ELECTRON LOCALIZATION AND DELOCALIZATION IN MIXED-VALENCE TRANSITION-METAL CLUSTERS - STRUCTURAL AND SPECTROSCOPIC STUDIES OF OXO-CENTERED TRINUCLEAR COMPLEXES [FE3O(OOCCME3)(6)(PY)(3)](+ 0) AND [MN3O(OOCCME3)(6)(PY)(3)](+/0)/

Crystal structures are reported for the following complexes [Fe3O(OOCC Me3)(6)(py)(3)]ClO4 . 0.5py, 1; [Mn3O(OOCCMe3)(6)(py)(3)]ClO4 . MeCN, 2; [Mn3O(OOCCMe3)(6)(py)(3)], 4; [Fe3O(OOCCMe3)(6)(py)(3)], 5, Crystal data are as follows: 1, system monoclinic, space group P2(1)/n, a = 1 1.658(5) Angstrom, b = 35.450(5) Angstrom, c = 14.084(5) Angstrom, bet a = 100.10(1)degrees, V = 5730(3) Angstrom(3) Z = 4; 2, orthorhombic, P2(1)2(1)2(1), a = 18.592(11) Angstrom, b = 25.924(11) Angstrom, c = 1 3.290(5) Angstrom, V = 6405(3) Angstrom(3), Z = 4; 4, monoclinic, P2(1 )/c, a = 17.941(4) Angstrom, b = 14.945(3) Angstrom, c = 21.655(4) Ang strom, beta = 96.50(3)degrees, V = 5769(2) Angstrom(3), Z = 4: 5, mono clinic, P2(1). a = 11.800(3) Angstrom, b = 20.238(7) Angstrom, c 12.00 3(2) Angstrom, beta = 106.97(2)degrees V = 2741.6(13) Angstrom(3), Z = 2. In 1 and 2, the triangular metal ion clusters are close to equilat eral geometry, but the coordination polyhedra of the metal ions are si gnificantly different. Those of the fully oxidized triiron(III) cluste r are close to the ideal tetragonal symmetry. Those of the trimanganes e(III) cluster show strong Jahn-Teller distortions, the four Mn-carbox ylate bonds defining a long and a short O-Mn-O axis and these axes aro und the three Mn centers being correlated so as to define an approxima te 3-fold screw axis for the whole cluster. In the mixed-valence trima nganese complex 4 the coordination implies localized valence states cl ose to (2Mn(3+) + Mn2+) but in the triiron complex 5 the bond distance s are dose to those expected for a partially averaged state (2Fe(2.5+) + Fe3+). Mossbauer spectra of the mixed-valence iron complex indicate a phase transition to a localized (2Fe(3+) + Fe2+) state below T = 96 K. In the high-temperature phase. IR spectra support a model in which the electron transfer takes place between two adjacent iron centers a t a rate which remains lower than that of the infrared time scale up t o room temperature.