ALKYNE OR ALKENE BINDING VERSUS HYDROGENATION IN REACTIONS WITH [IR2H(CO)(2)(MU-H)(2)(PH2PCH2PPH2)(2)][BF4] - STRUCTURE OF [IR-2(C2H4) (CO)(2)(MU-H)(PH2PCH2PPH2)(2)][BF4]

The reactions of [Ir2H(CO)(2)(mu-H)(2)(dppm)(2)][BF4] (1) (dppm = Ph2P CH2PPh2) with the alkynes, dimethyl acetylenedicarboxylate, hexafluoro -2-butyne or acetylene yield the respective products [Ir-2(H)(2)(CO)(2 )(mu-H)(mu-RCCR)(dppm)(2)][BF4] (R=CO2Me, CF3, H) in which the alkyne bridges the metals, bound in a parallel orientation. With tetrafluoroe thylene the analogous olefin-bridged product [Ir-2(H)(2)(CO)(2)(mu-H)( mu-C2F4)(dppm)(2)][BF4] is obtained. The reactions of 1 with diphenyla cetylene or ethylene result in hydrogenation, to yield cis-stilbene an d ethane, respectively, with subsequent reaction of the resulting unsa turated intermediate with additional substrate to give [Ir-2(CO)(2)(mu -H)(mu-PhCCPh)(dppm)(2)][BF4] or [Ir-2(C2H4)(CO)(2)(mu-H)(dppm)(2)][BF 4]. In the diphenylacetylene adduct this group is bound in a parallel position bridging the metals whereas the ethylene ligand binds to one metal in a terminal position opposite the Ir-Ir bond. The structure of this ethylene adduct has been determined by X-ray crystallography. Th is compound crystallizes in the monoclinic space group P2(1)/n, having a=13.6872(7), b=15.459(2), c=23.813(2) Angstrom, beta=97.870(5)degree s and Z=4. The carbonyl and ethylene ligands are disordered, but their positions have been adequately resolved and the structure has refined (on F-2) to R-1(F-o)=0.0344 (observed data) for 8699 unique reflectio ns (6638 observations) and 653 parameters varied.