Transition metal coordinated CO ligands are usually attacked by carbanionic nucleophiles at the carbon atom[''. In the case of phosphorus ylides there is only indirect evidence of such an attack, since the primary metalcarbonyl-ylide adduct is immediately deprotonated by further ylide to give the acyl metalate'''.We have now found that reaction with the silyl-substituted ylide (CH3)3P=CHSi(CH3)3 takes a very different course: On starting with hexacarbonyl compounds of chromium, molybdenum and tungsten, neutral carbene-pentacarbonylmetal complexes of type (2), whose carbene carbon atom bears a trimethylphosphoranediylmethyl-and a trimethylsiloxy-group as substituents, are formed directly and quantitatively.In accord with this mode of product formation-to be regarded formally as addition of the ylide fragments (CH3)3P=CH and Si(CH3)3 to a CO ligand-the zwitterionic adduct ( 1 ) generated by initial carbanion attack is not stabilized, as in other cases, by transylidation, but by transfer of the silyl group to the negatively charged acyl oxygen. The 1,3-silyl shift leading to regeneration of the ylide function proceeds so rapidly that (1) is neither isolable under the reaction conditions (10°C) nor detectable by NMR spectroscopy at low temperatures (-30"Cf31. Accordingly the carbon bound silyl group in ( 1 ) displays extraordinary mobility, due to the possibility of formation of the stable Si-0 bondL4].The pale-yellow, readily crystallizable carbene complexes (2) are stable under N', but rapidly decompose in solution to, inter alia, M(C0)6 and hexamethyldisiloxane. The composition and structure of the complexes have been confirmed by analytical and spectroscopic findings. The H-NMR and I3C-NMR spectra show, respectively, the signal of the ylide proton and of the carbene carbon as doublets with characteristic splitting ('JHCp=34Hz; 'Jeep= 10Hz) at lowest field (6H=4.7; 6C=250). The presence of the C-0, P-C, and Si-0 bonds follows from the position of the stretching absorptions at 1062 (vs), 968 (s), and 752 (m) cm-'. The pentacarbonyl structure is proved from the v C 0 bands, which indicate both high steric demand and high o-donor/n-acceptor ratio for the ylide carbene ligand (5 IR-active vC0, low wavelengths).While, analogously to the M(C06) reaction, Fe(C0)5 and the silyl ylide afford the complex (C0)4FeC[OSi(CH3)3]-CH=P(CH3)3 (3), which is trigonal bipyramidal configurated and bears the carbene ligands in the axial position [local C3"
