A new series of pseudooctahedral Fe(I1) complexes derived from the hexadentate ligand tr.isj4-[ (6-R)-2-pyridyl]-3-aza-3-butenyl)amine, where R is either H or CH3 (Figure l), has been synthesized and studied. Magnetic susceptibility and Mossbauer measurements demonstrate that compounds 11, 111, and IV as their PF6-salts undergo a high -low "spin equilibrium" in the solid state. Additionally, Evans' method solution measurements indicate that I1 and I11 also exhibit the same phenomenon in solution. Thus, these complexes are among the first Fe(I1) chelates to provide evidence for the existence of the spin equilibrium process in solution. An X-ray structural study of compound IV performcd by Delker and Stucky indicates that a steric interaction between the methyl groups and adjacent pyridine rings is largely responsible for the observed changes in the magnetic behavior of the series as the number of methyl substituted ligand arms is varied. Furthermore, the variable temperature structural data for IV shows an average decrease of -0.12 A in the six Fe-N bond distances in going from the fully high spin state ( p e f f -5.0 BM at 300'K) to a state of intermediate spin (peff -2.3 BM at 205°K). The results reported here indicate that multidentate ligands can be of general utility in designing new spin equilibrium systems where ligand substituent effects may be employed to "fine-tune" the ligand field strength around the crossover region.Transition metal complexes exhibiting spin equilibria between two electronic ground states have been recognized and studied for more than 10Usually these studies have focused on interpretation of the anomolous magnetic behavior which arises at or near the crossover region where electron spin pairing and ligand field splitting energies become competitive. To date, most studies of this nature have been restricted to the solid state, presumably due to the disappearance of the necessary equilibrium conditions in solution or to compound instability in the solution state. In particular for Fe(II), the only spin equilibrium system to be well characterized in both the solid and solution states is the poly( 1-pyrazoly1)borate Fe(I1) complex reported by Jesson and coworker^.^ It would seem desirable to develop and study spin equilibrium processes in the solution state as well as in solids since: (1) reports of such studies in solution have been rare, (2) comparative solution-solid state studies should be mutually complimentary in understanding lattice and solution environment contributions to the overall equilibrium process, and (3) the spin equilibrium dependency of electron transfer reactions in solution is of fundamcntal importance in modern reaction mechanism theory,I2 as well as being implicated in some metalloprotein electron transfer activity.As an initial approach toward obtaining solid vs. solution state data for spin equilibrium processes, we have synthesized a series of hexadentate ligands designed to support an ' A , , -5 T~g spin equilibrium for Fe(I1). Multidentate ligands were thought...

Intersystem crossing rates for 14 (low-spin) ^(high-spin) spin-equilibrium metal complexes of six-coordinate ironill) ( ^5T; AS = 2), iron(III) (2T ==^6A; AS = 2), and cobalt(II) (2E^4T; AS = 1) have been investigated in solution by Raman laser temperature-jump kinetics. Measurable first-order rate constants for the forward (ki) and reverse (/r-i) processes range from 4 X 105 to 2 X 107 s~1 , with four of the complexes having rate constants too large to measure (52 X 107 s"1) by the technique. The results are interpreted in terms of a model in which the spin multiplicity change is treated as an internal electron-transfer reaction.A number of first-row transition metal complexes with d5, d6, and d7 electronic configurations exhibit magnetic and spectroscopic behavior consistent with a thermal equilibrium between two states of differing spin multiplicity. For complexes in which the coordination number is constant and the donor atom set remains intact this phenomenon has come to be known as a "spin-equilibrium", with the state of lesser multiplicity being the low-spin (Is) state and that of higher multiplicity the high-spin (hs) state. Recent efforts have provided a variety of systems in which the spin equilibrium is present in solution.5™10 As a consequence, spin-equilibrium systems can be studied under conditions where perturbing influences from the lattice are minimized, and where the dynamic nature of the spin interconversion process is least likely to be inhibited. In solution

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