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

Abstract

The alkali-metal trihalides MX (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 structure as the global minimum for all MX species. Much higher level CCSD(T) computations show a moderate to strong distortion of the X anion by the M cation in the respective equilibrium geometries. Most obviously, for LiCl, the two Cl-Cl distances are separated by 0.786 Å. Even for CsI, the structure least distorted from the MX 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 MX 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" X anions, relatively localized and mutually perturbed X-X and M-X stretches are calculated. The suggestion emerges that the MX system may be alternatively described as an MX-X complex rather than the MX 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 X moieties. The thermochemistry of fragmentations of MX to MX + X versus M + X also supports the alternative viewpoint of the bonding in this class of molecules.

Unraveling the Bonding Complexity of Polyhalogen Anions: High-Pressure Synthesis of Unpredicted Sodium Chlorides Na2Cl3 and Na4Cl5 and Bromide Na4Br5

“…The discrete trichloride anions [Cl 3 ] − in the structure of hP 24-KCl 3 were found to be linear and symmetric (at 41 GPa, the intramolecular Cl1–Cl2 distances are of 2.301(3) Å, while the shortest intermolecular ones are of 2.800(5) Å (Figure S1b)). The [Cl 3 ] − anion in hP 24-KCl 3 is an example of 22-valence electron systems, which are well known in inorganic chemistry − (XeF 2 and trihalide anions being among them). As several so far known trichloride [Cl 3 ] − anions are asymmetric, , the [Cl 3 ] − anion in hP 24-KCl 3 provides the first example of a symmetric trichloride anion, similar to the symmetric [Br 3 ] − and [I 3 ] − known at ambient pressure. , This agrees with the empirical rule of high-pressure crystal chemistryelements behave at HP like the elements below them in the periodic table at lower pressure …”

“…These delocalized electrons contribute to the conduction bands making a σ-conjugated system in these materials metallic (see the inserts in Figures c,d and S9). In contrast, the [Cl 3 ] − ion in hP 24-KCl 3 , which is isoelectronic to XeF 2 with the 4-electron 3-center (4e–3c) hypervalent bonding system, − is obviously non-metallic (Figure S12). In addition, the −pCOHP curves explain the difference in the ELFs of cP 8-NaX 3 and hP 18-Na 4 X 5 .…”

Tracking structural phase transitions via single crystal x-ray diffraction at extreme conditions: advantages of extremely brilliant source

“…Cesium triiodide is an archetypical example of asymmetric trihalide [40][41][42]. It is one of the few inorganic trihalides stable at ambient conditions.…”