Many transition-metal complexes ML decompose diazo compounds N=CR 1 R 2 generating metal-carbenes LM=CR 1 R 2 which transfer the carbene group to other substrates, constituting an important tool in organic synthesis. All previous reports have shown that the CR 1 R 2 fragment at the metal-carbene remains intact from the parent diazo compound. Herein we report the detection and isolation of a monosubstituted copper carbene where the CR 1 R 2 ligand has undergone a modification from the initial diazo reagent. When Tp Ms Cu(THF) (Tp Ms = hydrotris(3-mesityl)pyrazolylborate ligand) was reacted with N,N -diethyl diazoacetamide [N=C(H)(CONEt)], the stable copper carbene Tp Ms Cu=C(H)(NEt) was isolated, resulting from a decarbonylation process, with carbon monoxide being trapped as Tp Ms Cu(CO). The simultaneous observation of products derived from the intramolecular carbene insertion reaction into C-H bonds demonstrates that the expected Tp Ms Cu=C(H)(CONEt) complex is also formed. Experimental data, DFT calculations, and microkinetic models allow us to propose that the latter undergoes CO loss en route to the former.