Theoretical exploration of the Sn2H4 and Pb2H4 singlet potential energy surfaces led to a local minimum corresponding to an isomer with an unsymmetrical structure presenting a single X-H-X bridge and a short X-X distance. For tin and lead, this isomer lies at 8 and 15 kcal/mol, respectively, above the preferred doubly bridged structure and at 25 and 13 kcal/mol, respectively, below the dissociation products 2XH2(1A1). The molecule can be seen as two singlet XH2 moieties bound by a three-center two-electron bridge and a n-sigma --> P-pi dative bond HX-H...XH2. Electronic correlation, analyzed through a CASCF+OVB procedure, strengthens the X-X bond and makes it more covalent, suggesting another limiting from HX-XH3 with an internal H bridge. Several indexes support a direct Sn-Sn link stronger than that of Pb-Pb. These singly bridged forms occupy a key position on the group 14 X2H4 potential energy hypersurfaces since they can be topologically related to the doubly bridged forms (both trans C2h and cis C2v), the methylmethylene-like forms HX - XH3 and the doubly bonded forms H2X = XH2. With germanium and silicon the singly bridged arrangement is caught in the well of HX-XH3, but it should remain a crossing point between the four isomers. This provides a new global view of the H2xH4 potential surfaces for which the largest number of true minima - five - is observed only for Sn2H4.