Fluxionality of M2C2 tetrahedral clusters containing cyclopentadienyldicarbonylmetal vertices: Unexpectedly different behaviour of molybdenum and tungsten analogues

The syntheses of a series of molecules [Cp′M(CO)2]2(RCCR′), where Cp′ = C5Me5, C5H4Me; M = Mo, W; R = Ph, R′= H, Et, CO2Me, SnMe3 are described. The molybdenum complexes possess a semi-bridged carbonyl thus rendering the molecules chiral; their 13C NMR spectra show four carbonyl resonances and two cyclopentadienyl environments. The barrier to racemisation depends on the bulk of the substituents on the ring. In contrast, the NMR spectra of the tungsten analogues show only single carbonyl and cyclopentadienyl environments even at −90°C and 9.4 T.