Wolfram(0)-komplexe des π-aciden N-methylpyrazinium-kations und des N-methylpyrazinium-radikals. Kristall- und molekülstruktur von [(C4H4N2-Me)W(CO)3(PCy3)2] (PF6), Cy cyclohexyl

Abstract Bonding characteristics of the dihydrogen-coordinating 16 valence electron species W(CO) 3 (PR 3 ) 2 (R  i Pr or Cy (cyclohexyl)) were determined by studying stable complexes with the poorly 2e-donating but strongly π-accepting cationic ligand N - methylpyrazinium (mpz + ). In comparison with the related complex with W(CO) 5 , the 1 H NMR and electrochemical data suggest a very strong degree of π back-donation from W(CO) 3 (PR 3 ) 2 to the cationic acceptor in the ground state. This view is supported by the crystal structure analysis of [(mpz)W(CO) 3 (PCy 3 ) 2 ](PF 6 ) which reveals a trans,mer -configuration with an unusually short WN bond (210.1(10) pm). Reduction to the N -methylpyrazinium radical complexes yields highly resolved ESR spectra which show the loss of one phosphine ligand in the case of the tricarbonyl complexes. However, even the W(CO) 3 (PR 3 ) fragment induces considerable spin delocalization from the pyrazinium radical ligand to the metal.