The not-so-simple coordination chemistry of alkali-metal cations Li+, Na+ and K+ with one carbonate anion: A study using density functional and atoms in molecules theories

A total of 23 new complexes in the series (CO3)M-y(y-2) (M = Li, Na or K; y = 1-5) have been optimized using DFT, adding to known planar complexes (CO3)M-y(y-2) (M = Li or Na; y = 1 or 2) and (CO3)M-y(y-2) (M = Na; y = 3). Planar structures are the most stable for: M = Li, y = 1-4; M = Na, y = 1-3; M = K, y = 1-3. For M = Na, three non-planar structures occur over a range in electronic energy of ca. 9 kJ/mol when y = 4, while for M = K only a single, non-planar structure occurs for y = 4. For M = Li, Na or K, two non-planar structures were the only ones found for y = 5. The CO3 moiety was found to be planar in all new species except for new isomers of (CO3)M- (M = Na or K) that have perpendicular (pi-like) CO3: M bonding. No evidence was found for any stable species when y = 6. Atomic charges were calculated by the AIM method for most of the new species and confirm that the CO3: M bonding is highly ionic. Stepwise complexation energies have been calculated for the addition of M+ to the most stable form of (CO3)My-1 y(-) (3) to give the most stable form of (CO3)M-y(y-2), for y = 1-5. The stepwise formation reactions are exothermic for all the metals when y = 1-3, but endothermic for all the metals when y = 4 or 5. However, addition of four or five M+ to one CO32- is exothermic overall for all three metals. (C) 2017 Elsevier B.V. All rights reserved.