FREE DOUBLY NEGATIVE TETRAHALIDES

The existence and properties of the closely related tetrahedral tetracoordinated dianions BeF42-, MgF42- and CaF42- in the gas phase are discussed. It is shown that these dianions exist in long-lived intermediate states bound by a barrier to dissociation. To definitely establish whether the added electrons remain bound, the outer-valence ionization potentials have been computed via the Green's function formalism which takes account of relaxation and electron correlation effects beyond the one-particle approximation. The results show that the potential barrier as well as the binding of the excess electrons of the dianions increase along the sequence BeF42-, MgF42-, and CaF42-. The possibility of dynamic autodetachment is analyzed by computing the potential energy surfaces of the lowest anionic and dianionic states of the tetrafluorides at both the independent-particle and the correlated levels. Strong effects of hole localization as a symmetry breaking phenomenon are observed accompanying the outer-valence ionization of the dinegative system sharing most of the outer-valence electron density on the symmetry equivalent fluorine ligands. Attempts are made to interrelate the nature or character of the chemical bonding with the thermodynamic stability of the dianion which also increases in going from BeF42- to MgF42- and finally to CaF42-. In this connection, some observations have been made which do not fit into expectations as taught by chemical experience. Calculations on the tetrachloride dianions BeCl42-, MgCl42-, and CaCl42- have also been performed. The major findings are briefly discussed.