Diphosphine Isomerization and C−H and P−C Bond Cleavage Reactivity in the Triosmium Cluster Os<sub>3</sub>(CO)<sub>10</sub>(bpcd):  Kinetic and Isotope Data for Reversible Ortho Metalation and X-ray Structures of the Bridging and Chelating Isomers of Os<sub>3</sub>(CO)<sub>10</sub>(bpcd) and the Benzyne-Substituted Cluster HOs<sub>3</sub>(CO)<sub>8</sub>(μ<sub>3</sub>-C<sub>6</sub>H<sub>4</sub>)[μ<sub>2</sub>,η<sup>1</sup>-PPhCC(PPh<sub>2</sub>)C(O)CH<sub>2</sub>C(O)]<sup>†</sup>

The coordination and reactivity of the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-dione (bpcd) with Os<sub>3</sub>(CO)<sub>10</sub>(MeCN)<sub>2</sub> (<b>1</b>) has been explored. The initial substitution product 1,2-Os<sub>3</sub>(CO)<sub>10</sub>(bpcd) (<b>2b</b>) undergoes a nondissociative, intramolecular isomerization to furnish the bpcd-chelated cluster 1,1-Os<sub>3</sub>(CO)<sub>10</sub>(bpcd) (<b>2c</b>) over the temperature range of 323−343 K. The isomerization reaction is unaffected by trapping ligands, yielding the activation parameters Δ<i>H</i><sup>⧧</sup> = 25.0(0.7) kcal/mol and Δ<i>S</i><sup>⧧</sup> = −2(2) eu. Thermolysis of <b>2c</b> in refluxing toluene gives the hydrido cluster HOs<sub>3</sub>(CO)<sub>9</sub>[μ-(PPh<sub>2</sub>)CC{PPh(C<sub>6</sub>H<sub>4</sub>)}C(O)CH<sub>2</sub>C(O)] (<b>3</b>) and the benzyne cluster HOs<sub>3</sub>(CO)<sub>8</sub>(μ<sub>3</sub>-C<sub>6</sub>H<sub>4</sub>)[μ<sub>2</sub>,η<sup>1</sup>-PPhCC(PPh<sub>2</sub>)C(O)CH<sub>2</sub>C(O)] (<b>4</b>). Time−concentration profiles obtained from sealed-tube NMR experiments starting with either <b>2c</b> or <b>3</b> suggest that both clusters are in equilibrium with the unsaturated cluster 1,1-Os<sub>3</sub>(CO)<sub>9</sub>(bpcd) and that the latter cluster serves as the precursor to the benzyne-substituted cluster <b>4</b>. The product composition in these reactions is extremely sensitive to CO, with the putative cluster 1,1-Os<sub>3</sub>(CO)<sub>9</sub>(bpcd) being effectively scavenged by CO to regenerate <b>2c</b>. Photolysis of cluster <b>2c</b> using near-UV light affords <b>3</b> as the sole product. These new clusters have been fully characterized in solution by IR and NMR spectroscopy, and the molecular structures of clusters <b>2b</b>,<b>c</b>, and <b>4</b> have been determined by X-ray crystallography. Reversible C−H bond formation in cluster <b>3</b> is demonstrated by ligand trapping studies to give 1,1-Os<sub>3</sub>(CO)<sub>9</sub>L(bpcd) (where L = CO, phosphine) via the unsaturated intermediate 1,1-Os<sub>3</sub>(CO)<sub>9</sub>(bpcd). The kinetics for reductive coupling in HOs<sub>3</sub>(CO)<sub>9</sub>[μ-(PPh<sub>2</sub>)CC{PPh(C<sub>6</sub>H<sub>4</sub>)}C(O)CH<sub>2</sub>C(O)] and DOs<sub>3</sub>(CO)<sub>9</sub>[μ-(PPh<sub>2</sub>-<i>d</i><sub>10</sub>)CC{P(Ph-<i>d</i><sub>5</sub>)(C<sub>6</sub>D<sub>4</sub>)}C(O)CH<sub>2</sub>C(O)] in the presence of PPh<sub>3</sub> give rise to a <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> value of 0.88, a value that supports the existence of a preequilibrium involving the hydride (deuteride) cluster and a transient arene-bound Os<sub>3</sub> species that precedes the rate-limiting formation of 1,1- Os<sub>3</sub>(CO)<sub>9</sub>(bpcd). Strong proof for the proposed hydride (deuteride)/arene preequilibrium has been obtained from photochemical studies employing the isotopically labeled cluster 1,1-Os<sub>3</sub>(CO)<sub>10</sub>(bpcd-<i>d</i><sub>4,</sub><i><sub>ortho</sub></i>), whose bpcd phenyl groups each contain one ortho hydrogen and deuterium atom. Generation of 1,1-Os<sub>3</sub>(CO)<sub>9</sub>- (bpcd-<i>d</i><sub>4</sub><i><sub>,ortho</sub></i>) at 0 °C gives rise to a 55:45 mixture of the corresponding hydride and deuteride clusters, re- spectively, from which a normal KIE of 1.22 is computed for oxidative coupling of the C−H(D) bond in the ortho metalation step. Photolysis of 1,1-Os<sub>3</sub>(CO)<sub>10</sub>(bpcd-<i>d</i><sub>4,</sub><i><sub>ortho</sub></i>) at elevated temperature and thermolysis of the low-temperature photolysis hydride/deuteride mixture afford an equilibrium mixture of hydride (67%) and deuteride (33%), yielding a <i>K</i><sub>eq</sub> value of 0.49, which in conjunction with the <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> ratio from the C−H(D) ortho-metalation step allows us to establish a <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> value of 0.60 for the reductive coupling from the participant hydride/deuteride clusters. These data, which represent the first isotope study on ortho metalation in a polynuclear system, are discussed relative to published work on benzene activation at mono- nuclear rhodium systems. UV−vis kinetic data on the transformation <b>3 </b>→<b> 4</b> provide activation parameters consistent with the rate-limiting formation of the unsaturated cluster 1,1-Os<sub>3</sub>(CO)<sub>9</sub>(bpcd), preceding the irreversible P−C cleavage manifold. The ortho metalation of the bpcd ligand in <b>3</b> and formation of the benzyne moiety <b>4</b> are discussed relative to ligand degradation reactions in this genre of cluster.