Colors charge-transfer

Charge-transfer complexes as intermediates in metal hydride additions to tetracyanoethylene (TCNE). Strong charge-transfer colors are observed when a colorless solution of TCNE is exposed to various metal hydrides owing to the formation of the [D, A] complex188 (equation 49). 

X-ray crystallographic analysis of the crystalline [bicumene, NO+] charge-transfer salt confirms that the charge-transfer color arises from a close approach of NO+ to the centroid of the phenyl moiety (see Fig. 10) with a non-bonded contact to an aromatic carbon of 2.63 A.194 The orange solution of bicumene bleaches slowly over a long period in a thermal reaction at room temperature (in the dark) or rapidly via irradiation of the CT band at low temperature. In both cases, l,l,3-trimethyl-3-phenylindane is obtained as the principal organic product (equation 63). 

The dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

The deep violet color of pentaphenylbismuth and certain other pentaarylbismuth compounds has been the subject of considerable speculation. It has been shown by x-ray diffraction (173) that the bismuth atom in pentaphenylbismuth is square—pyramidal. WeU-formed crystals are dichromic, appearing violet when viewed in one plane but colorless in another plane. The nature of the chromophore has been suggested to be a charge-transfer transition by excitation of the four long equatorial bonds ... 

Color from Charge Transfer. This mechanism is best approached from MO theory, although ligand field theory can also be used. There are several types of color-producing charge-transfer (CT) processes. 

The copper(I) ion, electronic stmcture [Ar]3t/ , is diamagnetic and colorless. Certain compounds such as cuprous oxide [1317-39-1] or cuprous sulfide [22205-45 ] are iatensely colored, however, because of metal-to-ligand charge-transfer bands. Copper(I) is isoelectronic with ziac(II) and has similar stereochemistry. The preferred configuration is tetrahedral. Liaear and trigonal planar stmctures are not uncommon, ia part because the stereochemistry about the metal is determined by steric as well as electronic requirements of the ligands (see Coordination compounds). 

The structure of the complex of (S)-tryptophan-derived oxazaborolidine 4 and methacrolein has been investigated in detail by use of H, B and NMR [6b. The proximity of the coordinated aldehyde and indole subunit in the complex is suggested by the appearance of a bright orange color at 210 K, caused by formation of a charge-transfer complex between the 7t-donor indole ring and the acceptor aldehyde. The intermediate is thought to be as shown in Fig. 1.2, in which the s-cis conformer is the reactive one. 

Equilibrium constants for complex formation (A") have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

Charge-transfer complexes with pyrimidine and purine bases as well as with solvents like hexa-methylphosphoramide and dimethyl sulfoxide are reported in Ref 66. The action of aromatic amines (primary, secondary, or tertiary) resulted in fume-offs or unidentifiable tars, in all cases purple or red colors developed prior to more violent reactions (Ref 66)... 

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