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ChemInform Abstract The title complexes (I) (space group P21/n, Z=4) and (II) (Pca21, Z=4) are prepared by reaction of Mn(III) acetate of Mn(II) acetate with the corresponding ligand in the presence of acetate followed by addition of HClO4 or -in the case of (II) -of Na perchlorate and air oxidation. These complexes are characterized by X-ray structure analysis, variable-temp. magnetic susceptibility, novel EPR data, 1H NMR, and UV/VIS spectra. Cyclic voltammetric studies show that both complexes undergo two one-electron quasi-reversible redox processes (Mn(II)Mn(III) → Mn(II)2 and Mn(II)Mn(III) → Mn(III)2).

The crystal structure of dicarbonylpentamethylcyclopentadienylchromium dimer has been determined by a single-crystal three-dimensional X-ray diffraction study. The structure consists of discrete centrosymmetric [(7r-(CH3)sCs)Cr(CO),]2 units, each with a short Cr-Cr bond distance [2.280 (2) A], The Cr-Cr triple bond length in [(7r-(CH3)5C5)Cr(CO)2]2 is greater than the Cr-Cr quadruple bond lengths in Li4[Cr2(CH3)8]-4C4H80 [1.98 A] and Cr2(C3Hs)4 [1.98 A] but less than the Cr-Cr distance in Cr2(02CCH3)4-2H"0 [2.362 A] for which a quadruple bond has also been suggested. Carbonyl groups, two per chromium, are staggered with respect to the Cr-Cr axis and the Cr-C-0 atoms are nearly collinear [angles 170.8 (6), 174.8 (6)°]. Detailed analysis of bond distances and angles suggests that the carbonyl groups are essentially terminal. The (CH3)SC5 rings exhibit nearly perfect Csv symmetry and are trans with respect to the Cr-Cr axis. However, Cr-C(ring) distances are nonequivalent; this is ascribed to steric crowding between the ring and carbonyl groups which forces the latter to lie over the Cr-Cr bond [Cr'-Cr-CQ angles 73.0 (3), 79.1 (3)°]. Short contacts are also observed between the carbonyl groups and the molecule appears to be quite crowded sterically. Crystallographic data are as follows: space group P2, ¡n; unit cell a = 8.717 (6) A, b = 14.716 (4) A, c = 9.867 (5) A, ß = 112.00 (5)°; U = 1173 A3; <2calcd for Z = 2 is 1.408 and dobsu is 1.41 (2) g/cm3. Diffractometer data (1411 independent reflections with F2 > 3 ) were used to refine the structure anisotropically to final values of RF = 0.053 and RwF =

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