Ab initio calculations were performed to study the stability of various pyrophosphate species in the gas phase : H4P2O7, H3P2O7-, H2P2O72-, HP2O73-, P2O74-, and their complexes with Mg2+. It is found that the metal cation allows the existence of highly charged anions in the gas phase. We also study the isomerization reactions Mg . H2P2O7 --> (H2PO4 . Mg . PO3), (Mg . HP2O7)(-) --> (HPO4 . Mg . PO3)(-), and (Mg . P2O7)(2-) --> (PO4 . Mg . PO3)(2-), at the self-consistent-field (SCF) and second-order perturbation (MP2) levels of the theory, using a 6-31+G** basis set with diffuse and polarization functions. Other basis sets, including one of valence triple zeta plus polarization (vTZP) quality, were employed to check for the convergence of the results. It is found that the same mechanism occurs for the isomerizations of the three species : one of the P-O bridging bonds of the reactant is longer than the other, and the route to the products proceeds through its elongation. : H4P2O7, H3P2O7-, H2P2O72-, HP2O73-, P2O74-, and their complexes with Mg2+. It is found that the metal cation allows the existence of highly charged anions in the gas phase. We also study the isomerization reactions Mg . H2P2O7 --> (H2PO4 . Mg . PO3), (Mg . HP2O7)(-) --> (HPO4 . Mg . PO3)(-), and (Mg . P2O7)(2-) --> (PO4 . Mg . PO3)(2-), at the self-consistent-field (SCF) and second-order perturbation (MP2) levels of the theory, using a 6-31+G** basis set with diffuse and polarization functions. Other basis sets, including one of valence triple zeta plus polarization (vTZP) quality, were employed to check for the convergence of the results. It is found that the same mechanism occurs for the isomerizations of the three species : one of the P-O bridging bonds of the reactant is longer than the other, and the route to the products proceeds through its elongation. : H4P2O7, H3P2O7-, H2P2O72-, HP2O73-, P2O74-, and their complexes with Mg2+. It is found that the metal cation allows the existence of highly charged anions in the gas phase. We also study the isomerization reactions Mg . H2P2O7 --> (H2PO4 . Mg . PO3), (Mg . HP2O7)(-) --> (HPO4 . Mg . PO3)(-), and (Mg . P2O7)(2-) --> (PO4 . Mg . PO3)(2-), at the self-consistent-field (SCF) and second-order perturbation (MP2) levels of the theory, using a 6-31+G** basis set with diffuse and polarization functions. Other basis sets, including one of valence triple zeta plus polarization (vTZP) quality, were employed to check for the convergence of the results. It is found that the same mechanism occurs for the isomerizations of the three species : one of the P-O bridging bonds of the reactant is longer than the other, and the route to the products proceeds through its elongation.