Metal ions bond distances

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). 

Speculation is made about why epi-inositol flips into the triaxial form to form a tridentate complex with borate, but complexes without flipping with metal ions. The distances between the three axial oxygen atoms are similar to those between the three oxygen atoms in the ax-eq-ax sequence. However, in the triaxial borate complexes (3) the bonds are tetrahedral if one were formed at the ax-eq-ax oxygen atoms, the bonds would be considerably distorted. However, the angles have much less effect on complex formation with cations which does not involve covalent bonds. 

Another factor comes into play here. In the first coordination sphere, deviations in metal-ligand bond distances are normally small enough to be neglected. The nearest 14 K+ ions lie at seven separate distances, from 4.3 to 6.0 A. The variation in the magnitude of the perturbation with distance must be considered, and it is not straightforward. Classical crystal field theory shows an inverse fifth power dependence of Dq on distance from the metal, but this result is specific to octahedral symmetry, in which lower order dependencies drop out. For individual perturbers we have proposed a dependence of the form aR-3 + bR-5 for both e and e [7]. For counterions we also suggest that the n contribution and the R-5 part of the a contribution can be neglected. 

Because of the higher charge and polarizing power of Ce4+ ion the metal-oxygen bond distances in its complex are shorter than the M-O bond distances in the Ce(III) nitrato complex. 

Noteworthy also is the large difference in the metal-aqua bond distance in the molybdenum(IV) and rhenium(V) complexes The rhenium-aqua bond is 0.13 A shorter than the molybdenum-aqua bond, which is more than one would expect from the ionic radii of these metal ions and is probably a result of the higher positive charge on the rhenium atom. The relative strong rhenium-aqua bond resulted in a relative strong acidic water ligand compared with that of the molybde-... 

Crystal field theory can deal with the observed splitting between the d-orbital sets increasing with increasing oxidation state of the central metal atom. Since the ionic radius of an ion decreases as ionic charge (which equates with oxidation state for a metal ion) increases, the surface charge density increases, metal-ligand bond distances (r) decrease and the splitting A 0 (which varies with r-5) increases. However, CFT has some obvious limitations, which led to development of alternate models. It cannot easily explain why A0... 

The difficulty with this approach is estimation of r. The mean value can be obtained from wave functions calculated from free atoms and ions, but this is not completely satisfactory since the interaction is proportional to r . This approach, however, gives more realistic values than the point-dipole approximation when the metal-ligand bond distances are less than 2 A. 

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. 

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