Covalent character of three-center, two-electron systems – and

MP2/6-311++G(3df,3pd) calculations were performed on C 2 H 3 + and C 2 H 5 + three-center, two electron (3c–2e) systems with T-shaped approach of proton to acetylene and ethylene, respectively. The Quantum Theory of Atoms in Molecules (QTAIM) was applied to analyze these moieties. It was found that H+⋯π interactions in C 2 H 3 + and C 2 H 5 + possess the characteristics typical for covalent bonds. Two complexes: C 2 H 5 + ⋯ C 2 H 2 and C 2 H 3 + ⋯ H 2 were also considered where C 2 H 5 + and C 2 H 3 + act as proton donating systems while C2H2 and H2 as proton acceptors, respectively – these complexes possess the characteristics of H-bonded species.