Thermodynamic N-Donor trans Influence in Labile Pseudo-Octahedral Zinc Complexes: A Delusion?

While the forces responsible for the chelate effect are well-established in coordination chemistry, the origin and implementation of the related thermodynamic trans influence remains debatable. This work illustrates a simple approach for quantifying this effect in labile pseudo-octahedral [Zn(<bold>Lk</bold>)(3)](2+) complexes lacking stereochemical preferences (<bold>Lk</bold> = <bold>L1L4</bold> are unsymmetrical didentate a,a'-diimine ligands). In line with statistics, the triply degenerated meridional isomers mer-[Zn(<bold>Lk</bold>)(3)](2+) are stabilized by 0.8 = Delta G(exch)(mer)<= = 4.2 kJ/mol over their nondegenerated facial analogues fac-[Zn(<bold>Lk</bold>)(3)](2+) and therefore display no apparent trans influence at room temperature. However, the dissection of the free energy terms into opposite enthalpic (favoring the facial isomers) and entropic (favoring the meridional isomers) contributions reveals a trans influence assigned to solvation processes occurring in polar solvents. Altogether, the thermodynamic trans influence operating in [Zn(alpha,alpha'-diimine)(3)](2+) complexes is 12 orders of magnitude smaller than the chelate effect. A weak templating effect provided by a noncovalent lanthanide tripod is thus large enough to produce the wanted facial isomer at room temperature.