SINGLE-CRYSTAL ABSORPTION-SPECTROSCOPY OF BINUCLEAR COMPLEXES OF IRON(III) AND MANGANESE(III) WITH THE MU-OXO-BIS(MU-ACETATO)DIMETAL CORE

Single-crystal absorption spectroscopy at variable temperature is used to determine exchange couplings between transition metal centers in b oth the electronic ground and excited states in two new homobimetallic complexes with the formula [LM(mu-O)(mu-CH3CO2)(2)ML'](ClO4)(2), wher e M is iron(III) or manganese(III). L and L' denote 1,4,7-triazacyclon onane and 1,4,7-trimethyl-1,4,7-triazacyclononane, respectively. Value s for the ground state exchange coupling constant J are -295 cm(-1) an d +10 cm(-1) for the iron and manganese compounds, respectively, using H-ex JS(1) . S-2. Exchange interactions in excited states are qualita tively analyzed, indicating that a spin-forbidden transition of the Fe -Fe binuclear unit occurs with significant intensity by the single-ion mechanism, and not as expected by the Tanabe pair intensity mechanism for spin-forbidden transitions, the dominant mechanism for isoelectro nic complexes of manganese(II).