexes ligands having low-lying π* orbitals, aromatic ligands. transition occur @ low energy if metal ion has low oxidation number, d orbitals relatively high in energy.

examples of such ligands taking part in mlct include 2,2 -bipyridine (bipy), 1,10-phenanthroline (phen), co, cn , scn. examples of these complexes include:

photoreactivity of mlct excited states

the photoreactivity of mlct complexes result nature of oxidized metal , reduced ligand. though states of traditional mlct complexes ru(bipy)3 , re(bipy)(co)3cl intrinsically not reactive, several mlct complexes characterized reactive mlct states have been synthesized.

vogler , kunkely considered mlct complex isomer of ground state, contains oxidized metal , reduced ligand. thus, various reactions electrophilic attack , radical reactions on reduced ligand, oxidative addition @ metal center due reduced ligand, , outer sphere charge-transfer reactions can attributed states arising mlct transitions. mlct states’ reactivity depends on oxidation of metal. subsequent processes include associative ligand substitution, exciplex formation, , cleavage of metal---metal bonds.

color of charge-transfer complexes

fig. 1 i2•pph3 charge-transfer complexes in ch2cl2. left right: (1) i2 dissolved in dichloromethane - no ct complex. (2) few seconds after excess pph3 added - ct complex forming. (3) 1 minute later after excess pph3 added, ct complex [ph3pi]i has been formed. (4) after excess i2 added, contains [ph3pi][i3].

many metal complexes colored due d-d electronic transitions. visible light of correct wavelength absorbed, promoting lower d-electron higher excited state. absorption of light causes color. these colors quite faint, however. because of 2 selection rules:

the spin rule: Δ s = 0

on promotion, electron should not experience change in spin. electronic transitions experience change in spin said spin-forbidden, such in spin forbidden reactions.

laporte s rule: Δ l = ± 1

d-d transitions complexes have center of symmetry forbidden - symmetry-forbidden or laporte-forbidden.

charge-transfer complexes not experience d-d transitions. thus, these rules not apply and, in general, absorptions intense.

for example, classic example of charge-transfer complex between iodine , starch form intense purple color. has widespread use rough screen counterfeit currency. unlike paper, paper used in currency not sized starch. thus, formation of purple color on application of iodine solution indicates counterfeit.