The Transmission of Substituent Effects in Transition Metal Complexes

Abstract

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...

Isotropic Nuclear Magnetic Resonance Shifts of Co(II) Complexes with Mixed Ligands

“…We have been interested of late in utilizing this method to study the transmission of substituent effects in complexes with mixed ligands. Such studies have been reported involving octahedral complexes of V(II1) (1,2) and of Cr(I1) (3) and tetrahedral complexes of Ni(I1) (4). The detailed interpretation of the results has varied in the different cases but all show the common feature that the effects observed have been attributed to contact rather than dipolar interactions.…”

Nitroxide derivatives of acetylacetone as spin-labelled ligands

“…Some years pated from the methyl groups. At the same time, ago (5) we discussed the use of 4-pyridyl-t-butyl earlier nmr studies (6) have shown that relatively nitroxide as a spin-labelled ligand. The use of mall modifications in metal-ligand bonding can pyridine as the ligand moiety has some disadvan-give rise to large substituent effects at they position tages.…”