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...
The effect of ligand substitution on bonding to other ligands has been investigated in a series of V(II1) complexes. Nuclear magnetic resonance (n.m.r.) contact shifts have been utilized for this purpose. These shifts are interpreted as arising from metal to ligand charge transfer. The results obtained are contrary to those expected from simple considerations of the electron withdrawing or donating abilities of the substituents. Electron withdrawing substituents decrease the charge transfer to the substituted ligand and increase the charge transfer to the remaining ligands. Electron donating substituents have the opposite effect. A simple interpretation is proposed to account for these results. An electron withdrawing substituent on a ligand in the xy plane increases the occupancy of d,, which cannot interact with the .rr system of the ligand in the xy plane but which is available for .n bonding with the remaining ligands. Effects of this type are to be anticipated whenever the filling of the t2, orbitals is not symmetrical.Canadian Journal of Chemistry, 49, 56 (1971) IntroductionThe n.m.r. spectra of paramagnetic complexes, The nature of the bonding in transition metal when interpretable, offer a very convenient complexes has been the subject of extensive in-method of studying metal ligand bonding (3). In vestigations. A subsection of this field deals with favorable cases admixture of metal and ligand the effect of changes in the structure of one ligand orbitals leads to delocalization of the unpaired on the bonding to another ligand. The aspect of electrons and this is manifested in the form of this latter area which has received the most atten-"contact" shifts of the resonances of ligand tion has been the so called "trans effect" particu-protons or other magnetic nuclei. This method larly as it applies to square planar complexes (1). can be very sensitive since rather small spin Examples of similar effects in octahedral com-densities give rise to substantial shifts and, for plexes have also been reported (2) but have been this reason, is particularly suitable for studying less well documented. The detailed interpretation the type of problem outlined above. Our choice of the trans effect is still to some extent an open of system was dictated by some practical limitaquestion, but in any event a selective transmission tions of the method. Sharp n.m.r. resonances are of electronic effects through the central metal only observed if the electron spin relaxation time atom is involved. Our interest in this area derives is short. Vanadium(II1) complexes fulfil this conmainly from a concern with the mechanisms of dition very well (4). Since we wish to study homogeneous catalysis and of ligand exchange systems involving a mixture of ligands a multireactions. It is a common finding in this area that plicity of resonances is expected and the basic rather smallchangesin the structureofligandsnot ligand system should therefore be as simple as directly involved in the reaction can considerably possible if unambiguous ana...
An empirical method for investigating the effect of substituents on the charge distribution of transition metal complexes is presented. This method is based on the measurement of n.m.r. chemical shifts of paramagnetic compounds and is applied to a series of V(II1) complexes. Measurements are reported on the sixteen isomers which result from the systematic replacement of CH3 groups by CF3 groups in V(II1) trisacetylacetonate. Ninety-nine of the possible 100 ' H and I9F chemical shifts have been measured. Substituent shifts of several thousand Hz are observed and have been interpreted using a model invoking charge redistribution in both a a n d norbitals. Similar experiments involving phenyl substituents are also reported. The model adopted enables the data to be fitted with a precision of around 2%. The substituent parameters derived from this model are discussed in terms of changes in electron density and the results compared with substituent effects in aromatic compounds. The important differences arise from the three dimensional nature of metal complexes which leads to interactions between the a and .rr electrons of a type not found in simple aromatic compounds. The significance of these results to the rationalization of ligand effects on ligand exchange and catalytic reactions is discussed.Afin d'Ctudier I'effet des substituants sur la distribution des charges dans des complexes de rnCtaux de transition, on a &labor& une methode empirique. Cette mCthode est basee sur la mesure des diplacements chimiques en r.m.n. de composes paramagnetiques et est appliquCe a une sCrie de complexes du V(II1). Des mesures furent effectuees sur les seize isomeres obtenus lors de la substitution systkrnatique des groupes CH3 par des groupes CF3 dans le tris V(1II)acCtylacCtonate. On a mesure 99 des 100 deplacements chimiques possibles du ' H et du I9F. Des deplacements induits par les substituants de plusieurs milliers de Hz ont CtC observes et ont 6tC interprCtCs en utilisant un modele impliquant des redistributions des charges dans les orbitales v e t n. On a rapport6 aussi des experiences similaires impliquant des substituants phenyles. Le modele adopte a permis d'obtenir des donnees avec une precision d'environ 2%. Les parametres des substituants obtenus de ce modele sont discutCs en fonction des changements dans la densite Clectronique et les resultats sont comparCs avec les effets de substituants dans les composCs aromatiques. Les diffkrences importantes proviennent de la nature tridimensionelle des complexes metalIiques qui implique des interactions entre les Clectrons a et nqui ne se retrouvent pas chez les composCs aromatiques simples. On discute de l'importance de ces resultats en ce qui a trait a la rationalisation des effets des coordinats sur I'tchange des coordinats ainsi que sur les rCactions catalytiques.[Traduit par le journal]Can. J . Chern., 51, 2260Chern., 51, (1973 Introduction We have previously reported (I) n.m.r. studies of complexes of V(II1) with mixed ligands and have suggested that experiments of th...
The Lewis acid-base interaction between pyridine bases and various O-alkyl dithiocarbonates and P-diketonates of Ni" has been studied by n.m.r. spectroscopy. There is correlation between the isotropic contact shifts of the H2 and H3 protons of pyridine bases and the Taft o* values of the substituent group R in [Ni(S,COR),].An explanation, based on a detailed study of the change in energy levels of the individual metal d orbitals, is proposed for the observed correlation which implies the relative strength of the Lewis acid-base interaction can be inferred from contact-shift studies. The observed trend in relative isotropic shifts of the protons at the 3-Me, 4-Me, and H4 positions and the absolute isotropic shifts of the H2 protons on the spin-delocalization mechanism is also discussed
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