Two Types of σ‐Allenyl Complexes from Reactions of Silylenes and Germylenes with Chromium Fischer Alkynyl(alkoxy)carbenes

The reactivity of amidinatotetrylenes of the type E( t Bu 2 bzm)R 1 (E=Si, Ge; t Bu 2 bzm= N , N ′‐bis(tertbutyl)benzamidinate; R 1 =alkyl or aryl) with the chromium Fischer alkynylcarbene complexes [Cr{C(OEt)C 2 R 2 }(CO) 5 ] (R 2 =Ph; ferrocenyl, Fc) has been studied. At room temperature, two different reaction pathways have been identified: (a) attack of the amidinatotetrylene to the alkynyl C 2 atom (γ‐attack), which leads to σ‐allenyl complexes in which the original C carbene atom maintains its attachment to the Cr(CO) 5 and OEt groups (compounds 3E - R1 - R2), and (b) attack of the amidinatotetrylene to the C carbene atom (α‐attack), which ends in σ‐allenyl complexes in which the original C carbene atom is not attached to the metal atom and has been inserted into an E−N bond of the amidinatotetrylene forming an E‐C‐N‐C‐N five‐membered ring (compounds 4E - R1 - R2). It has been found that compounds 3E - R1 - R2are thermodynamically less stable than their corresponding 4E - R1 - R2isomers and that some of the former (E=Ge; R 1 =CH 2 SiMe 3 ) can be transformed into the latter upon heating. At high temperatures (>70 °C) the reactions involving bulky amidinatotetrylenes (R 1 =Mes, t Bu) end in the carbene‐substitution products [Cr{E( t Bu 2 bzm)R 1 }(CO) 5 ].