Diyne and phosphorus-carbon bond reactivity in the reaction between [Co-2(CO)(6)](2)(PhC(4)Ph) and 2,3-bis(diphenylphosphino)maleic anhydride (bma). Syntheses, molecular orbital properties, and x-ray diffraction structures of Co-2(CO)(4)[mu-eta(2):eta(2):eta(1):eta(1)-(Z)-Ph(2)P(Ph)C=C(PhC(2))-C=C(Ph(2)P)C(O)OC(O)], Co-2(CO)(2)(bma)(2)center dot CH2Cl2, and Co-2(CO)(2)(bma)[mu-C=C(Ph(2)P)C(O)OC(O)](mu(2)-Ph(2)P)center dot C6H14

Thermolysis (>80 degrees C) of the diyne complex [Co-2(CO)(6)](2)(PhC(4)Ph) (1) with 2,3-bis(diphenylphosphino)maleic anhydride (bma) affords the new compounds Co-2(CO)(4)[mu-eta(2):eta(2):eta(1):eta(1)-(Z)-Ph(2)P(Ph)C=C(PhC(2))C=C(Ph(2)P)C(O)OC(O)] (3), Co-2(CO)(2)(bma)(2) (4), and Co-2(CO)(2)(bma)[mu-C=C(Ph(2)P)C(O)OC(O)](mu(2)-Ph(2)P) (5) in low yields, while [Co-2(CO)(6)](2)(PhC(4)Ph) reacts with added bma in either refluxing Ch(2)Cl(2) or in the presence of Me(3)NO to give the thermally sensitive complex [Co-2(CO)(4)(bma)(PhC(4)Ph)Co-2(CO)(6)]. (2). Independent experiments reveal that 3 arises from 2 by loss of the Co-2(CO)(6) group, coupled with P-C bond cleavage and diyne functionalization by the transient phosphido and maleic anhydride moieties, and the reaction between 2 and excess bma leads to both 4 and 5, with 5 originating from 4. The kinetics for the reaction of 4 to 5 have been measured by UV-vis spectroscopy, and on the basis of the first-order rate constants and the activation parameters (Delta H double dagger = 27.0 +/- 0.6 kcal mol(-1) and Delta S double dagger = 1.0 +/- 0.3 eu), a mechanism involving dissociative CO loss as the rate-determining step is presented. Binuclear 4 is extremely photosensitive and is converted cleanly to 5 by 366 nm light with a quantum efficiency of 0.0043. Compounds 2-5 hav been isolated and characterized in solution by IR and P-31 NMR spectroscopy. The solid-state structures of 3-5 have been established by X-ray crystallography. The X-ray structure of 4 reveals the presence of two bma ligands that are attached to the Co-2 (CO)(2) unit in a head-to-tail fashion via the PPh(2) groups and maleic anhydride pi bond, and the X-ray structure of 5 supports the existence of a mu(2)-PPh(2) moiety and a noncomplexed maleic anhydride pi bond, the result of P-C(maleic anhydride) bond activation. The redox properties of 2-5 were explored by cyclic voltammetry in CH2Cl2, and the oxidation/reduction behavior is discussed with respect to redox stabilization that each complex experiences, as modulated by the bma ligands(s). The orbital composition of the HOMO and LUMO levels in 3-5 has been studied by extended Huckel calculation, and the data are discussed relative to the observed electrochemistry. A plausible mechanism for the activation of Co-2(CO)(2)(bma)(2) and the formation of Co-2(CO)(2)(bma)[mu-C=C(Ph(2)P)C(O)OC(O)](mu(2)-Ph(2)P) is presented.