Alkali metal atoms can repopulate their valence electrons
toward
solvation due to impact from solvents or microsurroundings and provide
the remaining alkali metal cations for coordinating with a variety
of specific solvents, forming various electron-expanded complexes
or solvated ionic pairs with special interactions. Such special solute–solvent
interactions not only affect their electronic structures but also
enable the formation of entirely new species. Taking Na(THF)
n
(n = 1–6,
THF = tetrahydrofuran) and Na2@THF complexes as typical
representatives, density functional theory calculations are carried
out to explore the solvation of a sodium atom and its dimer in THF
and characterize their complexes as solvent-incorporated supramolecular
entities and particularly valence electron presolvation due to their
interaction with solvent THF. Electron presolvation is caused by the
Pauli repulsion between THF containing a coordinating O atom with
a lone pair of electrons and the alkali metal Na or Na2 containing valence electrons, and THF coordination to them forces
their valence electrons to redistribute, which can be easily realized
in such solvents. Compared with strongly bound valance electrons of
alkali metal atoms, THF coordination enables Na or Na2 electrons
to exhibit much more active states (i.e., the presolvated states)
featuring small vertical detachment energies of electrons and distorted
diffuse distributions in the frames of the generally structured metal
cation complexes, acting as the electron-expanded chemical entities.
Furthermore, the degree of electron diffusion and the polarity of
the Na–Na bond are proportional to the coordination number
(n) and the coordination number difference (Δn) between two Na centers in Na2@THF. The unique
properties of such entities are also discussed. This work offers a
theoretical support to the supramolecular entities formed by alkali-metal
atoms or their dimers with ligands containing O or N and uncovers
the unique electron presolvation phenomena and also enriches our understanding
of the novel metal atom complexes.