Equilibria Between Complexes with Different Coordination Numbers

4 Equilibria Between Complexes with Different Coordination Numbers 

Equilibria between hexa- and pentacoordinated sUicon complexes have been examined carefully by Kost et al. Exemplaiily the following works might be cited here Neutral hexacoordinated siUcon complexes derived from hydrazide chelating ligands with imino-donor groups form pentacoordinated Si complexes in the course of ionic dissociation reactions. Such complexes were shown to undergo facile intramolecular aldol-type condensation [176]. In a related system 

Hexamethylphosphoramide (HMPA) adducts of tetrachlorosilane (SiCU) were investigated with NMR spectroscopy in solution and solid-state structures [93]. In solution, the meridional and facial isomers of the hexacoordinated cationic silicon complex [SiCl3(HMPA)3] Cl predominate at all HMPA concentrations and are in equilibrium with the hexacoordinated neutral tram- and cA-[SiCL((HMPA)2] complexes, as well as the pentacoordinated cationic Si-complex cis- [SiCl3(HMPA)2] Cr [93]. 

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A change in the spin state of a metal ion also can accompany a change in coordination number. Again, in some cases conditions may be established in which an equilibrium exists between two complexes with different coordination numbers and different numbers of unpaired electrons. Some of the concepts which are used to describe intramolecular spin equilibria can be extended to the description of these coordination-spin equilibria. Examples include equilibria among four-, five-, and six-coordinate nickel(II) complexes and equilibria involving coordination number changes in iron porphyrin complexes and in heme proteins. 

Thermodynamic effects Equilibrium constants for replacing Y by another ligand Z or, in certain cases, for the formation of a complex with lower coordination number, i.e., the difference in free energy between two ground states of known structure. 

Ito and coworkers found that chiral ferrocenylphosphine-silver(I) complexes also catalyze the asymmetric aldol reaction of isocyanoacetate with aldehydes (Sch. 26) [51]. It is essential to keep the isocyanoacetate at a low concentration to obtain a product with high optical purity. They performed IR studies on the structures of gold(I) and silver(I) complexes with chiral ferrocenylphosphine 86a in the presence of methyl isocyanoacetate (27) and found significant differences between the iso-cyanoacetate-to-metal coordination numbers of these metal complexes (Sch. 27). The gold(I) complex has the tricoordinated structure 100, which results in high ee, whereas for the silver(I) complex there is an equilibrium between the tricoordinated structure 101 and the tetracoordinated structure 102, which results in low enantioselectivity. Slow addition of isocyanoacetate 27 to a solution of the silver(I) catalyst and aldehyde is effective in reducing the undesirable tetracoordinated species and results in high enantioselectivity. 

Cryptands have been somewhat deceptive for both coordination chemistry (Sastri et al., 2003) and photophysical properties of the resulting lanthanide complexes despite some commercial uses (Mathis, 1998), in particular of Lehn s Eu cryptate with cryptand 23a (fig. 28). The latter has been tested for the sensitization of the NIR luminescence of Nd and Yb. Characteristic emission from these two ions is seen upon excitation of the bipyridyl chro-mophores at 355 nm. Emission from Yb is reported to be much more intense than the one from Nd and the authors propose that the excitation mechanism depicted in fig. 9 is operative in this case since no transient absorption corresponding to the formation of the triplet state could be detected (Faulkner et al., 2001). Analysis of lifetime measurements in both water (r( F5/2) = 0.52 ps) and deuterated water (5.21 ps) gives a hydration number q = 1.5. Since fitting the luminescence decays to a double exponential function did not improve noticeably the resulting fit, the authors concluded that the non-integer value does not reflect an equilibrium between two different hydration states but, rather, that the distance of close approach of two water molecules is longer note that comparable experiments on Eu and Tb ... 

With many metal complexes, for a given coordination number, the energy difference between the two possible geometries in solution is low. Consequently, in solution the two structures may coexist in equilibrium. Such complexes are often referred to as fluxional complexes. Fluxionality with five coordinated complexes is particularly common. Note that in 2.3-2.S, all Ls need not be identical, in which case the symmetry properties of the metal complex would he affected. 

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