Conformational preferences of 34 valence electron A 2 X 4 molecules: An ab initio Study of B 2 F 4 , B 2 Cl 4 , N 2 O 4 , and C 2 O 4 2−

Ab initio molecular orbital structures and energies of B 2 F 4 , B 2 Cl 4 , N 2 O 4 , and C 2 O   4 2−have been calculated for both perpendicular D 2 d and planar D 2 h rotamers. The experimental trend toward greater preference for the D 2 d forms in going from B 2 F 4 to B 2 Cl 4 is reproduced. N 2 O 4 favors the planar conformation, although the rotation barrier is overestimated at the theoretical levels used. The oxalate dianion is calculated to be more stable in the D 2 d conformation; the experimental planar arrangement in the solid may be due to crystal packing forces. The preferences for one conformation over another are small; analysis indicates that different effects may predominate in each case: π stabilization for B 2 F 4 , hyperconjugation for B 2 Cl 4 , lone‐pair interactions for N 2 O 4 , and electrostatic repulsions for C 2 O   4 2− .