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cis-Dicarbonyl-bis(2-methyl-1,3-butadiene)molybdenum(0) (1) and -tungsten(0) (2) were prepared photochemically from tricarbonylmesitylene-molybdenum(0) and aceto-nitrile-pentacarbonyl-tungsten(0), respectively, as thermolabile yellow oils. 2 is obtained as a mixture of two diastereomeric pairs of enantiomers 2a (2a') and 2c (2c'). From  1 H and  13 C NMR spectroscopic data 2 a (2a') is shown to have C2 symmetry, whereas 2 c (2c') is asymmetric. The NMR signals of 2 c (2 c') are temperature dependent due to an intramolecular 2c⇄2c'-isomerization (⊿G≠ 250  = 47 kJ · mol -1 ). A trigonal prismatic transi-tion state with C s  symmetry explains the D-NMR spectroscopic results. The NMR signals of 2 a (2a') are temperature dependent, too. This is taken as evidence for a ligand movement which brings 2a (2a') on the way to a third diastereomeric form 2b' (2b), with no detectable population. The molybdenum complex 1 forms only the isomers la (la') with C 2  symmetry.

zeitschrift für naturforschung b

Gehinderte Ligandbewegungen in Übergangsmetall-Komplexen, XXI [1]. Isomeric und Dynamik von cis-Dicarbonyl-bis(η 4  -2-methyl-1.3-butadien)wolfram(0) / Hindered Ligand Movements in Transitionmetal Complexes, XXI [1]. Isomerism and Dynamics of cis-Dicarbonyl-bis(η 4  -2-methyl-1,3-butadiene)tungsten(0)