Charge-transfer complexing lone pair

Charge-Transfer Compounds. Similat to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the iatense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a TT-electron system as ia the case of aromatic compounds or from lone-pairs of electrons as ia ethers and amines. 

The skeletal nitrogen atoms in cyclophosphazenes possess a lone pair of electrons and, hence, they have long been viewed as potential donor sites to bind a proton or to form complexes with electron-acceptor molecules. The possibility of formation of anion-cation complexes by release of a halogen ion to a Lewis acid and charge-transfer complexes has also been studied. In addition, some cyclopho-sphazene derivatives form crystalline inclusion clathrates with a variety of guest molecules. Allcock (21, 22) has reviewed these aspects in detail. 

Sulfur dioxide has lone pairs and can act as a Lewis base it can also act as a Lewis acid. With certain amines, crystalline 1 1 charge-transfer complexes are formed in which electrons from nitrogen are presumably transferred to antibonding acceptor orbitals localized on sulfur. One of the most stable is Me -SC (12-XXIII) where the dimensions of the S02 molecule appear to be unchanged by complex formation. 

Note the significance of the term charge-transfer complex. When the S— X2 complex forms there is actual mixing of the lone pair orbital on S with the 2au... 

The second mechanism of SERS enhancement consists in the formation of a charge-transfer complex between the metal surface and the molecule. Molecules with lone pair electrons or n clouds, such as aromatic amines or phenols, show the strongest SERS effect. The effect can also be seen in other electron-rich compounds like carboxylic acids. 

It has recently been observed that the main band in the spectrum of triphenylarsine is not affected when the arsine forms a 1 1 charge transfer complex with iodine 441a). A significant change would be expected if the arsenic lone pair is involved in the bonding. 

IP s can be useful in two ways. First, they give an idea of the availability of the halogen lone pairs for molecular associations. Second, they run parallel to the frequency of the lowest ultraviolet absorption band. The latter are, in these molecules, of the (C—type in which an electron is excited from a halogen lone pair orbital to an orbital antibonding in the C—Xbond [24] [9]. This frequency measures the height above the ground state of the lowest empty orbital available for charge transfer complex formation in which the fluorocarbon is the electron acceptor. 

Solutions of I2 in donor solvents, such as pyridine, ethers or ketones, are brown or yellow. Even benzene acts as a donor, forming charge transfer complexes with I2 and Br2. The colours of these solutions are noticeably different from those of I2 or Br2 in cyclohexane (a non-donor). Whereas amines, ketones and similar compounds donate electron density through a a lone pair, benzene uses its vr-electrons. This is apparent in the relative orientations of the donor (benzene) and acceptor (Br2) molecules in Fig. 17.6b. The fact that solutions of the charge transfer complexes are... 

The reaction of carbenes with X2 (X = F, Cl, Br, or I) can result in three potential complexes depending on the electronic properties of the carbene ligand and the halogen used. Initially, the carbene lone pair interacts with the o orbital of the X-X bond to form a charge transfer complex (C, Scheme 5.34). This charge transfer complex is typically in equilibrium with a cationic halo-substituted complex with a halide counterion D. If the halide counterion is strongly Lewis basic, or if cation D is sufficiently Lewis acidic,... 

Charge-Transfer Forces. An electron-rich atom, or orbital, can form a bond with an electron-deficient atom. Typical examples are lone pairs of electrons, eg, in nitrogen atoms regularly found in dyes and protein and polyamide fibers, or TT-orbitals as found in the complex planar dye molecules, forming a bond with an electron-deficient hydrogen or similar atom, eg, —0 . These forces play a significant role in dye attraction. 

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