Fluxional behaviour

In addition to acting as an rf ligand, CgHg can coordinate in other modes, " some of which are illustrated in Fig. 19.33. Many of these complexes show fluxional behaviour in solution (p. 935) and the distinction between the various types of bonding is not as clear-cut as implied by the limiting structures in Fig. 19.33. 

Fe(CO)s is a highly toxic substance discovered in 1891, the only previously known metal carbonyl being Ni(CO)4. Like its thermally unstable Ru and Os analogues, its structure is trigonal bipyramidal (Fig. 25.10a) but its C nmr spectrum indicates that all 5 carbon atoms are equivalent and this is explained by the molecules fluxional behaviour (p. 914). 

The double doublet corresponds to Pfi, with splitting owing to phosphorus A (cis) (/(P-P) 25 Hz) and rhodium (/(Rh-P) 172 Hz). The fluxional behaviour is consistent with a rapidly rearranging (at room temperature) square planar structure rather than a tetrahedral one (Figure 2.11). 

On first consideration it may be concluded that if suitable crystals are available X-ray crystallography is the ideal method to decide unambiguously if a candidate compound is, in fact, homoaromatic (Childs et at., 1986a). The bond equalization and planarization associated with homoaromaticity should be readily detected by this means. However, the degree of bond equalization and the size of the homoconjugation gap necessary for homoaromaticity are open to debate (vide infra) (Childs et al., 1986a Haddon, 1988a). In addition, for systems capable of fluxional behaviour, dynamic or static disorder may lead to erroneous conclusions in the interpretation of the X-ray data (see Jackman et al., 1989). In suitable cases very careful X-ray studies can probably avoid this confusion (Dunitz et al., 1988). 

Scheme 1.10 Mechanism for the fluxional behaviour of the model Zr complex 98.

NMR spectroscopy ( Se, /= 1/2, 7%) is a powerful technique for identifying cyclic selenium molecules, especially the heteroatomic ring systems that contain sulfur or tellurium in addition to selenium, for which several isomers are possible for most compositions (Section 12.1.2). Solutions of monoclinic selenium in CS2 have been shown by high-performance liquid chromatography to form an equilibrium mixture of cyclo-Seg, cyclo-Sey and cyclo-Se6. The Se NMR spectra of such solutions show two singlets that are attributable to cyclo-Seg and cyclo-Se with relative intensities that correspond to a molar ratio of ca 6 No resonance is observed for cyclo-Sey presumably as a result of the fluxional behaviour (pseudorotation) of the seven-membered ring (Section 12.1.2). 

Fluxional behaviour for (0EP)Ti(02) and (TPP)Ti(02) has been shown from 3H and 13C NMR studies. At temperatures down to -50 °C the peroxo ligand undergoes fast exchange between two equivalent sites the barrier to rotation of the peroxo ligand at the coalescence temperature is 43.9 2.1 kJ mol-1 for (TPP)Ti(C>2). 

Several further experimental results lend support to the mechanisms postulated to explain the formation of the photoproducts. Thus irradiation of thiophene and substituted thiophenes in the presence of -propylamine leads to pyrroles, presumably via cyclopro-penylthiocarbonyl, or Dewar thiophene, intermediates (75T785). An extremely interesting development is the isolation of l,2,3,4-tetrakis(trifluoromethyl)-5-thiabicyclo[2.1.0]pent-2-ene, or perfluoro(tetramethyl Dewar thiophene) (2) by vapor-phase irradiation of perfluorotetramethylthiophene (81ACR76) the yield is about 58%. The half-life of thermal rearomatization of (2) at 160 °C is 5.1 h. The fluxional behaviour of the Dewar thiophene... 

Fluxional behaviour, 73-allyl palladium complexes, 8, 371 Formamidinates, in chromium complexes, 5, 355 Formates, with Ru and Os half-sandwiches, 6, 483... 

Relaxation studies on organo-metallic species have shown that fluxional behaviour can be observed.67... 

The full account of the crystal structure of [MoH4(PMePh2)4] has been published and the fluxional behaviour of this and related molecules discussed with reference to this structural information.347... 

Pt—N(2) = 2.843 A. First example of unidentate phen. observed in the solid state. In solution there is a possible fluxional behaviour see text... 

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