DYNAMICS OF COBALT(II) SPIN-EQUILIBRIUM COMPLEXES

Ultrasonic absorption measurements have been made on solutions of three cobalt(II) spin-equilibrium complexes. For the bis(terpyridine)cobalt(II) ion, no excess absorption was observed either in aqueous or methanol solutions, which implies a spin-equilibrium relaxation time of less than 2 ns, consistent with previous laser T-jump observations. For the bis(2,6-N-NHCH3-pyridinedicarboxaldimine)cobalt(II) ion again no excess sound absorption occurs in aqueous solution. Laser T-jump observations show a spectral change within the 13 ns heating risetime. For this complex in methanol and acetonitrile, however, a large excess sound absorption was observed which is described by overlapping double relaxation curves. A similar absorption was found as well for methanol solutions of the bis(2,6-t-butylpyridinedicarboxaldimine)cobalt(II) ion which is fully high-spin. This implies that the observed relaxations do not arise from the spin equilibrium but from some other process, possibly the dechelation of the ligand. Variable temperature solution magnetic moments and associated thermodynamic parameters are reported for the two spin-equilibrium complexes: for bis(terpyridine)cobalt(II) ion the thermodynamic parameters were calculated to be Delta H degrees = (2.085 +/- 0.007) kcal mol(-1) and Delta S degrees = (6.33 +/- 0.03) cal deg(-1) mol(-1) in methanol, and Delta H degrees = (3.93 +/- 0.02) kcal mol(-1) and Delta S degrees = (13.12 +/- 0.07) cal deg(-1) mol(-1) in water; for bis(2,6-N-NHCH3-pyridinedicarboxaldimine)cobalt(II) ion the thermodynamic parameters are Delta H degrees = (2.74 +/- 0.01) kcal mol(-1) and Delta S degrees = (7.29 +/- 0.04) cal deg(-1) mol(-1) in methanol, and Delta H degrees = (4.11 +/- 0.05)kcal mol(-1) and Delta S degrees = (10.85 +/- 0.15) cal deg(-1) mol(-1) in water.