Cyclische isomere vier‐ und fünfgliedrige heterodinucleare Fischer‐Carbenkomplexe — gebildet aus einem Vinylidentitanocen‐Fragment und Metallcarbonylen

Cyclic Isomeric Four‐ and Five‐Membered Heterodinculear Carbene Complexes — Formed from a Titanocene Vinylidene Fragment and Metal Carbonyls The four‐membered heterodimetallic carbene complexes Cp * 2 16 [L n M: Cr(CO) 5 ( a ), Mo(CO) 5 ( b ), W(CO) 5 ( c ), CpRe(CO) 2 ( f ), Mn 2 (CO) 9 ( g ), Re 2 (CO) 9 ( h ), Fe(CO) 4 ( i ), Rh(acac)(CO) ( i )] are easily prepared by trapping the vinylidenetitanocene [ Cp * 2 TiCCH 2 ] fragment 6 with the corresponding metal carbonyls. The vinylidenetitanocene 6 is generated in a favourable manner from Cp * 2 Ti(CH 3 )CHCH 2 ( 13 ) or Cp * (Fv)TiCH = CH 2 ( 14 ) [Fv: C 5 (CH 3 ) 4 CH 2 ] by a 1,3‐H shift (α elimination) at room temperature. The formation of 16 is characterized by a preferred suprafacial [2 + 2] cycloaddition. Starting from Cp * 2 ( 5 ) or 14 and transition metal carbonyls unexpected isomeric five‐membered titanacycles Cp * 2 8 [ML n = a–c , MnCp(CO) 2 ( d ), MnCp′(CO) 2 ( e ), f–i ] are obtained at higher temperatures (70–100°C). These oxatitanacyclopentenes 8 are formed also upon heating via the kinetic product 16 with high yield, as a result of a 1,2‐hydrogen shift, proved for c , g , h . From the spectroscopic data an η 1 ‐vinylcarbene structure is deduced, exhibiting a cis ‐ ( 16 ) or trans ‐configurated ( 8 ) L n MCCC sequence. An X‐ray structure analysis shows the importance of the acyl resonance form in 8a , typically observed for such heterodimetallic carbene complexes. The reversible liberation of the metal carbonyl is a remarkable property of complexes 16 in solution, whereas the thermodynamic product 8 shows the typical properties of Fischer carbene complexes.