Quantitative Estimates of Steric Effects: lntramolecular Strain Energy Effects on the Activation Energy for Aquation of Some transDichlorotetraamine Cobalt(III) Complexes
Inorganica Chimica Acta
In an effort to quantitatively estimate steric contributions to the aquation rates of a series of structurally related cobalt(III) tetraamine complexes, strain energy minimization calculations have been performed on the reactant and some plausible transition state structures. Free energies of activation ΔG*obs, are factored as: ΔG*obs, = ΔG*bb + ΔG*strain + ΔG*CF + ΔG*solvation + … where ΔG*bb is the free energy change associated with bond breaking, ΔG*solvation is the solvation free energy difference between the reactant and a proposed transition stare, ΔG*CF is the difference in crystal field stabilization between the reactant and a proposed transition state, and ΔG*strain is the strain energy difference between the reactant complex and a proposed transition state. The activation energy for the aquation of a hypothetical ‘strain free’ complex is defined as ΔG*int and reflects the energy required for the bond breaking step with all other terms. For the cations trans-(RR,SS)-dichloro-1,8- diamino-3,6-diazaoctanecobalt(III)(trans [Co(2,2,2- tet)Cl2]+), trans-(RR,SS)- or trans-(RS)-dichloro-1.9- diamino-3,7-diazanonanecobalt(III)(trans [Co(2,3,2- tet)Cl2]+ and trans-(RS)-dichloro-1,10-diamino-4,7- diazadecanecobalt(III)(trans[Co(3,2,3-tet)Cl2]+) ΔG*intis found to be a constant 123 kJ/mol. For the trans-dichlorocobalt(III) complexes with the ligands 1,4,7,10-tetraazacyclotridecane(-ane-N4), 1,4,8, 11-tetraazacyclotetradecane(-ane-N4), 1,4,8,12- tetraazacyclopentadecane(-ane-N4) and 1,5,9,13- tetraazacyclohexadecane(-ane-N4), ΔG*int lies in the range 133–139 kJ/mol.
University of Dayton Research Council
Johnson, David W. and Brubaker, George R., "Quantitative Estimates of Steric Effects: lntramolecular Strain Energy Effects on the Activation Energy for Aquation of Some transDichlorotetraamine Cobalt(III) Complexes" (1986). Chemistry Faculty Publications. 30.