The NaDyBr4 complex; its molecular structure and thermodynamic properties

Abstract

The molecular geometry, vibrational frequencies, relative stabilities, and some thermodynamic properties of different isomers of the NaDyBr 4 complex were determined by computation. We investigated, to our knowledge for the first time for such a complex, the possible effect of the partially filled 4f orbitals on the molecular properties of an MLnX 4 complex by using a "small core" effective core potential of the Stuttgart group. The tridentate complex, with three Na-Br bridges, was found to be the ground-state structure with the bidentate one only about 4-6 kJ/mol higher in energy. The relative abundance of these isomers changes with temperature and at the high-temperature conditions of a metal halide lamp the bidentate isomer is the more abundant isomer.

Chemistry, commentary and community: Discussion of “The NaDyBr4 complex: its molecular structure and thermodynamic properties” by Varga and Hargittai

“…Study [1] reproduced the adiabatic ionization potential; the bi-and tridentate isomers had values of 8.9 and 9.0 eV while refs. [2] and [3] reported measured values of 9.3 and 8.9 eV showing good agreement.…”

“…This means that the reaction energy of the NaBr + ion with DyBr 3 is only about 77 kJ/mol, i.e., about one third, smaller than that of the neutral species NaDyBr 4 . 1 It is worthwhile to compare the above relationship between the dissociation energies of the NaDyBr 4 -NaDyBr 4 + pair to that of the NaBr-NaBr + pair. In the case of NaBr and its ion, the corresponding analysis of the dissociation energies shows the neutral species to be some 305 kJ/mol more stable than its ion.…”

“…The dissociation energy of NaBr, as deduced from the enthalpy of formation data [7,8] for NaBr, Na and Br, is 363 kJ/mol. This means that only 58 kJ/mol, some 20%, of the bond energy of NaBr remains upon loss of 1 We could have considered the reaction of NaBr with the ion DyBr 3 + and its corresponding dissociation energy. However, since the ionization potential of DyBr 3 [2,6] is higher than that of NaBr, we are ignoring this process for now in favor of the thermodynamically preferred reaction pathway, whether or not it is more kinetically favored.…”

“…As a diatomic molecule, NaBr is the simplest possible case of an ionic system; it is understandable that the NaBr + ion is so much less stable than the neutral molecule. In the complex, however (both in the neutral molecule and in the ion) there is a DyBr 4 "subunit" in which there is a noticeable covalent bonding between the Dy and the Br atoms, with an about 0.3 Wiberg bond index (see the original article [1]). However, the interaction of this ion with sodium is essentially ionic (the Wiberg bond index of the Na-Br b "bond" is about 0.05 and the charge on Na is over + 0.9).…”

“…[1] but left undiscussed at the time was the calculation of the structure of the bidentate and tridentate forms of the NaDyBr 4 + ion and an accompanying NBO analysis. The tridentate ion, apparently, is not stable at all and it optimizes into the bidentate one.…”