DYNAMICS OF COBALT(II) SPIN-EQUILIBRIUM COMPLEXES

Ultrasonic absorption measurements have been made on solutions of thre e cobalt(II) spin-equilibrium complexes. For the bis(terpyridine)cobal t(II) ion, no excess absorption was observed either in aqueous or meth anol solutions, which implies a spin-equilibrium relaxation time of le ss than 2 ns, consistent with previous laser T-jump observations. For the is(2,6-N-NHCH3-pyridinedicarboxaldimine)cobalt(II) ion again no ex cess sound absorption occurs in aqueous solution. Laser T-jump observa tions show a spectral change within the 13 ns heating risetime. For th is complex in methanol and acetonitrile, however, a large excess sound absorption was observed which is described by overlapping double rela xation curves. A similar absorption was found as well for methanol sol utions of the bis(2,6-t-butylpyridinedicarboxaldimine)cobalt(II) ion w hich is fully high-spin. This implies that the observed relaxations do not arise from the spin equilibrium but from some other process, poss ibly the dechelation of the ligand. Variable temperature solution magn etic 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) c al 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 is(2,6-N-NHCH3-pyridinedicarboxaldimine)cobalt(II) ion the thermodynamic parameters are Delta H degrees = (2.74 +/- 0.01) kc al mol(-1) and Delta S degrees = (7.29 +/- 0.04) cal deg(-1) mol(-1) i n methanol, and Delta H degrees = (4.11 +/- 0.05)kcal mol(-1) and Delt a S degrees = (10.85 +/- 0.15) cal deg(-1) mol(-1) in water.