ChemInform Abstract Aerobic oxidation of Cr(II) (Cr(II) perchlorate or bromide) in the presence of the aromatic amine ligands tris(2-pyridylmethyl)amine (II), 2,2'-bipyridine, or 1,10-phenanthroline leads to the bis(µ-hydroxo)bis(amine-Cr(III)) complexes (III) and (IV). Both spectroscopic and magnetic characteristics of (IIIa) are consistent with expectations for a weakly antiferromagnetically coupled dimer. Hydroxo bridge ionization constants for (IIIa) are determined. The µ-oxo-µ-hydroxo species (V) is isolated pure in the solid state, but the bis(µ-oxo) analogue undergoes facile base hydrolysis to (CrL(OH)2)+. Kinetic studies of this base hydrolysis are discussed with respect to the mechanism. The structure of bis(µ-hydroxo)-Cr(III) dimer (IIIb) (space group C2/c, Z=4) is determined by X-ray analysis.

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Five linear, oxo-bridged dimers of the form [L(tmpa)CrOCr(tmpa)LI2+ (L = NCS-, NCO-, N3-, CN-, C1-) have been prepared through the nucleophilic displacement of a hydroxo bridge from [(t~npa)Cr(OH)~Cr(tmpa)]~+ by L-. A mechanism involving the migration of a bridging oxo function to a cis coordination position is proposed to account for the change in stereochemistry that accompanies the conversion of the chromium diol to [(tmpa)(SCN)CrOCr(NCS)(tmpa)]2+. Electronic spectra, magnetic susceptibilities, and one-electron-oxidation half-wave potentials of the dimers are reported along with acid ionization constants of the corresponding hydroxo-bridged [L(tmpa)Cr(0H)Cr(tmpa)Ll3+ complexes. Large singlet-triplet energy gaps determined from the temperature dependence of the magnetic susceptibilities of the oxo-bridged dimers with L = NCS-(509 cm-I) and CN-(580 cm-I) support the formulation of diamagnetic ground states with the R electronic configuration (e,b)4(e,)4(b2,)2, where cub, e,, and bzs represent the *-bonding level, a nonbonding Cr-centered R MO and a weakly &bonding orbital in the approximate D, , , symmetry of the NSCrOCrNS coordination sphere, respectively. The assignment of two strong [L(tmpa)CrOCr(tmpa)LI2+ dimer absorptions near 28 500 and 24000 cm-I to e,%,* and b2,-eu* transitions (eu* designates the r-antibonding level) is supported in part by a study of the e,-e,* band energy dependence on solvent dielectric constant for L = SCN-. Consistent with expectations from the R molecular orbital model, pK, values of the hydroxo-bridged complexes fall off sharply with increasing lODq while EIl2 for the [L(tmpa)CrOCr(tmpa)LI2+/'+ couple follows an increasing trend as the L-substituent moves from weaker to stronger fields within the spectrochemical series.

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