Synthesis, Reactivity, and Theoretical Studies of the η²(4e)-Bonded Phosphaalkyne Complex [CpMo{P(OMe)₃}₂{η²(4e)-P⋮CBu^t}][B(C₆F₅)₄] and the Molybdenum-Mediated Cyclocotrimerization of Alkyne and Phosphaalkyne Ligands

Whereas the cations [CpMo{P(OMe)3}2{η2(4e)-alkyne}]+ do not react with alkynes or P⋮CBut, the newly synthesized isostructural phosphaalkyne complex [CpMo{P(OMe)3}2{η2(4e)-P⋮CBut}][B(C6F5)4], which is unreactive towards PhC2Ph, readily reacts via an associative stepwise process with P⋮CBut to give [CpMo{P(OMe)3}2{η4-1,3-P2C2But2}][B(C6F5)4]. A further interesting difference in alkyne and phosphaalkyne chemistry was observed when it was found that CpMoCl(CO){η2(4e)-PhC2Ph} reacts with TlPF6 and P⋮CBut to give the unusual 16-electron cyclocotrimerization product identified by single-crystal X-ray crystallography.