Activation parameters for directly determined first order rate constants for outer sphere electron transfer reactions, and crystal structure of a pertinent pentaammine of Co III

Abstract

The outer sphere reductions of Co(NH 3) 5B 3+ by Fe(CN) 5A 3− have been studied. The observed pseudo first order rate constants (Co complex in excess) obey the dependence k obs= K os k et[Co]/(1 + K os[Co]), as expected for outer sphere electron transfer reactions. Values of the fundamental electron transfer rate constants k et have been determined, along with the equilibrium constant K os for a range of reactions in which A and B are pyridyl ligands of different sizes. The first order electron transfer rate constants vary in a manner that is consistcnt with adiabatic electron transfer. The outer sphere ion pairing equilibrium constants K os have been calculated: K os=8.6 ± 0.1 × 10 2 M −1 when A and B=pyridine; K os=1.07 ± 0.09 × 10 3 M −1 where A=pyridine, B=1-phenyl-3-(4-pyridyl)propane; K os=1.86 ± 0.11 × 10 3 M −1 when A=4,4′-bipyridine, B=pyridine; K os=1.27 ± 0.08 × 10 3 M −1 when A=4,4′-bipyridine, B=4-phenylpyridine. Distances of closest approach between the metal centers in the reactive ion pairs are compared, and it is concluded that there is a common mechanism, in which the ammonia side of the cobalt complex approaches the cyano side of the iron complex in each reactive ion pair. The distance of closest approach between the two metal centers ( a) was calculated from the experimental values for the ion pairing equilibrium constant K os at 25 °C: 5.2 Å when A=4,4′-bipyridine, B=pyridine; 5.4 Å when A=4,4′-bipyridine, B=4-phenylpyridine; 5.5 Å when A=pyridine, B=1-phenyl-3-(4-pyridyl)propane; 5.7 Å when A=B=pyridine. These relatively short metal-metal distances, when compared to the X-ray structure of the compound [Co(NH 3) 5(4-phenylpyridine)] 2[S 2O 6] 3· 4H 2O, do not support an ion pair orientation in which the two substituted pyridine ligands A and B are oriented toward each other. [ P2 1/ c, a=7.399(3), b=22.355(10), c=13.776(4) Å, β=92.02(3)°, R=0.070.] The crystallographic results show that if the two pseudo-octahedral coordination spheres are oriented in the reactive ion pair so that an ammonia face of the cobalt complex is at hydrogen bonding distance from a cyano face on the iron complex, the metal-metal distance is 5.3 Å, a distance which is in agreement with the kinetic results.