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Inorganic Exotic Molecules 821

B. Univalent Anions, Hydride, on another alkali metal atom, as we have said; and so much
and Alkalide Salts easierthattheelectronmaybetakenoffandleftasananion
unto itself. These are so-called electride salts. While these
Hydrogen also forms the anion H , and numerous hydride
−
species may be recognized as a form of “expanded metal”
salts such as LiH and CaH 2 are known. The alkali metals
and we are also used to writing e in an electrochemi-
−
likewise form alkalide ions. The binding of an electron by
cal or otherwise redox context, it is nonetheless a surprise
the alkali metals is of comparable strength to that to atomic
to see a free electron as part of a structure or crystalline
hydrogen; the electron affinities of H and the alkali metals
lattice.
−
−
are very similar. Salts of Na ,K ,Rb , and Cs are all
−
−
+
−
known. One such salt is [Li(CH 3 NH 2 ) n ] Na , known as
both solid and liquid with varying n, and formed from a E. Ions of Gold
1:1 mixture of Li and Na with methylamine. Since sodium
Such metal clustering as found for the above neutral
is less electronegative than lithium, and both the ionization
polylithium species is found for cationic polygold species
energy and electron affinity of Na are smaller than those
in salts and/or solution with stoichiometries such as
of Li, it is surprising that the isomer [Na(CH 3 NH 2 ) n ] +
+
+
[(LAu) 3 O] , [N(AuL)] , and even [C(AuL) 5 ] + and
−
Li is unobserved, much less preferentially formed. What +2 4
[C(AuL) 6 ] , where L is a triarylphosphine, typically
−
is also surprising is that there are seemingly no Li salts
Ph 3 P,whereinPh = C 6 H 5 = phenyl.Aha!Carbonsingreat
−
known, while Na salts arise from all alkali metal/sodium
−
numbers appear. And oh, yes, there are also Au salts that
combinations, including species that contain both Na and
+
might otherwise be considered “merely” alloys. Interest-
−
Na . This may look familiar from the finding that, as de-
ingly, gold has another anionic form, [AuF 6 ] , in which
−
fined by EMF potentials in water, lithium is more reduc-
the oxidation state is +5, otherwise unprecedented for
ing than sodium, but then again, the aqueous order for
either the alkali metals or the so-called coinage metals
the alkali and other metals runs counter to any periodic
(Cu, Ag, Au).
trends.
C. Polylithium Species III. CATIONS AND/OR ANIONS:
PLEMEIOELECTRONIC AND
Perhaps to appease the chemist for the seeming paucity
HERMAPHRODITIC SPECIES
of anionic salts, lithium forms a collection of polar bi-
nary compounds with exotic stoichiometries, at least when
A. Transition Metals
compared with their formal binary counterparts with hy-
drogen. For example, paralleling CH 4 ,NH 3 ,H 2 O, HF, and This section attempts to answer why the aforementioned
H 2 S are CLi 4 ,Li 3 N, Li 2 O, LiF, and Li 2 S, but there are also alkalide salts are at all exotic. We suspect the answer is
the unprecedented CLi 3 and CLi 5 , NLi 4 ,Li 3 O and Li 4 O, because we are used to the alkali metals being cations, to
Li 2 F and Li 3 F, and Li 4 S all with close Li Li contacts metals being cations, and to not having a given species
andseeminglyintermetallicbonding.However,beforeone appear in both positive and negative charge states. Thus
concludes that interlithium bonding is always stabilizing, a choice of charge states is not a surprise. That is es-
one should recall that Li 2 O, as a triatomic molecule and sentially the hallmark of transition metals, for example,
not salt, is linear. It is not bent like its hydrogen analogue, the long-known sets of aquated cations Fe 2+ (green) and
water. Mixed hydrogen/lithium species are rare. We note Fe 3+ (yellow) and the tetrahedral anions [MnO 4 ] (pur-
−
that one such species, triatomic molecular and not salt ple), [MnO 4 ] 2− (green), and [MnO 4 ] 3− (blue) with their
LiOH, is also linear. central metals with positive oxidation states. That the same
heavy (nonhydrogen) atom skeleton is roughly maintained
as the number of electrons is varied is also not a surprise,
D. Electride Salts
sets of so-defined plemeioelectronic species are not par-
We extend the discussion of these alkalide salts and note ticularly rare.
the existence of numerous species containing a second
alkali metal ion but now found as complexed (with a
B. Nonmetal Ions
crypt and/or crown ether) cation. These cations may be
understood as solvated but with a well-defined solvation It is rare that ions of both signs are found. Some of
shell, both in terms of structure and stoichiometry. The sol- the very few salt-forming hermaphroditic (and so like-
vation has made it easier to remove an electron from the wise exotic) species include the following: [O 2 ] +/− (e.g.,
+
alkali metal—so much easier that the electron may be put the dioxygenyl salt [O 2 ] [PtF 6 ] − and the superoxide