Crystalline phosphino-functionalized mesoionic olefins (p-MIOs) Merschel, Arne Merschel Arne Vishnevskiy, Yury Vishnevskiy Yury Neumann, Beate Neumann Beate Stammler, Hans-Georg Stammler Hans-Georg Ghadwal, Rajendra Ghadwal Rajendra Phosphino-functionalized mesoionic olefins (p-MIOs), (iMIC)CHR (iMIC = PhC{N(Dipp)}<sub>2</sub>C(PPh<sub>2</sub>)C, Dipp = 2,6-iPr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>; R = H 4a or Ph 4b), derived from a 1,3-imidazole-based mesoionic carbene (iMIC) are reported. The p-MIOs 4a (blue) and 4b (green) are crystalline solids and have been characterized by NMR spectroscopy and X-ray diffraction as well as DFT calculations. The calculated proton affinities of 4a (291.3 kcal mol<sup>−1</sup>) and 4b (288.1 kcal mol<sup>−1</sup>) support their remarkable basicity that exceeds that of classical NHCs. Treatment of 4a with (Me<sub>2</sub>S)AuCl (and AgOTf) affords the dinuclear gold compounds [(4a)Au](X)<sub>2</sub>(5-X) (X = Cl or OTf) with a short Au⋯Au distance of 3.130(2) Å for 5-Cl. The reaction of 4a with the main-group reagent PhBCl<sub>2</sub>yields the boronium cation [(4a)BPhCl]Cl (6). 51 21 8217-8222 8217-8222 Royal Society of Chemistry (RSC) 2022 1