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Spectroscopic and theoretical studies of a binuclear cis µ-1,2 peroxide-bridged Fe(III) complex, [Fe 2 (O 2 )(OBz) 2 {HB(pz′) 3 } 2 ], where HB(pz′) 3 represents hydrotris(3,5-diisopropyl-1-pyrazolyl)borate and OBz is benzoate, are presented and discussed. It is shown that the high O-O stretching frequency of 876 cm -1 does not reflect a strong O-O bond (k O-O ) 3.1 mdyn/Å) but rather results from substantial mechanical coupling between the Fe-O and O-O stretch motions. The coupling strength in µ-1,2 peroxide-bridged complexes is mainly determined by the metal-O-O bond angle, and this angle is larger in the three structurally characterized Fe-peroxide dimers than in related Cu and Co dimers due to a strong π-bonding interaction in the former. Using absorption, magnetic circular dichroism, and resonance Raman spectroscopies we can identify four peroxide-to-Fe charge transfer (CT) transitions contributing to the optical absorption spectrum below 35 000 cm -1 . These transitions give rise to two similarly intense bands centered at ∼14 600 and 26 200 cm -1 ( ∼ 3800 M -1 cm -1 ) and to weaker features at 12 500 cm -1 (∼400 M -1 cm -1 ) and 24 650 cm -1 (∼1600 M -1 cm -1 ). A complete band assignment based on the experimental data and assisted by density functional calculations is presented and provides significant insight into Fe-peroxide bonding. From a quantitative analysis of the optical absorption and resonance Raman data within the framework of time-dependent theory of electronic spectroscopy it follows that the π contribution to the peroxide f Fe charge donation amounts to 50-65%. The σ contribution, however, is smaller than in the well-characterized trans µ-1,2 peroxide-bridged Cu(II) dimer, and the charge on peroxide is thus similar in the two complexes, suggesting that the bridging peroxide in [Fe 2 (O 2 )(OBz) 2 {HB(pz′) 3 } 2 ] has a rather basic/nucleophilic character. The mechanistic implications of these results for the conversion of the putative cis µ-1,2 peroxide-bridged diiron(III) intermediate (P or H peroxo ) of methane monooxygenase to a high-valent diiron(IV)-oxo species are discussed.

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Rhodium-diene complexes with the Tp iPr ligand, Tp iPr Rh(diene) (Tp iPr ) hydridotris(3,5-diisopropylpyrazolyl)borate; diene ) 1,5-cyclooctadiene (cod) (1), norbornadiene (nbd) (2)), are prepared by treatment of [Rh(µ-Cl)(diene)] 2 with KTp iPr or TlTp iPr . The structure of 1, characterized by X-ray crystallography, contains the κ 2 -coordinated Tp iPr ligand, and its central Rh atom adopts a square-planar geometry, whereas the unit cell of 2 contains two independent molecules: a square-planar structure with a κ 2 -coordinated Tp iPr ligand similar to 1 and a trigonal-bipyramidal one with a κ 3 -coordinated Tp iPr ligand. Although 1 H and 13 C NMR spectroscopy does not provide any information concerning the coordination geometry due to the interconversion between the square-planar and trigonal-bipyramidal structures, which occurs at a rate much faster than the NMR time scale, IR analysis reveals that the bands observed around 2470 and 2540 cm -1 are assigned to the B-H stretching vibrations of the square-planar and trigonal-bipyramidal structures, respectively. Thus, the ν(B-H) value can be used as an indicator of the hapticity of a Tp iPr ligand (κ 2 (∼2470 cm -1 ) vs κ 3 (>2530 cm -1 )) as compared with related Tp iPr ML n -type complexes.

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