Selenobenzaldehyd-Komplexe von Chrom und Wolfram: Synthese, Struktur und η1/η2-Isomerie

Article abstract of DOI:10.1016/0022-328X(89)80056-7

The benzylidene(pentacarbonyl)chromium and -tungsten complexes (CO)5M (II) (M=Cr: R=H (a), OMe (b); M=W: R=CF3 (c), H (d), Me (e), OMe (f), OEt (g)) react with SeCN by insertion of the selenium atom into the metal-benzylidene carbon bond to give pentacarbonyl(selenobenzaldehyde) complexes, (CO)5M (IIIa-g).The IR spectra of η¹-III and η²-III differ significatly.In solution the compounds are present in the from of a dynamic equilibrium between the η²- and the η¹-isomers.At room temperature the interconversion of the isomers is rapid on the 1H NMR time scale.Two isomers for IIIe can be detected below -113 deg Celsius.The η¹/η¹/η²-isomeric equilibrium shifts toward the η²-isomer (a) with decreasing electron-donating ability of the para-substituent R, (b) with decreasing temperature, (c) with decreasing polarity of the solvent and, (d) by replacement of chromium with tungsten in analogous complexes. the structure of η²-IIId was established by X-ray crystallography.The difference in enthalpy (ΔH) and entropy (ΔS) between the η¹- and the η²-isomers in toluene solution has been determined. ΔS is small and almost independent of the substituents R. ΔH for the rearrangement η²-III -> η¹-III decreases in the series IIIc, d, e and is negative for IIIf.The equilibrium constants are mainly determined by the donor/acceptor properties of R.