Ligand Field Information from Charge-Transfer Spectra of Substituted Tris(1,3-diketonato)iron(III) Chelates. Spectrochemical Series for 1,3-Diketones

A series of tris(1,3-diketonato)iron(III) chelates has been prepared in which the 1,3-substituents are CH5 CF5 and C6H5. The spectra of this series in the 20-38-kK region were studied in order to make assignments based on the expected electronic effect of the substituents. Three electronic transitions can be successfully treated using a simple one-electron molecular orbital scheme. The data presented support the assignment of the most intense band between 30 and 37 kK to the intraligand pi-pi* transition. The other two bands in this region are assigned to a metal-to-ligand (t(2g)) charge transfer between 24 and 29 kK, and a ligand-to-metal (pi -> eg) charge transfer between 20 and 23 kK. These assignments are justified on the basis of the relative energy separations in the simplified molecular orbital diagram. In connection with these energy separations, a spectrochemical series for the substituted 1,3-diketones was developed from the visible spectra of the corresponding Cr(III) chelates. The octahedral splitting parameter, a, for the Fe(III) chelates was then calculated using the relationship Delta = g(metal ion)f(ligand). The energy separations (sr tag) and (e(g)r -) were determined from the charge-transfer spectra. The sum (pi t(2g),) + A + (e, vs) agrees very well with the observed pi pi* transition energy. In terms of ligand field effects, the CFI, substituent on the chelate ring destabilizes all the metal levels relative to CH:. The phenyl substituent stabilizes the metal levels relative to CH3.