Alkali-Metal Trihalides: M⁺X₃^– Ion Pair or MX–X₂ Complex?

The alkali-metal trihalides MX₃ (M = Li, Na, K, Rb, and Cs; X = Cl, Br, and I) are systematically studied using coupled-cluster methods. Benchmarks using CCSD­(T) against diatomic experimental results suggest satisfactory performance for the weighted core-valence basis sets (new basis sets for K, Rb, and Cs) selected for predicting reliable structures and harmonic vibrational frequencies. An isomer search using the B3LYP functional yields a planar, yet asymmetric T-shaped C_s structure as the global minimum for all MX₃ species. Much higher level CCSD­(T) computations show a moderate to strong distortion of the X₃^– anion by the M⁺ cation in the respective equilibrium geometries. Most obviously, for LiCl₃, the two Cl–Cl distances are separated by 0.786 Å. Even for CsI₃, the structure least distorted from the M⁺X₃^– model, the two I–I distances differ by 0.243 Å. It does not take much energy to distort the parent anions along an antisymmetric stretch, so this is no surprise. The normal modes of vibration of the MX₃ molecules are in better agreement with matrix isolation experiments than previous calculations. And these normal modes reveal that, instead of the well-established antisymmetric and symmetric stretches of the “free” X₃^– anions, relatively localized and mutually perturbed X–X and M–X stretches are calculated. The suggestion emerges that the MX₃ system may be alternatively described as an MX–X₂ complex rather than the M⁺X₃^– ion pair. This perspective is supported by bonding analyses showing low electron densities at the bond critical points and natural bond orders between the MX and X₂ moieties. The thermochemistry of fragmentations of MX₃ to MX + X₂ versus M⁺ + X₃^– also supports the alternative viewpoint of the bonding in this class of molecules.