TUNGSTEN(0) COMPLEXES OF PI-ACCEPTING N-METHYLPYRAZINIUM CATION AND THE N-METHYLPYRAZINIUM RADICAL - CRYSTAL AND MOLECULAR-STRUCTURE OF [(C4H4N2-ME)W(CO3(PCY3)2](PF6), CY = CYCLOHEXYL

Bonding characteristics of the dihydrogen-coordinating 16 valence electron species W(CO)3(PR3)2 (R = (i)Pr or Cy (cyclohexyl)) were determined by studying stable complexes with the poorly 2e-donating but strongly pi-accepting cationic ligand N-methylpyrazinium (mpz+). In comparison with the related complex with W(CO)5, the H-1 NMR and electrochemical data suggest a very strong degree of pi back-donation from W(CO)3(PR3)2 to the cationic acceptor in the ground state. This view is supported by the crystal structure analysis of [(MPZ)W(CO)3(PCy3)2](PF6) 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(PR3) fragment induces considerable spin delocalization from the pyrazinium radical ligand to the metal.