Two different syntheses of N-heterocyclic carbene complexes of P(I) cations are presented; the structural features of these phosphamethine cyanine salts, in conjunction with the results of computational investigations, provide insight into the nature of the bonding of these heavy allene analogues.

Ethylene's cycloadditions to unsaturated hydrocarbons occupy well-established ground in classical organic chemistry. In contrast, its reactivity toward alkene and alkyne analogs of carbon's heavier-element congeners silicon, germanium, tin, or lead has been little explored. We show here that treatment of the distannynes Ar(iPr4)SnSnAr(iPr4) [Ar(iPr4) = C6H3-2,6(C6H3-2,6-iPr2)2, 1] or Ar(iPr8)SnSnAr(iPr8) [Ar(iPr8) = C6H-2,6(C6H2-2,4,6-iPr3)2-3,5-iPr2, 2] with ethylene under ambient conditions affords the cycloadducts Ar(iPr4) Sn(mu2:nu1:n1-C2H4)2Sn Ar(iPr4 (3) or Ar(iPrs) Sn(mu2:nu1:nu1-C2H4)2Sn AriPrs (4) that were structurally and spectroscopically characterized. Ethylene incorporation in 3 and 4 involves tin-carbon sigma bonding and is shown to be fully reversible under ambient conditions; hydrocarbon solutions of 3 or 4 revert to the distannynes 1 or 2 with ethylene elimination under reduced pressure or upon standing at approximately 25 degrees C. Variable-temperature proton nuclear magnetic resonance studies showed that the enthalpies of reaction were near -48 (3) and -27 (4) kilojoules per mole.

The reactions of hydrogen or ammonia with germylenes and stannylenes were investigated experimentally and theoretically. Treatment of the germylene GeAr(#)(2) (Ar(#) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Me(3))(2)) with H(2) or NH(3) afforded the tetravalent products Ar(#)(2)GeH(2) (1) or Ar(#)(2)Ge(H)NH(2) (2) in high yield. The reaction of the more crowded GeAr'(2) (Ar' = C(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr(2))(2)) with NH(3) also afforded a tetravalent amide Ar'(2)Ge(H)NH(2) (3), whereas with H(2) the tetravalent hydride Ar'GeH(3) (4) was obtained with Ar'H elimination. In contrast, the reactions with the divalent Sn(II) aryls did not lead to Sn(IV) products. Instead, arene eliminated Sn(II) species were obtained. SnAr(#)(2) reacted with NH(3) to give the Sn(II) amide {Ar(#)Sn(mu-NH(2))}(2) (5) and Ar(#)H elimination, whereas no reaction with H(2) could be observed up to 70 degrees C. The more crowded SnAr'(2) reacted readily with H(2), D(2), or NH(3) to give {Ar'Sn(mu-H)}(2) (6), {Ar'Sn(mu-D)}(2) (7), or {Ar'Sn(mu-NH(2))}(2) (8) all with arene elimination. The compounds were characterized by (1)H, (13)C, and (119)Sn NMR spectroscopy and by X-ray crystallography. DFT calculations revealed that the reactions of H(2) with EAr(2) (E = Ge or Sn; Ar = Ar(#) or Ar') initially proceed via interaction of the sigma orbital of H(2) with the 4p(Ge) or 5p(Sn) orbital, with back-donation from the Ge or Sn lone pair to the H(2) sigma* orbital. The subsequent reaction proceeds by either an oxidative addition or a concerted pathway. The experimental and computational results showed that bond strength differences between germanium and tin, as well as greater nonbonded electron pair stabilization for tin, are more important than steric factors in determining the product obtained. In the reactions of NH(3) with EAr(2) (E = Ge or Sn; Ar = Ar(#) or Ar'), the divalent ArENH(2) products were calculated to be the most stable for both Ge and Sn. However the tetravalent amido species Ar(2)Ge(H)NH(2) were obtained for kinetic reasons. The reactions with NH(3) proceed by a different pathway from the hydrogenation process and involve two ammonia molecules in which the lone pair of one NH(3) becomes associated with the empty 4p(Ge) or 5p(Sn) orbital while a second NH(3) solvates the complexed NH(3) via intermolecular N-H...N interactions.

Treatment of the stannylenes SnAr'2 (Ar' = C6H3-2,6(C6H3-2,6-Pri2)2), SnAr2# (Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2, or Sn{N(SiMe3)2}2 with H2, D2, or NH3 under identical, mild (1 atm, 65 degrees C) conditions showed that SnAr2' reacted readily to afford the products {Ar'Sn(mu-H)}2, {Ar'Sn(mu-D)}2, or {Ar'Sn(mu-NH2)}2 with elimination of Ar'H or Ar'D. The less crowded stannylenes SnAr2# and Sn{N(SiMe3)2}2 were recovered unreacted. The differences in reactivity were accounted for in terms of the n-p energy separations which is lowest in the case of AnAr2'. The low energy separation in SnAr' is consistent with enhanced singlet diradical character of the ground state which increases its reactivity.