BINDING OF PI-ACID LIGANDS IN DIIRIDIUM AND RHODIUM IRIDIUM IODO COMPLEXES, INCLUDING RARE EXAMPLES OF ETHYLENE COORDINATION IN A-FRAME COMPOUNDS - STRUCTURE OF [IR2I2(CO)(MU-CO)(PH2PCH2PPH2)2].CH2CL2

The diiodo species [MIrI2(CO)(mu-CO)(dppm)2] (M = Ir (1), Rh (3), dppm = Ph2PCH2PPh2) are prepared by the reaction of the corresponding chloro complexes, [MIrCl2(CO)2(dppm)2], with KI. Abstraction of one iodide ion in 1 and 3, with use of AgBF4, yields the corresponding A-frame complexes [MIr(CO)2(mu-I)(dppm)2][BF4] (M = Ir (2), Rh (4)). All four species react with CO to give tricarbonyl complexes, but further CO addition is not observed. Reaction of 1 with dimethyl acetylenedicarboxylate (DMAD) yields four isomers of [Ir2I2(CO)2(mu-DMAD)(dppm)2], although upon heating of this mixture three of these species transform into the fourth, in which each metal has one iodo and one carbonyl ligand attached, having a structure much like that previously determined for the dichloro analogue. Reaction of the cationic species 2 and 4 with DMAD yields [MIr(CO)2(mu-I)(mu-DMAD)(dppm)2][BF4], and reaction of the mixed-metal product with CO yields [RhIrI(CO)2(mu-CO)(mu-DMAD)(dppm)2][BF4], which displays unusually high C-13-C-13 coupling of 47.8 Hz between the bridging carbonyl and the terminally bound carbonyl on Ir. Compounds 1 and 2 react with ethylene to yield the unusual ethylene adducts [Ir2(CO)(C2H4)(mu-I)(mu-CO)(dppm)2][X] (X- = I-, BF4-), in which ethylene is terminally bound to one iridium center. The structure of 1 has been determined by X-ray techniques. This compound crystallizes, as the CH2Cl2 solvate, in the space group P2(1)/c with a = 20.241 (4) angstrom, b = 14.153 (2) angstrom, c = 20.446 (2) angstrom, beta = 112.76 (1)-degrees, V = 5400.7 angstrom 3, and Z = 4 and has refined to R = 0.041 and R(w) = 0.049 on the basis of 5949 observations and 346 parameters varied. This complex has the unusual structure in which one carbonyl is terminally bound and one is bridging. One iodo ligand is opposite the Ir-Ir bond on one metal, while the other iodo group is cis to the Ir-Ir bond on the adjacent metal. Both carbonyl groups are mutually cis.