Metal—ligand bond compression

It is thus evident that the experimental results considered in sect. 4 above are fully consistent with the interpretation based on absolute reaction rate theory. Alternatively, consistency is equally well established with the quantum mechanical treatment of Buhks et al. [117] which will be considered in Sect. 6. This treatment considers the spin-state conversion in terms of a radiationless non-adiabatic multiphonon process. Both approaches imply that the predominant geometric changes associated with the spin-state conversion involve a radial compression of the metal-ligand bonds (for the HS -> LS transformation). 

As the flexibility of the macrocycle increases, then mismatch hole-size effects are expected to be moderated. In any case, as discussed in Chapter 1, a metal ion which is too large for the cavity may be associated with folding of a flexible macrocycle thereby allowing normal metal-ligand bond distances to be achieved. However, this is not always the case, and a number of examples of unfolded macrocyclic complexes containing compressed metal-donor distances are known (Henrick, Tasker Lin-doy, 1985). 

If the metal-ligand bond distances change considerably with oxidation state this can also retard the rate of electron transfer. For example the Fe—O distance in [Fe(H20)g] is 2.21 A but that in [FefHjOI ] " is 2.05 A so if electron transfer took place from [FefHjO) ] to Fe(H20)6, with both complexes in their ground states, the product would be a compressed Fe(II) ion and a stretched Fe(Ill) ion. It is therefore necessary that the molecules become vibrationally excited before electron transfer takes place i.e. the Fe(III) and Fe(II) geometries must be expanded and compressed, respectively. 

In a later study, bis-NHC ruthenium-alkylidene complex was activated under compressive strain [87] (Fig. 16). In order to initiate Ru-mediated polymerisation of norbomene in solid state, polymer catalyst (34 kg mol ) and a norbomene monomer were incorporated in a high molecular weight poly(tetrahydrofuran) (pTHF) matrix (Mn=170 kDa, PDI=1.3) which provided the physical cross-linking through the crystalline domains and allowed macroscopic forces to be transferred to the metal-ligand bonds. Consecutive compressions showed that up to 25% of norbomene monomer was polymerised after five loading cycles. 

Fig. 13.8 Extension and compression of the Fe—O bonds in [Fe(H20)nJ and [Fe(H20)6]". respectively, to form an activated complex in which all metal-ligand distances are identical, a prerequisite for electron transfer between the two complexes. [From Lewis, N. A. J. Chem. Educ. 1980. 57, 478-483. Used with permission.]...

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