Rajendra S. Ghadwal

We report the room temperature dimerization of carbon monoxide mediated by C4/C5‐vicinal anionic dicarbenes Li(ADC) (ADC = ArC{(Dipp)NC} 2 ; Dipp = 2,6‐ i Pr 2 C 6 H 3 ; Ar = Ph, DMP (4‐Me 2 NC 6 H 4 ), Bp (4‐PhC 6 H 4 )) to yield ( E )‐ethene‐1,2‐bis(olate) (i.e. − O−C=C−O − = CO en ) bridged mesoionic carbene (iMIC) lithium compounds CO en ‐[(iMIC)Li] 2 (CO en ‐[iMIC] 2 = [ArC{(Dipp)NC} 2 (CO)] 2 ) in quantitative yields. CO en ‐[(iMIC)Li] 2 are highly colored stable solids, exhibit a strikingly small HOMO–LUMO energy gap, and readily undergo 2e‐oxidations with selenium, CuCl (or CuCl 2 ), and AgCl to afford the dinuclear compounds CO on ‐[(iMIC)E] 2 (E = Se, CuCl, AgCl) featuring a 1,2‐dione bridged neutral bis‐iMIC (i.e. CO on ‐[iMIC] 2 = [ArC{(Dipp)NC} 2 (C=O)] 2 ). CO en ‐[(iMIC)Li] 2 undergo redox‐neutral salt metathesis reactions with LiAlH 4 and (Et 2 O) 2 BeBr 2 and afford CO en ‐[(iMIC)AlH 2 ] 2 and CO en ‐[(iMIC)BeBr] 2 , in which the dianionic CO en ‐moiety remains intact. All compounds have been characterized by NMR spectroscopy, mass spectrometry, and X‐ray diffraction. Stereoelectronic properties of CO on ‐[iMIC] 2 are quantified by experimental and theoretical methods.