Exchange interactions, charge delocalization, and spin relaxation in a mixedvalence diiron complex studied by Mössbauer spectroscopy J. Chem. Phys. 99, 6421 (1993); 10.1063/1.465881Effect of high pressure on the exchange interactions in binuclear chromium (III) complexesIn this paper we present the characterization by UV -VIS, Mossbauer, and EPR spectroscopy of [L 2 Fe 2 (Il-OHh] (CI04)2'2CH30H '2H 2 0, with L = N,N',N" -trimethyl-l,4,7-triazacyclononane, a novel dimeric iron compound, which is shown to possess a central exchange-coupled delocalizedvalence Fe(II/III) pair. Complete delocalization of the excess electron in the dimeric iron center is concluded from the indistinguishability of the two iron sites in Mossbauer spectroscopy. Mossbauer, EPR, and magnetic susceptibility data imply a system spin S, = 9/2 for the ground state. This finding is explained as being a consequence of the double-exchange interaction which is generated by the delocalized electron. Experimental values obtained from UV-VIS, Mossbauer, and EPR spectroscopy are for the double-exchange parameter B = 1300 cm -I, the g factors gx,y = 2.04 and gz = 2.3, the parameters for zero-field splitting D = 4 cm -I and E::::;O cm -I, and for the hyperfine parameters AEQ = -2.14 mm s-I, Ax,y = -21.2 T, A z = -27 T, and {) = 0.74 mm S-I. From our temperaturedependent studies we assign to the first excited state a spin-octet with an excitation energy A> 175 cm -I. From this value a lower bound of -235 cm -I has been deduced for the exchange-coupling constant J. In the framework of a simplified description of the iron atoms by unperturbed 3d orbitals, the values of the A tensor components as well as the quadrupole splitting AEQ can be interpreted in a consistent manner by assuming the excess electron being delocalized over two d a orbitals centered at the two iron sites of the dimer and directed along the iron-iron axis as the z direction.178