Multisite Reactivity of the Central Mo<sub>2</sub>CP Core in the Unsaturated Carbyne-Bridged Complex [Mo<sub>2</sub>(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>(μ-CPh)(μ-PCy<sub>2</sub>)(CO)<sub>2</sub>]

The title compound reacted with HBF<sub>4</sub>·OEt<sub>2</sub> at room temperature to give a mixture of the agostic-like, phosphine-bridged complex [Mo<sub>2</sub>Cp<sub>2</sub>(μ-CPh)(μ-κ<sup>1</sup>:η<sup>2</sup>-PHCy<sub>2</sub>)(CO)<sub>2</sub>]BF<sub>4</sub> (major) and the carbene-bridged complex [Mo<sub>2</sub>Cp<sub>2</sub>(μ-η<sup>1</sup>:η<sup>2</sup>-CHPh)(μ-PCy<sub>2</sub>)(CO)<sub>2</sub>]BF<sub>4</sub> (minor). It readily added a molecule of HCCCO<sub>2</sub>Me or a single Se atom at its Mo<sub>2</sub>C(carbyne) center to give with high yield the corresponding propenylylidene- or selenoacyl-bridged derivatives [Mo<sub>2</sub>Cp<sub>2</sub>{μ-η<sup>2</sup>:η<sup>3</sup>-CPhCHC(CO<sub>2</sub>Me)}(μ-PCy<sub>2</sub>)(CO)<sub>2</sub>] and [Mo<sub>2</sub>Cp<sub>2</sub>{μ-η,κ:η,κ-C(Ph)Se}(μ-PCy<sub>2</sub>)(CO)<sub>2</sub>], respectively. In contrast, the addition of a neat donor at the metal site can induce a reversible carbyne–carbonyl coupling, as observed in the reaction with N<sub>2</sub>CPh<sub>2</sub> to give the ketenyl derivative [Mo<sub>2</sub>Cp<sub>2</sub>{μ-η<sup>1</sup>:η<sup>2</sup>-C(Ph)CO}(μ-PCy<sub>2</sub>)(CO)(κ<sup>1</sup>-N<sub>2</sub>CPh<sub>2</sub>)].