Reactions of the molybdenum bis(alkyne) complex [mo(SC₆F₅)(F₃CC≡CCF₃) ₂(η⁵-C₅H₅)] with alkynes leading to isomeric and fluxional η⁴-butadienyl complexes

Reactions of the bis(hexafluorobut-2-yne) complex [Mo(SC₆F₅)(F₃CC=CCF₃) ₂(?⁵-C₅H₅)] 1 with alkynes RC=CR' give the novel ?⁴-butadienyl complex prone-[Mo{?⁴-C(CF₃)C(CF₃)=CRCR(SC ₆F₅)}(F₃CC=CCF₃)(? ⁵-C₅H₅)] 2 (R = R' = Me; R = H, R' = Me or Ph; R = Me, R' = CO₂Me) which undergoes isomerisation via an ?²-C,C vinyl intermediate [Mo{?³-C(CF₃)C(CF₃)CR=CR'(SC ₆F₅)}(F₃CC=CCF₃)(? ⁵-C₅H₅)] 3 (R = R' = Me, Et or Ph; R = Me, R' = Co₂Me or Ph; R = H, R' = Ph) to the supine isomer 4 (R = R' = Me or Et; R = Me, R' = CO₂Me or Ph; R = H, R' = Me or Ph) followed by a novel 1,4 thiolate migration to give supine-[Mo{?⁴-CR'CR;C(CF₃)C(CF ₃)(SC₆F₅)}(F₃CC=CCF ₃)(?⁵-C₅H₅)] 5 (R = R' = Ph; R = Me, R' = Ph). Fluorine-19 NMR studies established that with PhC=CPh 3 and 5 exist in solution as an equilibrium mixture. With PhC=CMe exchange between 3, 4 and 5 is faster which allowed study by dynamic ¹⁹F NMR and spin-saturation-transfer experiments.