Reaction of N,N′-dimesitylformamidine with dimethylmalonyl dichloride in dichloromethane in the presence of an excess of triethylamine gives the 2-chloro-4,5-dioxohexahydropyrimidine 1. The corresponding diamidocarbene 3 is generated in situ by further deprotonation with KHMDS at –40 °C and identified by trapping with S8 to give the fully characterized (including X-ray structure) sulfur adduct 4. It also reacts with [RhCl(cod)]2 to yield the NHC complex [RhCl(3)(cod)] (5) (characterized also by X-ray structure). The donor properties of 3 were evaluated against the established IR [ν(CO)] scale from [RhCl(3)(CO)2] (6). The average value of ν(CO) = 2045 cm–1 indicates that the diamidocarbene 3 is much less nucleophilic than structurally relevant six-membered NHCs including the anionic diaminocarbenes previously reported in our group.