Effect of the central metal ion

The effect of the central metal ion on the rates of square-planar substitutions can be very strong. Hence, the tendency to undergo substitution is in the same order as the tendency to form pentacoordinate complexes Ni Pd Pt . Thus, the substitution rates of chlorine by pyridine in EtOH depend on the metal, as follows  

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When the reaction was conducted at room temperature under the catalysis of Yb[(-)BNP]3, the asymmetric induction was improved to 73% ee. The effect of the central metal ion of the chiral catalysts on the optical yield of the product, 2-phenyl-2,3-dihydro-4H-pyran-4-one, is shown in Fig. 2. The degree of enanti-oselection is highly sensitive to and dependent on the ionic radius of lanthanide ions [31]. 

They further observed the stereoselective effect in several other systems containing other amino acids as the polymer ligands74, and identified certain prerequisites for effective resolution (i) mixed complexes are more stable and it is better to use Cu(II) or Ni(II), (ii) the sum of the dentation of the polymer ligands and the mobile ligands (amino acids) should be equal to the coordination number of the central metal ion. The difference in free energy of formation of the two dia-stereomeric complexes with L- and D-proline was estimated to be 0.4 to 0.5 kcal/mol for the L-proline-Cu resin. It was emphasized that the stereoselectivity of this process was sufficient for the complete resolution of racemic a-amino acids74. ... 

In the emission of substate I of Ir(4,6-dFppy)2(acac), distinctly more vibrational satellites are found in the range of M-L vibrations (up to 500/600 cm-1) than in the emission of the substates II/III of Pt(4,6-dFppy)(acac). The relatively high-intensity of the corresponding satellites is characteristic of a large participation of the central metal ion - in particular of the d-orbitals - in the emitting triplet state of Ir(4,6-dFppy)2(acac). Among other effects, these M-L vibrational satellites are responsible for a smearing out of the emission spectra at ambient temperature. This effect is more distinct for Ir(4,6-dFppy)2(acac) than for Pt(4,6-dFppy) (acac) (compare Fig. 9 to Fig. 6). 

The lifetime of the impurity center produced by electron trapping is obviously important to the photographic process. It is affected by the identity of the central metal ion, its valence state, the composition of the ligand shell and the composition of the host lattice. The first example of a ligand effect was reported for self-trapping of photoholes in bromide-doped AgCl (see above) [181]. The lifetimes and trap depths of the self-trapped hole states [AgClj jBr ]4- increased as x increased [181]. 

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