Describing the π bonding in ruthenium and osmium iminoxolenes as highly covalent allows one to experimentally dissect the electron distribution in these compounds.
Oxobis(iminoxolene)osmium(VI) compounds ( R ap) 2 OsO ( R ap = 2-(4-RC 6 H 4 N)-4,6-t Bu 2 C 6 H 2 O) are readily deoxygenated by phosphines and phosphites to give five-coordinate ( R ap) 2 Os(PR′ 3 ) or six-coordinate ( R ap) 2 Os(PR′ 3 ) 2 . Structural data indicate that this net two-electron reduction is accompanied by apparent oxidation of the iminoxolene ligands due to their greater ability to engage in π donation to the reduced deoxy form of the osmium complex. In ( R ap) 2 Os(PR′ 3 ) 2 , the HOMO is a ligand-based combination of the iminoxolene redox-active orbitals, while the LUMO is a highly covalent metal-iminoxolene π* orbital. In the trans isomer, the HOMO is required to be ligand-localized by symmetry, while in the cis isomer, the ligands adopt a conformation that minimizes metal−ligand π* interactions in the HOMO. Kinetic studies indicate that the deoxygenations involve the ratedetermining attack of the phosphorus(III) reagent on the five-coordinate oxo complexes. Varying the substituents of the aryl groups on the iminoxolene ligands or on the triarylphosphines has little effect on the rate of oxygen atom transfer, with the best correlation shown between oxygen atom transfer rates and the HOMO−LUMO gap of the oxo complexes. This suggests that the osmium oxo group shows a balance between electrophilic and nucleophilic character in its oxygen atom transfer reactions with phosphorus(III) reagents.
Eight-coordinate MoO2(DOPOQ)2 can donate two oxygen atoms to substrates such as phosphines in a four-electron nonclassical oxygen atom transfer reaction.
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