Crystalline (NN)C-M(NN) complexes: synthesis, structure, bonding and lability [M = Si, Ge, Sn or Pb; (NN)=1,2-((BuCH2N)-C-t)(2)C6H4]

Each of the red-brown or red, isomorphous, crystalline adducts (NN)C-M(NN) [(NN)=1,2-((BuCH2N)-C-t)(2)C6H4 and M=Si, Ge, Sn or Pb] has been obtained f rom the carbene (NN)C and the appropriate silylene, germylene, stannylene o r plumbylene M(NN) in benzene and crystallisation from a hydrocarbon. They are monomeric, with the three-co-ordinate C-carb and M atoms in an almost p lanar (C) or pyramidal (M) environment. The C-M distances are more than ca. 10% longer than for a typical M-II-C bond in an MR2 molecule. Variable tem perature C-13 and M-x NMR spectra (M-x=Si-29, Sn-119 or Pb-207) in toluene- d(8), as well as for the appropriate M(NN) and C(NN) precursors, have been recorded. The C-carb and M chemical shift data show that the compounds read ily dissociate in solution, their stability decreasing in the sequence Sn > Pb > Si > Ge. From the magnitude of the chemical shifts, their conformatio n and C-M distances of the adducts, it is concluded that the C-M bond in ea ch adduct is best formulated as being electrostatic in nature, with the car bene moiety as electron donor and the M(NN) fragment as acceptor.