An electronic spectral study of some Fe(II) magnetic isomers in solution and a spectral-structural correlation with their Ni(II) analogs
Wilson, Lon J.
Master of Arts
Pseudooctahedral Fe(II) Complexes of the hexadentate ligand, tris4-[(6-R)-2-pyridyl]-3-aza-3-butenylamine, where R is either H or CH^ (Figure 1) , have been studied in solution by electronic spectroscopy. The complexes represent a magnetically interesting series with Fe-I being low-spin, Fe-IV high-spin, and Fe-II and Fe-III spin equilibrium species. The d-d electronic spectrum of high-spin Fe-IV has been interpreted in terms of a strongly distorted octahedral ligand field model with 1 Dq = 11,4 cm. As evidenced by X-ray structural results, a major contribution of the reduction from symmetry appears to arise from a static molecular distortion in which the average Fe-N(imine) and Fe-N(pyridine) bond distances differ by .14. As for other low-spin tris(a-diimine)Fe(II) complexes, the spectrum of Fe-I is characterized by only Fe (II) -» l igand charge transfer bands whose intensities and positions obscure any d-d transitions. The spectrum of the spin equilibrium Fe-III compound displays features commensurate with a molefraction-weighted population of both high-spin and low-spin magnetic isomers with 1Dqhs (Fe-III) being ~11,7 cm-1 when the two spin states are nearly equi-energetic. The analogous Ni(II) complexes of Figure 1 have also been synthesized and characterized by elemental analysis, infrared spectroscopy, and solution conductivity, magnetic susceptibility, pmr, and d-d electronic spectral measurements. From the spectral studies, 1Dq values for the series are found to increase according to the sequence: Ni-IV (1,55 cm-1) <Ni-III (11, cm-1) < Ni-I (11,24 cm-1). Using spectral and available X-ray structural data for tha two metal ion series, it has been empirically determined that lODq a r-^ with r the average M-N bond distance. From this structure-bonding relationship, a value of 1Dqs for Fe-I has been indirectly obtained and ratios for (Fe2)/ 1Dq(Ni2) and lODqgfFe2)/ 1Dq(Ni2+) estimated as 1.7 and 1.15 respectively. By this ratio method, values of Dqg for Fe-II and Dq^g for Fe-II and Fe-III have been obtained and a 3d electron mean pairing energy, 1, is calculated to be 12,8 + 4 cm ^ for this family of Fe(II) complexes.