The reaction of the bis(amino)allenylidene complex [(CO) 5 WdCdCdC(NMe 2 ) 2 ] with diethyldiazomethane yields two products, a cyclic carbene complex (2) by 1,2-addition of Et 2 C-N 2 to the C R -C β bond of the allenylidene ligand and the η 1 -butatriene complex [(CO) 5 W{C[C(NMe 2 ) 2 ]d CdCEt 2 }] (3). Complex 2 slowly eliminates N 2 and rearranges into 3. In contrast, only η 1 -butatriene complexes, [(CO) 5 M{C[C(NMe 2 )XR]dCdC(R 0 ) 2 }], are isolated from the reaction of diazoalkanes (R 0 ) 2 C-N 2 (R 0 = Me, Et, n Pr) with alkoxy(amino)allenylidene complexes [(CO) 5 MdCd CdC(NMe 2 )OR] (M = Cr, W; R = Me, Et, (-)-menthyl) or with the (alkylthio)(amino)allenylidene complex [(CO) 5 WdCdCdC(NMe 2 )S n Pr]. These new η 1 -butatriene complexes are related to C R phosphine adducts of allenylidene complexes such as [(CO) 5 W{C(PMe 3 )dCd C(NMe 2 )Ph}] (17) and might be regarded as C R carbene adducts. However, the PMe 3 substituent in 17 is not replaced when 17 is treated with an N-heterocyclic carbene. Vice versa, the "carbene" substituent "C(NMe 2 )OEt" in the η 1 -butatriene complex [(CO) 5 Cr{C[C(NMe 2 )OEt]dCdCEt 2 }] is not replaceable by PMe 3 . Free N-heterocyclic carbenes do not add to the C R atom of the allenylidene complex [(CO) 5 WdCdCdC(C 6 H 4 NMe 2 -p) 2 ] but instead to the C γ atom, giving the dipolar neutral alkynyl complexes [(CO) 5 W-CtCC(C 6 H 4 NMe 2 -p) 2 L] (L = SIMe, SIMes). DFT calculations on the reaction mechanism indicate that a cyclic carbene complex and two isomeric η 2 -butatriene complexes are intermediates in the reaction pathway to form η 1 -butatriene complexes. The structure of two representative examples of η 1 -butatriene complexes and of one C γ carbene adduct has been established by X-ray structure analyses. † Dedicated to Professor J€ urgen Heck on the occasion of his 60th birthday.