Structure of the Second Coordination Sphere of Transition Metal Complexes. IV. Solvent Effects

Abstract

The participation of various solvents in the second coordination sphere of Co(I1) pyrazolylborate has been investigated. Equilibrium constants relative to carbon tetrachloride as a second sphere ligand have been obtained. The ability of a solvent to participate in the second sphere depends predominantly on its dipole moment and it is concluded that Van der Waals forces are responsible for outer sphere binding. Can. J. Chem., 51,4137(1973) We have previously reported n.m.r. structural studies of the weak second sphere co~lplexes formed between coordinately saturated Co(I1) trispyrazolylborate compounds and other ligands present in solution (1, 2). These studies have also been extended to include the structures of ion pairs formed by analogous trispyrazolylmethane complexes (3). The motivation underlying this research is the belief that the efficiency of ligand exchange processes depends intimately on the ability of a metal complex to hold incoming ligands in a second coordination sphere where they are readily available for further reaction (4). The efficiency of a sequence of such ligand exchange reactions determines the overall efficiency of homogeneous catalysis. The Co(I1) trispyrazolylborates comprise a particularly favorable substrate system for studies of this kind since the strongly bound tridentate ligands cannot be replaced by the monodentate-ligands added which, if they are going to interact at all, can therefore only occupy positions in the second coordination sphere. Further, the large magnetic anisotropy of these Co(I1) complexes give rise to dipolar shifts in the n.m.r. spectra of surrounding molecules and hence provides a probe to investigate the structure of such a second sphere. Previous studies have demonstrated that anions tend to prefer to occupy positions close to the three-fold axis of the chelate whereas neutral molecules such as aniline and pyridine are located perpendicular to this axis-and have preferred orientations which may or may not be favorable for a subsequent ligand exchange reaction. It is apparent that the solvent plays a part in determining the detailed structure of the second sphere. Thus it was argued previously (3) that anions occupy positions on the symmetry axis because the more favorable positions perpendicular to the axis are preferentially taken by the dipolar solvent molecules. Similarly the observed n.m.r. shifts of neutral molecules depend strongly on the solvent used, indicating competition between added ligands and solvent molecules for the limited number of places in the second sphere. The present note reports some further studies with Co(I1) trispyrazolylborates aimed at clarifying solvent effects.The crux of the method used for studying second sphere structure lies in the equation (5)which relates the dipolar n.m.r. shift of a magnetic nucleus to its position in space relative to the cobalt chelate. Unfortunately this equation (the detailed form of which has been discussed recently (6)) cannot be used directly since it is impossible to disentangle Avo from t...

Metal complex catalyzed reactions of anils. III. Kinetic and mechanistic studies

“…In the present case we may write a rapid pre-equilibrium involving the replacement of acetone by aniline in the second sphere: i.e., By arguments analogous to those presented above the equilibrium 3 will not be affected by dilution with carbon tetrachloride providing CCl, does not compete with aniline or acetone for second sphere sites. Experimental evidence with other metal complexes does indeed indicate that CCI, is a very poor second sphere ligand (11). The rate expression 4 above will give the observed result in the dilution experiments.…”

Outer sphere complexes as intermediates in coordination chemistry

“…There is also relatively little difference in the enthalpies of formation of the second sphere complexes formed with the series of substituted anilines [53]. There are rather more substantial differences in the equilibrium constants for complex formation between cobalt boronpyrazolate and various solvents with the polar solvents being substantially better ligands than the nonpolar solvents [56]. The system described above comprises probably the most extensive series of studies of the geometry of second sphere complexes.…”

Solvent Effects on Solvent Exchange Reactions at Nickel(II) Centers

“…Two recent n.m.r. studies are important (24,25). They show that KE values for neutral molecule neutral complex encounters vary over more than a factor of ten and are solvent sensitive.…”