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Encyclopedia of Physical Science and Technology EN007D-343 July 10, 2001 20:13

Inorganic Exotic Molecules 823

−
to be considered. Even in the [PCS] ion the C S bond potassium and diamond or zinc-blende-like BP, respec-
˚
length (1.62 A) is shorter than the expected value for a typ- tively. The compounds Ca 3 Br 2 (CBN) and Sr 3 Cl 2 (CBN)
˚
◦
ical single bond (1.82 A); then again, both resonance struc- were synthesized at 950 C from the corresponding metal,
−
tures {P C S } and { P C S} look exotic with their its dihalide, boron nitride, and graphite. By analogy,
−
carbon–phosphorus triple and double bonds, respectively. Ca 3 Cl 2 C 3 was formed from calcium, CaCl 2 , and graphite
◦
An interesting comparison involves the difference of at 900 C. The synthesis of the first compound contain-
−
bond lengths between corresponding bonds in correspond- ing an isolated [PCS] ion was achieved in glyme (dme,
ing first and second row species. For example, the dif- 1,2-dimethoxyethane) solution,
ference of the CO and CS bonds in CO 2 and CS 2 is S C(OC 2 H 5 ) 2 + [(CH 3 ) 3 Si] 2 P Li
˚
1.55 − 1.16 = 0.39 A. By contrast, the difference of the
+
−
−
−
same bonds in [NCO] and [PCS] is enigmatically in- → [Li(dme) 3 ] [PCS] + 2C 2 H 5 OSi(CH 3 ) 3 . (1)
˚
creased to 1.62 − 1.13 = 0.49 A. Table II presents the col-
+
The [SNS] ion was classically obtained by oxidation of
lected bond lengths d(XY) and d(YZ), and the interrow
a mixture of tetrasulfur tetranitride, S 4 N 4 , and elemen-
differences d(XY) and d(YZ). No explanation is ap-
tal sulfur with AsF 5 as both the oxidant and fluoride-
parent for the spread of difference values.
accepting Lewis acid:
With the counterion-dependent exception of the [C 3 ] 4−
1
+
−
ion, all species quoted in Table I can be regarded as linear. S 4 N 4 + S 8 + 6AsF 5 → 4[SNS] [AsF 6 ] + 2AsF 3 .
2
In all ions isoelectronic to CO 2 the most electroposi- (2)
tive atom always occupies the central Y position in the However, two more recent laboratory syntheses are more
most stable X Y Z species. There is the pseudohalogen, convenient as they do not involve the use of explosive
CNO, with a central nitrogen of intermediate electronega- S 4 N 4 :
tivity to that of its bonding partners. That it is unstable 2+ −
[S 8 ] [AsF 6 ] + NaN 3
2
is reflected in its trivial name “fulminate,” as opposed
3
−
+
to the C-centered but variously (O, N)-attached isomer →[SNS] [AsF 6 ] + NaAsF 6 + N 2 + S 8 (3)
4
cyanate/isocyanate. The remaining isomer, CON, with the + −
[N(SCl) 2 ] [AlCl 4 ] + SnCl 2
electronegative oxygen in the center remains unknown and
−
+
may safely be assumed to be even less stable. →[SNS] [AlCl 4 ] + SnCl 4 . (4)
C. Some Synthetic Chemistry
V. ETHYLENE AND BEYOND: ANALOGUES,
Most of the anionic 16-valence-electron species that are CAGES, AND CLEAVAGE
isoelectronic to CO 2 and CS 2 were synthesized by high-
temperature solid-state reactions in ampules made of the We now consider another simple species and see where
refractory metal niobium or tantalum. For example, the our logic leads us. We start with the “organic compound”
compounds Sr 3 (BN 2 ) 2 and K 3 (BP 2 ) were prepared at ethylene, C 2 H 4 . With its double bond, its banana bonds,
◦
1000 C from strontium nitride and graphitic BN, or from its σ and π bonds, it is an archetype for numerous organic
species, as well as the inorganic species that constitute
TABLE II CO 2 and CS 2 and Their Ionic, Isoelectronic Ana- the purview of this article. Let us now proceed to diverse
logues XYZ, Experimentally Determined Bond Lengths d(XY ) exotica and with many forks in the road; there is no unique
and d(YZ ), and the Differences between the Distances for route or even destination.
Corresponding Bonds with First Row and Second Row
Elements ∆d(XY ) and ∆d(YZ ) a
A. Metal Complexes
XYZ d(XY) d(YZ) XYZ d(XY) d(YZ) ∆d(XY) ∆d(YZ)
Let us now consider metal complexes. Transition metal
OCO 1.16 SCS 1.55 0.39
complexes of olefins, alkynes, and arenes and other or-
[ONO] + 1.15 [SNS] + 1.46 0.31
ganic π systems have become quite common in both
[NNN] − 1.18
the textbook and research literature: some exotica of this
[NCO] − 1.21 1.13 [PCS] − 1.56 1.62 0.35 0.49
type will be discussed in a later section. We start with
[NCN] 2− 1.23
an ethylene complex with the simplest metal ion, Li .
+
[NBN] 3− 1.36 [PCP] 3− 1.77 0.41
The cation is definitionally electrophilic, the olefin has an
[CBN] 4− 1.44 1.38
energetically accessible pair of electrons, and the com-
[CCC] 4− 1.35
plex is really quite sensible. We may recognize it as
a
All distances are in angstroms. analogous to the better known olefin–silver complexes