The reaction of C 2(CO 2Me) 2 with trans-MeIr(CO)(PPh 3) 2 leads to a kinetic isomer which has been characterized by 1H and 31P NMR and infrared spectra and to a thermodynamic isomer which has been characterized by 1H and 31P NMR, infrared, microanalysis and X-ray crystallography. The isomerization occurs readily in solution at room temperature; somewhat more slowly at −20°C. The thermodynamically stable isomer of MeIr(CO)(PPh 3) 2[C 2(CO 2Me) 2] crystallizes in the centrosymmetric monoclinic space group P2 1/ c with a 14.847(2), b 16.648(2), c 15.656(3) Å, β 90.595(14)°, V 3869.7(11) Å 3 and Z = 4. Single-crystal X-ray diffraction data were collected with a Syntex P2 1 automated diffractometer (Mo- K α radiation, 2θ 5–40°) and the structure was solved and refined to R F 8.6% for all 3631 independent data ( R F 4.0% for those 2318 data with | F o| > 6σ(| F o|)). The Ir I center has a trigonal-bipyramidal environment with the methyl ligand and one PPh 3 ligand occupying axial sites (Ir-Me 2.193(14), Ir-P(1) 2.425(4) Å). The C 2(CO 2Me) 2 ligand is π-bonded to the iridium atom and lies with its triple bond parallel to the equatorial coordination plane; the equatorial ligands are completed by the second PPh 3 ligand (Ir-P(2) 2.402(3) Å) and a CO ligand (Ir-CO 1.812(15) Å).