REVERSIBLE CHELATE-TO-BRIDGE LIGAND-EXCHANGE IN CO2(CO)4(MU-PHC-EQUIVALENT-TO-CPH)(BMA) AND ALKYNE DIPHOSPHINE LIGAND COUPLING - SYNTHESIS,REACTIVITY, AND MOLECULAR-STRUCTURES OF CO2(CO)4(MU-PHC-EQUIVALENT-TO-CPH)(BMA), CO)4(MU-PHC-EQUIVALENT-TO-CPH)((Z)-PH2PCH=CHPPH2), AND ETA-1,ETA-1-(Z)-PH2PC(PH)=(PH)CC=C(PPH2)C(O)OC(O))

The reaction of the alkyne-bridged binuclear complex Co2(CO)6(mu-PhC=C Ph) (1) with the unsaturated diphosphine ligands 2,3-bis(diphenylphosp hino)maleic anhydride (bma) and (Z)-Ph2PCH=CHPPh2 has been investigate d. It is shown that the bma ligand reacts with 1 to give Co2(CO)4(mu-P hC=CPh)(bma) (2) with a chelating bma ligand. Chelate-to-bridge bma li gand equilibration is promoted by photolysis and thermolysis condition s. This transformation proceeds by dissociative CO loss and with a K(e q)(bridge/chelate) of approximately 1 at 45-degrees-C. The binuclear c omplex Co2(CO)4(mu-PhC=CPh){(Z)-Ph2PCH=CHPPh2} (3) exhibits a chelatin g diphosphine ligand with no evidence for the analogous chelate-to-bri dge exchange that was observed with 2. The alkyne complex 2 undergoes P-C(maleic anhydride) ligand cleavage coupled with alkyne-C(maleic anh ydride) and alkyne-PPh2 bond formation at 83-degrees-C to give 2,eta1, eta1-(Z)-Ph2PC(Ph)=C(Ph)C=C(PPh2)C(O)OC(O)} (4). All complexes have be en isolated and characterized in solution by IR and NMR spectroscopy. VT C-13 NMR measurements indicate that the ancillary CO groups in 2-4 are static with respect to intramolecular metal exchange over the temp erature range -91-degrees-C to room temperature. The solid-state struc tures of 2 (bridging), 3 (chelating), and 4 have been established by X -ray crystallography. Binuclear 2, as the CH2Cl2 solvate, crystallizes in the triclinic space group P1BAR: a = 10.712(1) angstrom, b = 11.14 67(8) angstrom, c = 19.944(1) angstrom, alpha = 78.311(6)-degrees, bet a = 89.029(7)-degrees, gamma = 68.169(7)-degrees, V = 2160.4(3) angstr om3, Z = 2, d(calc) = 1.475 g.cm-3; R = 0.0697, R(w) = 0.0850 for 1711 observed reflections. 3 crystallizes in the triclinic space group P1B AR: a = 9.983(2) angstrom, b = 12.047(2) angstrom, c = 16.651(2) angst rom, alpha = 74.67(1)-degrees, beta = 77.99(1)-degrees, gamma = 84.52( 1)-degrees, V = 1889.2(6) angstrom3, Z = 2, d(calc) = 1.414 g.cm-3; R = 0.0587, R(w) = 0.0658 for 2425 observed reflections. Compound 4 crys tallizes in the orthorhombic space group P2(1)2(1)2(1): a = 10.352(1) angstrom, b = 15.883(1) angstrom, c = 23.437(2) angstrom, V = 3853.5(6 ) angstrom3, Z = 4, d(calc) = 1.507 g.cm-3; R = 0.0529, R(w) = 0.0608 for 1141 observed reflections. The eight-electron ligand nylphosphino) -ethenyl]-3-(diphenylphosphino)maleic anhydride in 4 is bound to the t wo Co(CO)2 moieties, forming an eight-membered dimetallocyclic ring. T he redox properties of 2-4 have been examined by cyclic voltammetry. I n the case of the chelating and bridging isomers of 2, three well-defi ned one-electron responses were found and assigned to the 0/+1, 0/-1, and -1/-2 redox couples. IR analysis reveals that the unpaired electro n in the reduced species 2-. (chelating) resides primarily on the bma ligand. The nature of the HOMO and LUMO levels in 2 and 3 has been det ermined by extended Huckel MO calculations, the results of which are d iscussed with respect to the redox chemistry.