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822 Inorganic Exotic Molecules

−
K + [O 2 ] ; the ozonide [O 3 ] − is known in numerous as well as disappointing that while the oxidation state of
+
salts, but no ozonium, [O 3 ] , salt is known); [NO 2 ] +/− iodine goes from −1to +7 and that of xenon from 0 to +8,
−
+
(nitronium/nitrite), but not [NO] or [NO 3 ] ; [ClO 2 ] +/− cesium, and a fortiori the other alkali metals, is limited to
but no hypochlorite, chlorate, or perchlorate-like cations; +1 ... and dare we forget, −1.
[XeF 5 ] +/− (but not XeF 3 , known only as the cation, and
not XeF 7 , known only as the anion); and the hexafluoro
species [EF 6 ] +/− , where E = Cl, Br, and I (E = Re is also IV. THREE-ATOM 16-VALENCE-ELECTRON
known). A plausible assumption is that one wants an inter- XYZ SPECIES: ISOELECTRONIC
mediately high oxidation state, but this is contradicted by ANALOGUES
the case of alkali metal cations and anions and the pair of
doubly charged ions [S 4 ] 2+ and [S 4 ] . Also note that the A. Old and New Species
2−
geometry of the cations and anions may be greatly differ-
Several isoelectronic analogues of CO 2 have been known
ent: [XeF 5 ] is a C 4v square pyramid, whereas [XeF 5 ] −
+
for a long time and are well characterized. These include
is one of but two binary species of the stoichiometry AB 5
neutral N 2 O and the ionic species [N 3 ] − (see below),
with D 5h pentagonal symmetry, and [S 4 ] 2+ and [S 4 ] 2−
[CN 2 ] , and [NO 2 ] . Interestingly, the list does not in-
+
2−
are, respectively, a 6-π-electron, square-shaped aromatic
clude [BO 2 ] ion. The so-called, long-known “metabo-
−
and an alkane-like (but gauche, not staggered) extended
rate” ion with this formula is found in numerous salts, but
chain. We parenthetically add that these two last species
structurally it appears only as a trimer or polymer.
may be also recognized as valence isoelectronic analogues
More recently, great progress has been made in the
of [O 4 ] 2+/2− , both unknown in these dimeric states, as op-
area of these simple 22-electron (16-valence-electron)
posed to their aforementioned monomers [O 2 ] +/− , while
n
X Y Z species. All of the possible [X Y Z] systems
the sulfur compounds are not known monomerically in
that contain X, Y, and Z elements of the first row (second
the condensed phase save as the [S 2 ] anion as a source
−
period) and net charges (−4 ≤ n ≤+4) were studied us-
of color for an uncommon mineral, “green ultramarine.”
ing quantum chemical methods and many new ions were
predicted to be locally stable. Meanwhile several of these
C. Can Alkali Metals Have Higher unknown species have been prepared and characterized,
Oxidation States? for example, [CBN] 4− and [C 3 ] , and these species to-
4−
gether with their heavier CS 2 analogues are discussed in
We close this section with a brief mention of higher va-
the following paragraphs.
lence alkali metal species. By this we do not mean highly
ionized atomic ions such as [Na] 11+ —verily any atom can
be stripped of any, many, and even all of its electrons,
B. Bond Lengths and Angles
but that is not what is meant. We do not mean species
like KO 2 that have univalent K accompanying superox- The structurally well-characterized ionic, isoelectronic
+
ide, [O 2 ] , and are not tetravalent oxides like the covalent analogues of CO 2 are summarized in Table I. We also
−
CO 2 and ionic UO 2 nor even divalent peroxides like BaO 2 . include three isoelectronic ions corresponding to CS 2 , the
What about CsF 3 ? Known as a matrix-isolated species, heavier, valence isoelectronic homologue of CO 2 . The
cesium trifluoride seemingly contains univalent, and thus [BAs 2 ] 3− anion, which is isoelectronic to CSe 2 , has also
˚
commonplace, Cs with the trifluoride ion, [F 3 ] , which, been reported, with d(B As) = 1.87 A.
−
+
while, it has a precedent in other trihalide ions such as In all cases the bond lengths in the ions isoelectronic
−
[I 3 ] , still qualifies as an example of exotica. There is no to CO 2 and CS 2 are suggestive of at least partial multiple
evidence for this being an ion pair of [CsF 2 ] and F , bonding, and therefore significant (p–p)π bonding has
−
+
the cation of which was suggested years ago as a plausi-
ble species because it is isoelectronic to XeF 2 . It is still a
− −
unrealized. TABLE I Structural Parameters for N 2 ,[N 3 ] ,
and [N 5 ] ++
N 2 [N 3 ] − [N 5 ] +
D. Trihalide Ions
Symmetry D ∞h D ∞h C 2v
Then again, similarly isoelectronic to XeF 2 is trihalide
d(N N, terminal) 1.10 1.18 1.11
ion, [IF 2 ] . It is a plausible, indeed known, but still hardly
−
d(N N, central) 1.30
explored species, unlike its better known valence isoelec-
θ(NNN) 180 110, 166
tronic chlorine and bromine analogues, both [ICl 2 ] and
−
−
[ClF 2 ] , and [IBr 2 ] and [BrF 2 ] . It is perhaps surprising a Bond lengths d in angstroms; angle θ in degrees.
−
−