Nature of the Arsonium-Ylide Ph3 As=CH2 and a Uranium(IV) Arsonium-Carbene Complex.

Treatment of [Ph3 EMe][I] with [Na{N(SiMe3 )2 }] affords the ylides [Ph3 E=CH2 ] (E=As, 1As; P, 1P). For 1As this overcomes prior difficulties in the synthesis of this classical arsonium-ylide that have historically impeded its wider study. The structure of 1As has now been determined, 45 years after it was first convincingly isolated, and compared to 1P, confirming the long-proposed hypothesis of increasing pyramidalisation of the ylide-carbon, highlighting the increasing dominance of E+ -C- dipolar resonance form (sp3 -C) over the E=C ene π-bonded form (sp2 -C), as group 15 is descended. The uranium(IV)-cyclometallate complex [U{N(CH2 CH2 NSiPri 3 )2 (CH2 CH2 SiPri 2 CH(Me)CH2 )}] reacts with 1As and 1P by α-proton abstraction to give [U(TrenTIPS )(CHEPh3 )] (TrenTIPS =N(CH2 CH2 NSiPri 3 )3 ; E=As, 2As; P, 2P), where 2As is an unprecedented structurally characterised arsonium-carbene complex. The short U-C distances and obtuse U-C-E angles suggest significant U=C double bond character. A shorter U-C distance is found for 2As than 2P, consistent with increased uranium- and reduced pnictonium-stabilisation of the carbene as group 15 is descended, which is supported by quantum chemical calculations.