Coordination equilibria in solution

Lei YB, Anson FC (1995) Dynamics of the Coordination equilibria in solutions containing copper(II), copper(I), and 2,9-dimethyl-l,10-phenanthroline and their effect on the reduction of O2 by Cu(I). Inorg Chem 34(5) 1083-1089... 

The dynamics of spin equilibria in solution are rapid. The slowest rates are those for coordination-spin equilibria, in which bonds are made and broken even these occur in a few microseconds. The fastest are the AS = 1 transitions of octahedral cobalt(II) complexes, in which the population of the e a antibonding orbital changes by only one electron these appear to occur in less than a nanosecond. For intramolecular interconversions without a coordination number change, the rates decrease as the coordination sphere reorganization increases. Thus the AS = 2 transitions of octahedral iron(II) and iron(III) are slower than the AS = 1 transitions of cobalt(II), and the planar-tetrahedral equilibria of nickel(II) are slower again, with lifetimes of about a microsecond. 

Studies of equilibria in which cyanoolefins are allowed to react with Ni[P(0-o-tolyl)3]3 have shown (36) that olefin coordination of 4PN is preferred over 3PN by a factor of 200, while olefin coordination of 2PN is preferred over 3PN by a factor of 1000. (Note the decreasing K for olefin complex formation in Table II in the series CH2=CH(CH2)BCN as n increases from 0 to 3 4 x 104,10,3.6, and 2.2. A value of 0.5 can be estimated for n - oo, based on the K for 1-hexene.) The relative amount of nitrile-coordinated nickel in solutions of cyanoolefins increases with added free phosphorus ligand, because of the different stoichiometries in Eqs. (14) and (15). 

In square-planar, 16-electron complexes (i.e. coordinatively unsaturated) as found for many group 9 (Co, Rh, Ir) and 10 (Ni, Pd, Pt) metals, the associative process is most common. The trans effect and trans influence ligand series [19-23] are also useful measures in the study of homogeneous catalysis. Apart from very small ligands, such as CO, H2, and NO, steric repulsion between ligands, as well as complexes and incoming substrates, plays a dominant role in determining the kind of intermediates and complexes formed and their equilibria in solution. 

In general the five- and six-coordinate silicon compounds have chemical shifts at unusually low frequency from — 50 to — 200 ppm, with the six-coordinate compounds appearing at lower frequency from —130 to —200 ppm while the five-coordinate compounds are in the —50 to —150 ppm region. Silicon-29 NMR provides a good method for studying the equilibria between normal four- coordinate silanes and higher-coordinate species in solution as well as characterizing these compounds as penta- or hexacoordinate silicon compounds. 

Diamagnetic Preformed ligand + Ni(C104)2 hydrate in SqPl R = Me SqPl five-coordinate EtOH equilibria in solution... 

R = H SqPl cis Oh equilibria in solution 3-22 As above -(- NaNj Five-coordinate... 

The planar complex reacts much faster than the other two, with a negative AS suggesting associative activation. The substitution of R-sal by H2-salen for the other two complexes appears to be dominated by rapid planar/octahedral equilibria in solution, and the octahedral form is kinetically inert. The rate differences span four orders of magnitude in acetone, but are reduced by a factor of 30 when MeOH is employed as solvent. This is probably caused by coordination of the more polar methanol, leading to more stable octahedral adducts. Interestingly, the observed rate constant for the reaction of the planar complex is greater in methanol than it is in acetone. The likely explanation is a contribution from an associative solvolysis pathway in the more polar solvent. 

The fact that at constant concentration of mercury the magnitude of the analytical response for different ligands correlates well with the magnitude of the formation constaiit implies that the relative strength of coordination exhibited in solution is also maintained at the surface. Thus, the affinity of the interface for mercury ions can be varied and controlled over a wide range in a predictable fashion by the appropriate choice of ligand. It is also important to note that the above correlation between the normalized response (at constant concentration) and the formation constant implies that the system reaches uilibrium. Since chemical speciation involves, at a fundamental level, competitive equilibria, the approach outlined here could be employed in such studies. 

The thermodynamic stability of a species is a measure of the extent to which this species will be formed from other species under certain conditions, provided that the system is allowed to reach equilibrium. Consider a metal ion M in solution together with a monodentate ligand L, then the system may be described by the following stepwise equilibria, in which, for convenience, coordinated water molecules are not shown ... 

Coordination equilibria were investigated by pH and potentiometric titration in aqueous solution. The measured e.m.f. values (E) are converted into hydrogen-ion concentrations using the modified Nernst equation ... 

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