Through the use of large and non-polarizable WCAs it becomes possible to prepare stable salts of cations initially only known from gas phase mass spectrometry or computational work in the condensed (and solid) phase. This phenomenon – induced by good WCAs – has been termed pseudo gas phase conditions. Thus, a series of such gas phase cations have been characterized as bottled compounds in the condensed phase. The analysis of the origin considers three intimately related domains: (i) energetic, (ii) structural and (iii) spectroscopic.
(Text and figures mainly taken from our review: Angew. Chem., Int. Ed. 2018, 57, 13982-14024.)
Energetic Domain: The Figure right top, provides a Born-Fajans-Haber Cycle (BFHC) that permits analysis of all questions on an energetics basis. The gas phase reaction is shown in blue. One realizes that, compared to the gas phase reaction, the solid-state chemistry (in the gray box) is further complicated by inclusion of a counterion and the standard state of the ligand for complexation.
But what influences the lattice energies that result from introduction of the counterion…? This is largely the ion size as follows from the classical Kapustinskii equation and related volume-based thermodynamics. Increasing the size of the anion results in the decrease in the lattice energy. This is readily demonstrated for several [A]– salts in the Figure, bottom, with increasing anion volumes from 0.05 to 1.11 nm3 ([A]– = [Cl]–, [BF4]–, [SbF6]–, [Al(ORF)4]–, or [(RFO)3Al−F−Al(ORF)3]−). The lattice energies associated
with the latter two are very low and only decrease little.
The energetic influence of such very low lattice energies induced by large WCAs can be demonstrated by comparison of the energetics of weakly bound complexes in gas and solid phase, for example as in the case of the published acetylene complexes [Ag(C2H2)n]+[Al(ORF)4]– (n = 1,3 or 4). The energetics of complex formation in the solid state is anion size dependent. This follows from the comparison of (calculated) ΔrG values for the sequential coordination of acetylene ligands at the Ag+ cation in the gas phase with those in the solid state, with both the large [Al(ORF)4]– and the smaller [BF4]– (Table right).
The ΔrG° values in the gas phase and the solid state entries with the excellent [Al(ORF)4]– WCA are very similar and only differ by a maximum of 15 kJ mol–1. This phenomenon represents the energetic domain of the pseudo gas phase conditions.
Formation of [Ag(C2H2)4]+ supported by the small [BF4]– anion is endergonic by an unassailable +35 kJ mol–1, for the much larger [Al(ORF)4]– anion it is only endergonic by +3 kJ mol–1. This small value can be overcome experimentally through temperature control: [Ag(C2H2)4]+[Al(ORF)4]– forms in a closed container under C2H2 atmosphere. In the closed system it loses acetylene above –20 °C. For X-ray crystal structure determination, it had to be mounted at –50 °C.
Structural Domain: The solid-state ion packing of true salts which include a very large WCA lead to the lattice in the crystal being essentially determined by the anion. Thus, the packing adheres to the radius ratio rules. Yet, since the WCA is large, the interstices in the lattice, which accommodate the cations, are also rather large and pretty much non-interacting. Under these conditions the cation experiences significantly reduced crystal packing effects (essentially negligible!) that for small and more strongly interacting anions often cause the solid-state structure to differ from that observed in the gas phase. Therefore, under these circumstances it is possible to crystallo-graphically characterize cations that are a close representation of what is observed in the gas phase.
Spectroscopic Domain: The availability of reliable gas phase spectroscopic data (e.g. IR and Raman) for cationic species prepared in the gas phase permits investigations into the legitimacy of the pseudo gas phase conditions within the spectroscopic domain. True pseudo gas phase conditions would result in the reproduction of gas phase vibrational data from solid state samples. In those, the cation is located in the interstices of a large WCA.
Two cases have to be distinguished: i) The effect of the preferably very low polarizability of the WCAs and ii) the effect of the size of the cation in relation to its interstice in the lattice.
"How does the Environment Influence a Given Cation? A Systematic Investigation of [Co(CO)5]+ in Gas Phase, Solution and Solid State." Chem. Eur. J. 2018, 24, 19348-19360.