The first example of a stable metallapyrylium complex, [CHdC(Me)CHdC(Me)OdIr(PEt 3 ) 3 ] + -BF 4 -(2), has been prepared, and its reaction chemistry has been explored. Compound 2 is obtained in ∼50% yield upon treatment of mer-CHdC(Me)CHdC(Me)OIr(H)(PEt 3 ) 3 (3) with silver tetrafluoroborate in tetrahydrofuran. The other major product of this reaction, [mer-CHdC(Me)CH 2 C(Me)dOIr(H)(PEt 3 ) 3 ] + BF 4 -(4), is readily converted back to 3 (by treating with base) and can be reused. Compound 2 exhibits downfield 1 H NMR chemical shifts for its ring protons, consistent with its characterization as an aromatic metallacycle. The iridium center in 2 is reactive toward a variety of 2edonor reagents, including hydride reagents, methyllithium, chloride reagents, and trimethylphosphine. The products of these reactions are octahedral Ir(III) compounds containing the iridaoxacyclohexa-1,3-diene ring skeleton.The chloride reaction product, mer-CHdC(Me)CHdC(Me)OIr(Cl)(PEt 3 ) 3 ( 6), has been characterized by X-ray diffraction. Compound 2 also undergoes [4 + 2] cycloaddition reactions with various unsaturated substrates, including acetone, alkynes, alkenes, and sulfur dioxide. In each of these reactions, the substrate adds across iridium and the central carbon atom of the ring (C3) to produce octahedral Ir(III) compounds containing the iridaoxacyclohexa-1,4diene ring skeleton. The X-ray crystal structure of the sulfur dioxide cycloadduct, {fac-[CHd C(Me)CHC(Me)dOIrS(O) 2 ](PEt 3 ) 3 } + BF 4 -(13), has been obtained. Finally, treatment of 2 with nitrosobenzene generates a novel adduct containing two fused five-membered rings, {fac-[C(Me)CHdC(Me)OIrON(Ph)dCH](PEt 3 ) 3 } + BF 4 -(14). This reaction probably involves initial [2 + 2] cycloaddition, followed by rearrangement. The structure of 14 has been confirmed by X-ray diffraction.

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The synthesis and blood glucose lowering properties of the first vanadium-vitamin B(12) bioconjugates are reported.

Treatment of [(PR3)2Rh(μ-Cl)]2 (R = Me or Et) with potassium oxapentadienide leads to the production of ((1−3-η)-5-oxapentadienyl)Rh(PR3)2 (1, R = Me; 2, R = Et) as equilibrium mixtures of anti and syn isomers. Similarly, treatment of [(PR3)2Rh(μ-Cl)]2 (R = Me, Et) with potassium 2,4-dimethyloxapentadienide generates ((1−3-η)-2,4-dimethyl-5-oxapentadienyl)Rh(PR3)2 (3, R = Me; 4, R = Et). Compounds 3 and 4 exist predominantly as the anti isomer but upon cooling exhibit two rotameric forms, the sickle-shaped and U-shaped rotamers. Treatment of 1 with an additional 1 equiv of PMe3 produces the 18e species ((1−3-η)-5-oxapentadienyl)Rh(PMe3)3 (5), which is stable at room temperature. In contrast, when 2 is treated with an additional 1 equiv of PEt3, no reaction can be detected by NMR at room temperature. However, upon cooling to −70 °C, the characteristic signals for the phosphine adduct ((1−3-η)-5-oxapentadienyl)Rh(PEt3)3 (6) are observed by NMR. Compound 3, like 1, reacts with an additional 1 equiv of PMe3 to produce ((1−3-η)-2,4-dimethyl-5-oxapentadienyl)Rh(PMe3)3 (7) at room temperature. Finally, treatment of 4 with PEt3 yields no phosphine adduct, even upon cooling to −70 °C. Treatment of 4 with methyl triflate (CH3O3SCF3) leads to methylation at the rhodium center and coordination of the oxapentadienyl CO group, producing the 18e species [(η5-2,4-dimethyl-5-oxapentadienyl)Rh(PEt3)2(Me)]+O3SCF3 - (8). Similarly, treatment of 4 with HBF4·OEt2 generates [(η5-2,4-dimethyl-5-oxapentadienyl)Rh(PEt3)2(H)]+BF4 - (9), together with a novel isomer (10). Isomer 10 results from hydride migration to C3 of the 2,4-dimethyloxapentadienyl ligand, followed by metal-mediated activation of a methyl (C5) C−H bond. Treatment of 8 with PPN+Cl- results in chloride attack at the rhodium center and production of ((1−3-η)-2,4-dimethyl-5-oxapentadienyl)Rh(PEt3)2(Me)(Cl) (11). In contrast, treatment of 9/10 with PPN+Cl- leads to hydride migration, dissociation of the resulting 2,4-dimethyloxapentadiene ligand, and production of [(PEt3)2Rh(μ-Cl)]2. Compounds 1, 5, and 8 have been characterized by single-crystal X-ray diffraction.