Cation-anion pair, chains contact

One can visualize a range of behavior from one extreme of a completely covalent species (I) to the other of completely free (and highly solvated) ions (V). The intermediate species include the tight or contact ion pair (II) and the solvent-separated or loose ion pair (III). The contact ion pair has a counterion (or gegenion) of opposite charge close to the propagating center (unseparated by solvent). The solvent-separated ion pair involves ions that are partially separated by solvent molecules. In cationic polymerization the chain end is cationic and has a negative counterion, while in anionic polymerization the chain end is anionic and has a positive counterion. 

Alkali metal anions are large and highly polarizable. With two electrons sharing the outer s-orbital, expansion occurs to yield radii of about 2.8,3.1. 3.2, and 3.5A for Na. K. Rb, and Cs, respectively.Crystal structure determination permitted the identification of a variety of structural motifs. In addition to isolated anions, various structures include contact ion pairs between and M. dimers of the anions. (M )2, and anionic chains. (M )n. - Although one might think that coulomb repulsion would prevent dimerization of M. a theoretical study showed that such dimers can be stabilized by adjacent cations. Dimers and chains presumably form by partial hybridization of the diffuse s-orbital with empty p-or J-orbitals. 

When rationalizing the significant difference of the hydrocarbon- and ether-bridged radical anions, the main aspect will certainly be the conformation of the oxyethylene chain, which brings the electrophores into closer contact. An additional aspect follows from the ability of the oxygen centres along the chain to chelate the counterion and thus to fix the cation between the electrophores. It is not possible from the available experimental evidence to discriminate between the two effects. The role of ion pairing and the relative position of the counterion and carbanion will be dealt with below. 

These short Au-Au contacts may be compared with distances of 2.88 A in metallic gold and 2.60 A in gaseous Au2. The term aurophilicity has been coined by H. Schmidbaur to describe the phenomenon [189,194], The interactions can occur as pairs, squares, linear chains or two-dimensional arrays of gold centres. Examples include the association between dimer units in the dithiocarbamates Au(dtc)2 (Figure 4.16) and the ionic tetrahydrothiophen complexes Au(tht)2 AuXJ (X = halogen), where cations and anions stack with Au—Au 2.97-2.98 A (X = I). The interaction is such that Au(S203)2 pair up, despite their charge, with Au-Au 3.24 A in the sodium salt. Likewise in Aupy AuCl, cations pair up at 3.42 A apart [10, 195]. 

Fe(Cp )2]2[Cu(mnt)2]. In this compound both cations from the D+D+ pair in the repeat unit are perpendicular to the chain direction as shown in Fig. 26b. Short intrachain contacts were observed and involve the Cu from the anion and a C from one of the Me groups in the cation, with a Cu-C distance of 3.562 A. The side-by-side cations are relatively separated and the closer C-C separations exceeds intrachain contacts were observed and the chains are essentially isolated. 

The crystal structure of [Fe(Cp)2]2[Ni(mnt)2]2[Fe(Cp)2] is composed by segregated stacks of pairs of cations, [Fe(Cp)2]+, and zig-zag dimerized anions stacks, with a neutral [Fe(Cp)2] molecule laying beside each anionic dimer [65]. In the case of [Co(Cp)2][Ni(dmit)2], the crystal structure consists of layers composed by two types of chains formed by the [Ni(dmit)2] anions, through short S—S contacts. Cation pairs are located between the anionic stacks [68], In the crystal structure of [Co(Cp)2][Ni(dmit)2]3 2MeCN, the partially oxidized anions form... 

The measurement of A vs concentration provides no evidence as to the nature of the ion pairs which form, i.e. whether they are contact or solvent separated species. Also, the mobility of the ion pairs does not influence the results. Contact ion pairs are likely to be more mobile than those separated by solvent since the latter include a section of at least one polymer chain. However, it is possible to envisage mechanisms, involving concerted motion of the cation and anion of a solvent separated pair, which would allow the effective movement of the neutral pair. This is also true for contact vs solvent separated triples. Measurements to be discussed below, involving the dc polarisation of cells, are capable of distinguishing between mobile and immobile pairs. 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

Another distorted variant of the NiAs structure occurs in NiP which is stable only above 850° C (159). In the orthorhombic NiP structure the distortions are stronger than in the MnP type (Fig. 44) but like in MnP the metal atoms form zig-zag chains with Ni—Ni = 2.53 A. The coordination of the nickel atoms is modified insofar as they are shifted towards a comer of the distorted anion octahedra. As a result there are only five phosphorus atoms in contact with the central nickel atom. The anions themselves are arranged in pairs with a P—P distance of 2.43 A, which roughly corresponds to the length of a half bond. In the absence of cation-cation bonds the P—P pairs would lead to divalent Ni and non-metallic properties would be possible. In the actual structure the Ni—Ni bonds exclude semiconductivity which, moreover, cannot be expected in a high-temperature phase. 

These centres are formed by the addition of monomer to a suitable anion. They are almost always simpler than their cationic reverse part. The counter ion is usually a metal cation able to interact with the electrons of the growing end of the macromolecule, and to bind in its ligand sphere monomer or solvent molecules or parts of the polymer chain. This changes the properties of the whole centre. Therefore, by selection of the metal, the stability of the centre, the tendency of the centres to aggregation, the position of the equilibrium between the contact and solvent-separated ion pairs and free ions, and the stereoselectivity of the centre [the ability to produce polymers with an ordered structure (tacticity, see Chap. 5, Sect. 4.1)] are predetermined. The chemical reactions of the metal cations are, however, very limited. Most solvents and potential impurities are of nucleophilic character. They readily solvate the cation, leaving the anion relatively free. The determination... 

As in the case of cationic polymerization, the presence of a metal atom can drastically change the electronic parameters of ring opening. In other words, the counterion may play the role of a symmetry switch , i.e. it can induce the reversion of stereospecificity of the active center. The possibility of anionic chain propagation on contact ion pairs via on intermediate stage of the formation of monomer-separated ion pairs was considered by Erusalimsky as far back as 1970 [61]. However, even 20 years later the author of the present paper does not attempt to discuss this problem in detail. 

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