The Role of the Metal Atom in the Reversible Uptake of Molecular Oxygen. The Structures of the Molecular Oxygen Complexes Formed by Bis [bis(diphenylphosphino) ethane]iridium (I) Hexafluorophosphate and Its Rhodium Analog

The crystal and molecular structures of the molecular oxygen complexes of bis[bis(diphenylphosphino)ethane]iridium(I) hexafluorophosphate, [lr(O₂){(C_6H5)₂PCH₂CH₂P(C₆H₅)₂}₂][PF₆], and bis[bis(diphenyl-phosphino)ethane]rhodium(I) hexafluorophosphate, [Rh(O₂){(C₆H₅)₂PCH₂CH₂P(C₆H₅)₂}₂][PF₆], have been determined from three-dimensional X-ray data. The two complexes are nearly isostructural, and the inner coordination around each metal atom is trigonal bipyramidal, considering the oxygen molecule as a monodentate ligand. The oxygen molecule is attached to the metal atom in the π-bonding fashion, with the two oxygen atoms equidistant from the metal. In the iridium complex, where addition of the oxygen is irreversible, the strong metal-to-oxygen bond is accompanied by a weak O-O bond, the O-O distance being 1.625 (23) Å, which is considerably longer than that expected for two oxygen atoms linked by a σ bond (1.49 Å). In the rhodium complex, where addition of oxygen isreversible, the weaker metal-to-oxygen bond results in a stronger O-O bond, the O-O distance being 1.418 (11) Å. Crystal data: the iridium complex crystallizes in space group P2₁/n, (C_2h⁵), of the monoclinic system, with four molecules in a unit cell of dimensions a = 17.24 (3), b = 16.35 (3), c = 16.98 (3) Å, and β = 96.64 (2)°. The structure has been refined by least-squares techniques to a conventional R factor of 6.7% on F, using 2177 reflections, collected by counter methods, for which F² > 2σ(F²). No crystallographic symmetry is imposed upon the discrete, monomeric ions. The Ir-P distances vary noticeably from 2.283 (7) to 2.452 (9) Å. The equivalent data for the analogous rhodium complex, prepared here for the first time, are a = 17.18 (2), b = 16.49 (2), c = 16.96 (2) Å, β = 95.20 (3)°, Z = 4, and space group P2¹/n. The least-squares refinement on F, using 2048 independent reflections, collected by counter methods, for which F² > 3σ(F²), converged to a conventional R factor of 5.2%. The Rh-P distances vary from 2.336 (4) to 2.362 (4) Å.