Strain metal-ligand distances

Fig. 10.1. Strain energy vs. metal-ligand distance plots, (a) Parameters used in the calculations, (b) Calculated curves for the three conformers of [Co(/ran.s-diammac)]3+/2+I1321.

On the other hand the estimation of the 3e>/3R quantities entails somewhat more effort. There are effectively two methods of approach of which the simpler uses the unsophisticated electrostatic model. On this basis Dq, and hence e, shows a 1/R5 distance dependence so that direct differentiation then indicates that at any given metal-ligand distance, R, de-JdR — -5 (ex/R). In justification of this rather naive model it may be noted that Bums and Axe13) found the strain dependence of Dq to be rather well described by the 1/R5 variation. 

This term vanishes by setting either k i = 0, or Tml = fo- While the first approach corresponds to a single-point calculation [347, 348], with the latter method the total strain energy mapped as a function of the metal-ligand distance can be obtained [210]. Remember here that an accurate treatment of the metal ion selectivity should... 

Table 2.4. Strain-free distances r0 for the metal-ligand bonding interaction.00...

Another technique used for the computation of metal ion selectivities, where plots of strain energy vs. metal ligand equilibrium distance r0 are produced and interpreted, is discussed in Chapter 8[°21. 

Figure 10. Ni(II) complex of the bispidine-based hexadentate ligand L8 (bispidine= 3,7-diazabicyclo[3.3.1]nonane) with the corresponding cavity size function, i.e. the strain energy as a function of the averaged metal donor distances.78...

Let us analyze the curve in Fig. 17.16.1 Spherical metal ions prefering metal-amine distances similar to those at the minimum of the curve (2.07 A) will fit best to the ligand and consequently lead to especially stable complexes. Metal ions preferring smaller bonds (<2.07 A, as is the case, e. g., for cobalt(III),) or larger bonds (>2.07 A, as, e. g., for copper(l) or zinc(II)) will have elongated or compressed bonds, respectively, and relatively lower stabilities (i. e., higher strain energies). 

Fig. 2 Illustration of strain, ensemble and ligand effects, (a) Adsorbate bonding is affected by a change in metal-metal bond distances, (b) Adsorbate bonding is affected by bonding to two different metals instead of one type of metal, (c) Adsorbate bonding is affected by hetero-metallic bonding of the active site atoms with neighboring atoms.

Table 3.4 Strain-free bond distances r for the metal-ligand bonding interaction."...

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