Page 167 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Inorganic Exotic Molecules 825

E. Nitroso and Isonitroso Compounds elements in their divalent state is an interesting study. Con-
and Some Sulfur Analogues sider, for example, tetrafluoroethylene and difluorocar-
bene, C 2 F 4 and CF 2 , not to be confused with carbon difl-
The next squashing generates the isomeric nitroso and
uoride. CF 2 is a rather stable example of divalent carbon.
isonitroso species RNO and RON. For most R, the former
While the carbon–carbon double bond in ethylene is con-
isomer is more stable. The relative stability depends on the
siderably stronger than the corresponding carbon–carbon
electronegativity of the R. For the electropositive R = Li,
single bond in ethane (no surprise!), the former bond in
the LiON form is the more stable. Isoelectronic analogues
tetrafluoroethylene is weaker than the carbon–carbon sin-
are known having one or the other structure, for example,
gle bond in hexafluoroethane. The bond is not that weak—
Me 2 NNS and NSF; there is a plethora of nitrogen–sulfur
CF 2 isstillrecognizedasaquitereactivecarbene.Proceed-
compounds, most of which lack corresponding analogues
ing down the column, we have less and less tendency for
with nitrogen and oxygen, such as S 4 N 4 ,N 4 S 4 F 4 , and
the essentially olefinic species, and, instead, polar, if not
N 3 S 3 Cl 3 , in counterpoint to monomeric NO and NOF, ac-
ionic, compounds. We thus generate the toothpaste addi-
tually ONF, and NOCl, actually ONCl. (Monomeric NSF
tive stannous fluoride, which does not contain Sn 2+ in the
and NSCl are also known but these are sulfur-centered,
crystal (unlike aqueous solution), but rather the tetrameric
not nitrogen-centered.)
fluoro-cation ring [(SnF) 4 ] 4+ with fluorides bridging the
tins within the oligomeric cation and between oligomers.
F. Elemental Oxygen, Elemental Sulfur,
and Sulfur–Oxygen Species
H. Di- and Tetravalent Oxides
The final squashing results in elemental dioxygen, O 2 ,
We see this behavior in the divalent oxides as well. Car-
an altogether normal, commonplace, well-known species
bon monoxide is a well-defined diatomic gas that has
that is one of the few non-transition-metal species with
the strongest bond in any neutral molecule and a dipole
unpaired electrons that do not polymerize or oligomer-
moment almost precisely zero. It shows no tendency to
ize under normal situations. Indeed, its polymers are all
dimerize—the formal ethylene-like species O C C O
but unknown. Whereas there is some indirect evidence for
has entertained, educated, and evaded numerous chemists;
O 4 and we know of none for O 8 , for the valence isoelec-
now, clearly, it cannot be the instability of the C O unit
tronically related sulfur, S 8 is the normal form for elemen-
per se, as this is found in so many of the metal carbonyls
tal sulfur, where S 2 is the exotic species and neutral S 4 re-
enunciated above. Traversing the length of the column
mains largely unknown, as opposed to the aforementioned
gets us to lead oxide, a long-known, quite ionic solid with
[S 4 ] 2− and [S 4 ] 2+ ions found in salts. Speaking of salts, but
at least formal Pb 2+ ions. This does not mean that group
now within the context of mixed sulfur–oxygen species,
14 oxides never want to associate. Carbon dioxide, CO 2 ,
2−
sulfites and sulfates, [SO 3 ] 2− and [SO 4 ] , are common-
is a well-defined triatomic gas (with a much more sensi-
place anions, while the “lower” oxoanions [SO] 2− and
ble zero dipole moment than the negligible one for CO
[SO 2 ] 2− remain essentially unknown, and this, despite the
because of the symmetry of the former). The triatomic
well-characterizedsituationfortheisoelectronicchlorine–
SiO 2 also exists, but as a high-temperature gas. It readily
oxygen anions, [ClO n ] for n = 1, 2, 3, and 4, and the im-
−
forms an extended solid with an extensive collection of
portance of the redox chemistry of sulfur–oxygen radical
forms with poetic-sounding names, tridymite, cristobalite,
anions [SO n ] •− for n = 2, 3, and 4 in at least the aqueous and coesite, as well as the more prosaic sand, amorphous
−
2−
2−
solution of [S 2 O 4 ] , HSO , and [S 2 O 8 ] , respectively.
3 silica, silicalite, and quartz. Because of this diversity of
forms, we consider silicon dioxide as an exotic species
G. Cleavage Reactions and despite its ubiquity in both the natural and human-derived
Fluorinated Derivatives environment.
Another source of interesting chemistry arises from con-
sideration of the formal cleavage reaction of ethylene into
VI. REVERSING THE SQUASHING:
methylene, of the parent olefin into the parent carbene,
BORON HYDRIDES
C 2 H 4 → 2CH 2 . (7)
A. Relating Organic and Inorganic Species
This reaction per se, like that of most simple derivatives
of ethylene, is not particularly relevant here—it is too “or- Let us return to ethylene and reverse the squash-
ganicky.” However, the conflict between weakly bonded ing procedure, going to diborane(6), from C 2 H 4 to
but tetravalent ethylene analogues with higher group 14 B 2 H 6 , and thereby from organic carbon- to inorganic