K-X bonds

The broken bonds (boldface = dissociated fragment) (boldface = recommended data reference in parentheses) Methods (reference in parentheses)  

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Solid solutions are essentially more convenient materials for studying the mutual influence of atoms because of the possible monotonic change in composition with retention of the structure of a whole series of compounds [212], It is well-known that between the cell parameters of a mixed crystal and the concentration of the components there is a linear relation (Vegard s law). If the KBr—KI system, which gives a continuous series of solid solutions, is taken as an example, then the picture of gradual change in the K-X bond ionicity as the composition of the solution is... 

Thus the products are determined by the strength of the C—X bond for X = Cl, Br, or I the rate of aromatization (k2) appears to be sufficiently high to compete with the reverse reaction (k t). 

We have also carried out preliminary experiments in which we have detected the laser desorption of ethylene, cyanogen, methanol, and benzene from the Pt(s)[7(111) x (100)] surface. These spectra are shown in Figure 9. In the experiments involving ethylene, cyanogen, and methanol only neutral species are desorbed. In the case of benzene we observe the molecular parent ion in the absence of the electron beam. We believe that this is due to resonance multiphoton ionization of the benzene by the laser after desorption (resonance multiphoton ionization of benzene is very efficient with 249 nm radiation). These spectra are in marked contrast to the results of SIMS experiments which produce a wide variety of complex metal-adsorbate cluster ions. In the case of ethylene, our experiments were performed at 140 K, and under these conditions ethylene is known to be a molecular x-bonded species on the surface. In SIMS under these conditions the predominant species is CH (15)t but in the laser desorption FTMS experiments neutral ethylene is the principal species detected at low laser power. 

The structure of K[TpBut2] has also been determined by x-ray diffraction thereby providing a rare example of a potassium complex in which the primary coordination number is three, with an average K-N bond... 

Isomerization sets in at higher temperatures than 273 K as apparent from NMR line broadening. The reason for a preference for the particular (cis O, N1) isomer is not obvious neither the O/N alternative for organosilyl migration nor the position of the cis/trans equilibrium with respect to the N-C(=X) bonds is easily predictable. 

Fig. 7.22 Comparison of calculated and observed (x-ray) mean N-C(a)-C bond angles for oligopeptides. Regions of and , of the most populated regions (N>3) of a set of oligopeptides selected as described in the reference quoted above.

For bromide and iodide, the nature of the countercation influences the structure of the anionic complex. In fact, when the [(18-C-6)-K]Br and [(18-G-6)-K]I salts are used, the anionic complexes ([ 163—X]—, X = Br, I) remain mononuclear and adopt a T-shaped structure (Figure 11). In both cases, the Hg-X bonds are shorter than those observed in the corresponding dinuclear complexes in agreement with the terminal location of the anion. The reaction of bis(pentafluoro)phenylmercury 164 with [(18-C-6)-K]Br and [(18-C-6)-K]I also afford T-shaped complexes [164-Br] and [164-1]. The Hg-Br (2.93 A) and Hg-I (3.12 A) bonds found in these complexes are longer than those observed in [163-Br] and [ 163—1] indicating that 164 is a weaker Lewis acid than 163.206... 

The electronic spin-state crossover in [Fe(HB(pz)3)2] has also been observed in the fine structure of its fC-edge x-ray absorption spectrum [38]. The changes in the x-ray absorption spectra of [Fe(HB(pz)3)2] are especially apparent between 293 and 450 K at ca. 25 eV, as is shown in Fig. 5. The 293 K x-ray absorption spectral profile observed in Fig. 5 for [Fe(HB(pz)3)2] has been reproduced [39] by a multiple photoelectron scattering calculation, a calculation that indicated that up to 33 atoms at distances of up to 4.19 A are involved in the scattering. As expected, the extended x-ray absorption fine structure reveals [38] no change in the average low-spin iron(II)-nitro-gen bond distance of 1.97 A in [Fe(HB(pz)3)2] upon cooling from 295 to 77 K. 

Several Ru(III) salen complexes of the type Ruin(salen)(X)(NO) (X=C1-, ONO-, H20 salen = N,AP-bis(salicylidene)-ethylenediamine dianion) have been examined as possible photochemical NO precursors (19). Photo-excitation of the Rum(salen)(NO)(X) complex labilizes NO to form the respective solvento species Ruin(salen)(X)(Sol). The kinetics of the subsequent back reactions to reform the nitrosyl complexes (e.g. Eq. (8)) were studied as a function of the nature of the solvent (Sol) and reaction conditions. The reaction rates are dramatically dependent on the identity of Sol, with values of kNO (298 K, X = C1-) varying from 5 x 10-4 M-1 s-1 in acetonitrile to 4 x 107 M-1 s-1 in toluene, a much weaker electron donor. In this case, Rum Sol bond breaking clearly... 

The order of reactivity of the C-X bond (generally I > Br > Cl > F) is consistent with its strength. For instance, the experimentally found dissociation energies for phenyl halides (DPh x) are 528, 402, 339, and 272 kj mol-1 at 298 K for X=F, Cl, Br, and I, respectively [2]. Consequently, catalytic defluorination in the literature is comparatively rare. The different reactivity of the C-X bonds renders possible the selective dehalogenation of compounds containing two dissimilar halides, leaving intact the stronger C-X bond. 

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