Stereochemical nonrigidity

Solid-state pressure effects on stereochemically nonrigid structures. J. R. Ferraro and G. J. Long, Acc, Chem. Res., 1975,8, 171-179 (57). 

The reactivity is additionally complicated by the stereochemical nonrigidity of carbon monoxide, and also by the metallic bonds which, in agreement with their low energy, can open and close with surprising ease as is shown in the following example178 ... 

Stereochemical nonrigidity is ubiquitous with seven and higher coordinated complexes because the geometries associated with them are easily interconverted by relatively small atomic displacements. Intramolecular rearrangements are complex but their understanding is helped considerably, but not solved, by nmr techniques. ... 

Zirconium and hafnium complexes are highly fluxional. NMR studies indicate stereochemical nonrigidity, facile exchange of terminal and bridging ligands, and rapid intermolecular ligand exchange. 

Molecular symmetry, and point groups, 53-59 Molecules reactivity of. 203 stereochemically nonrigid, 723-730... 

The temperature-dependent PMR spectra of the M(R2Dtc)3 complexes [M = Ga(III), In(III)] show them to be stereochemically nonrigid. Kinetic parameters for the intramolecular metal-centered rearrangement (by a trigonal... 

Variable-temperature PMR studies on the stereochemically nonrigid, (Cl4-n)Ti(R2Dtc) (n = 2, 3, or 4) complexes show the metal-centered rearrangements to be fast on the PMR time scale at temperatures higher than —90°. Hindered rotation about the C-N bonds was observed for R = r -Pr, and activation parameters were determined for this process. 

The (T s-Cp)Zr(Me2Dtc)3 complex was obtained by the reaction of (jf-Cp)2ZrCl2 with NaMe2Dtc in dry CH2C12 under N2 (89). Evidence for stereochemical rigidity (four Me resonances of relative intensity 2 1 2 1 at 37°C), and the obvious contrast with the stereochemically nonrigid ClTi(Me2Dtc)3 complex prompted Bruder et al. (89) to undertake a structural determination of this... 

Similar mixed-ligand complexes of the type (R, R2Dtc)2(MNT)Fe have been synthesized. The complexes were obtained initially as dianions, [(RiR2-Dtc)2(MNT)Fe]J", and were subsequently oxidized either by air or Cu(II) ions in acetonitrile (510). They also exhibit the singlet- triplet equilibrium however, they show a higher population of the triplet state than is found for the TFD analogues. The complexes are stereochemically nonrigid and display the same type of kinetic processes as their TFD counterparts. Thermodynamic activation parameters for inversion of the two complexes (TFD versus MNT) do not differ within experimental error. 

The stereochemical nonrigidity of the Fe(MePhDtc)3 was the first to be recognized (493, 495). At —108°C the complex is frozen out and two isomers are present (cis and trans). The cis isomer is unaffected upon warming. However, two of the three sites of the trans isomer are interchanged. No geometric cis—trans isomerization is observed, and at higher temperatures S2C N bond rotation occurs. 

One aspect of metal carbonyl chemistry that should be mentioned in surveying the more commonly found modes of CO coordination is the stereochemical nonrigidity of carbonyl clusters. This aspect has received considerable attention over the past decade, especially as 13C nmr instrumentation has become more readily available. In many carbonyl clusters, terminal and bridging carbonyls as established by x-ray structural studies are equilibrated on the nmr time scale (37, 39-41). The manner of equilibration takes place in a concerted way in order that each metal center maintains a constant electron count. For example, bridge terminal interconversion, (1), proceeds via complementary unsymmetrical CO bridges. 

In (dtc)2Fe(mnt) and (dtc)2 Fe(tfd) (30) and their Ru analogs several novel observations were made 45 the complexes are stereochemically nonrigid, they show a temperature dependent, singlet-triplet equilibrium and they can be subjected to oxidation and reduction over a series of four steps (2— to 1 +), which, in the case of the Ru complexes, are fully reversible. 

The observant reader will undoubtedly have noted that no evidence of any stereochemical nonrigidity (on the NMR time scale) has been obtained for any of the mononuclear complexes derived from OFCOT. However, the dinuclear complex 88 does exhibit fluxional behavior (151). If the solid-state structure of 88 were rigidly maintained in solution, the 19F-NMR spectrum should show eight resonances due to symmetry inequivalent fluorines. However, only four broad resonances are present in the 19F NMR spectrum at room temperature, indicative of a dynamic process that interconverts the two enantiomorphs of 88. Two mechanisms have been proposed for such interconversions in COT analogues a glide and a twitch (Scheme 3). The... 

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