The trans-influence and axial interactions in low spin, tetragonal cobalt(II) complexes containing macrocyclic and/or cyano ligands. Pulse radiolytic studies in fluid solution, electron paramagnetic resonance spectra at 77 K, and single-crystal x-ray structures

Abstract

Investigations of the weak interactions between low spin cobalt(II) species and their axial ligands have been performed. X-ray structural studies of six-coordinate Co(N/sub 4/)(OH/sub 2/)/sub 2//sup 2 +/ and Co(N/sub 4/)(ClO/sub 4/)/sub 2/ complexes, where (N/sub 4/) is an equatorially coordinated tetradentate macrocyclic ligand, reveal that the axial Co--ligand bond lengths are 0.3 to 0.5 A longer per bond than are the equatorial Co--ligand bond lengths. By means of the pulse radiolytic reductions of the corresponding cobalt(III) complexes, low spin cobalt(II) species of the types Co/sup II/(N/sub 4/)L/sub 2/ (L = NH/sub 3/, NO/sub 2//sup -/, CN/sup -/) and Co/sup II/(CN)/sub 5/X (X = Cl, CN) have been generated, and the rates of ligand exchange with water in the reduced species have been determined by a kinetic conductivity method. All the cobalt(II) species have been found to be very labile in two (axial) coordination positions, with an appreciable range of rate constants (k greater than or equal to 10/sup 3/ s/sup -1/) which depend on both the equatorial and the axial ligands. Electron paramagnetic resonance spectra have provided evidence for interactions with either one or two axial ligands (at 77 K) but on the whole have not provided definitive criteriamore » regarding the coordination number of low spin cobalt(II) species in fluid solution. Comparison of the results of this and other studies suggests a strong trans influence in the axial ligand interactions of low spin cobalt(II) complexes. A simple three-center bonding model has been used to describe the weak axial interactions. This model also provides a convenient basis for interpreting the absorption spectrum of a Co/sup II/(N/sub 4/)CN transient generated in the pulse radiolysis studies. It is inferred that the synergistic trans-influence deduced for low spin cobalt(II) species may be a significant factor in inner sphere electron transfer reactions.« less