Intramolecular fluxional process

The structure of (17) in solution is of interest because of Collman et al s proposal that the nitrosyl ligands undergo a rapid intramolecular fluxional process, (17a) (17b) (175). The evidence for this intercon-... 

The magnitudes of AS for intramolecular fluxional processes in organometallic complexes are frequently found to be between - -20 and —20 JK moR and all of the AS values calculated for (l)-(3) (Table 1) lie in this range. This observation is consistent with the proposed intramolecular nature of the fluxional processes which (l)-(3) undergo in solution. 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

For carbonyl complexes, a combination of IR (p. 11) and 13C NMR spectroscopy will often reveal the molecular symmetry and also provide further information about the nature of the metal centre to which the CO ligand(s) is bound. Flowever, the spectroscopic events involved occur within completely different time frames. Molecular vibrations (IR and Raman spectroscopy) are rapid relative to molecular fiuxional processes. NMR transitions, however, are slow, often comparable in rate to intramolecular fluxionality and even intermolecular ligand exchange processes. This can lead to time-averaged chemical environments being observed on the NMR timescale . So long as this is borne in mind, 13C NMR spectroscopy is a very valuable technique and can provide thermodynamic and kinetic data about such processes over a temperature range [variable temperature (VT) NMR]. 

It has been well established that the configurational stability of the tin centers in trioiganotin halides, which is usually very low, is enhanced to a considerable extent when intramolecularly coordinating substituents are present (123). The presence of a C,N-chelating ligand allows a detailed study concerning the fluxional processes that may be operative in solution. The two basic processes are (i) Sn—N dissociation/association and (ii) inversion of configuration of the tin center see Fig. 13. 

Rates for intramolecular A/A isomerisations have been reported for Cu(L)2+, where L = atropoisomer of di-imine benzimidazole-pyridine ligands. H, C and 31P NMR spectra suggest that MX[P(C7H7)3], where M = Cu or Ag, X = Cl, Br, are non-rigid in solution at room temperature.980 Dynamic 31P NMR spectroscopy was used to follow fluxional processes in [(3,5-di-/er/-butyl-l, 2,4-triphospholyl)Cu(PPh3)]. The preferred coordination mode at room temperature is r 5-7i coordination of copper. At low temperatures, two cr-isomers are seen.981... 

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