The reactions of hydrogen or ammonia with germylenes and stannylenes were investigated experimentally and theoretically. Treatment of the germylene GeAr2# (Ar-# = C6H3-2,6-(C6H2-2,4,6-Me-3)(2)) with H-2 or NH3 afforded the tetravalent products (Ar2GeH2)-Ge-# (1) or (Ar2Ge)-Ge-#(H)NH2 (2) in high yield. The reaction of the more crowded GeAr'(2) (Ar' = C6H3-2,6-(C6H3-2,6-Pr-i(2))(2)) with NH3 also afforded a tetravalent amide Ar'Ge-2(H)NH2 (3), whereas with H-2 the tetravalent hydride Ar'GeH3 (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. SnAr2# reacted with NH3 to give the Sn(II) amide {(ArSn)-Sn-#(mu-NH2)}(2) (5) and (ArH)-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 NH3 to give {Ar'Sn(mu-H)}(2) (6), {Ar'Sn(mu-D)}(2) (7), or {Ar'Sn(mu-NH2)}(2) (8) all with arene elimination. The compounds were characterized by H-1, C-13, and Sn-119 NMR spectroscopy and by X-ray crystallography. DFT calculations revealed that the reactions of H-2 with EAr2 (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 Sri lone pair to the H-2 sigma(star) 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 NH3 with EAr2 (E = Ge or Sn; Ar = Ar-# or Ar'), the divalent ArENH2 products were calculated to be the most stable for both Ge and Sn. However the tetravalent amido species Ar2Ge(H)NH2 were obtained for kinetic reasons. The reactions with NH3 proceed by a different pathway from the hydrogenation process and involve two ammonia molecules in which the lone pair of one NH3 becomes associated with the empty 4p(Ge) or 5p(Sn) orbital while a second NH3 solvates the complexed NH3 via intermolecular N-H center dot center dot center dot N interactions.