Page 173 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GPA Final Pages
Encyclopedia of Physical Science and Technology EN007D-343 July 10, 2001 20:13

Inorganic Exotic Molecules 831

ions are unbound. Helium and neon are particularly loathe it is surrounded by one or more metal fragments, with zir-
to lose electrons: their ionization energies (ionization conocenes [“Cp 2 Zr” or Zr(C 5 H 5 ) 2 ] being especially suited
potentials) exceed those of any other element or group. (9). The smallest (fewest number of electrons as well as
To date, helium and neon remain inert. Argon also has a fewest number of atoms) molecular species with a planar
−
high ionization energy and so ArF 2 remains unobserved. tetracoordinated carbon atom is [Al 4 C] . Other tetracoor-
By contrast, the proton affinity of argon is sufficiently dinated nonmetals may presumably also be coerced into a
−1
high (ca. 369 kJ mol ) to make HAr quite stable, both planar geometry. The square planar phosphonium cation
+
on its own and as part of a major resonance contributor to (10) was recently characterized, so far the only example
H Ar F. Accordingly, this last species enjoys existence of its kind.
under at least cryogenic conditions.
IX. TRANSITION METAL COMPLEXES
WITH EXOTIC LIGANDS
AND GEOMETRIES

The geometries and reactivities found for transition metal
complexes are very diverse. This can partly be attributed
to the fact that transition metals tolerate a variety of oxi-
C. Unusual Coordination Numbers
dation states and coordination numbers. Furthermore, the
for Main Group Elements
electronic situation of the participating ligands can change
dramatically upon coordination to the metal ion. It is there- In the presence of transition metals, main group elements
fore not surprising that many unusual molecules are found not only can adopt rare geometries but can also display
within this class of compounds. unusual coordination numbers. In 1995 the synthesis of
a phosphido complex with a terminal P 1 ligand was de-
A. Stabilizing Highly Reactive Molecules
scribed (11). A triple bond to a unicoordinate phospho-
The strong influence of transition metals on the reactiv- rus atom was proposed similar to the phosphaalkynes
ity of coordinated ligands is highlighted by the fact that a (R C P). Hypervalent carbon atoms with the coordina-
variety of molecules known to be intrinsically difficult tion number six were found for the aforementioned gold
to synthesize have been isolated in the form of metal complex (12). Structurally related are the carbido (13) and
complexes. The antiaromatic cyclobutadiene, for exam- thenitridocluster(14)inwhichacarbonandnitrogenatom
ple, is highly reactive and its isolation generally requires are surrounded by six ruthenium carbonyl and five iron
◦
low-temperature matrices (ca. −260 C). Coordinated to carbonyl fragments, respectively. Just about the time that
a metal fragment, the stability is significantly enhanced, precedents are being established for six-coordinated car-
as highlighted by the fact that the iron carbonyl complex bon, high-level computational theory has suggested that
◦
(7;mp27 C) can be distilled for purification (bp 48 C, its realization in a planar hexagonal geometry may be re-
◦
2−
3 mm). A similar situation is encountered with the ex- alizable in [CB 6 ] ,CB 6 H 2 , and (three isomers, no less!)
tremely strained alkyne benzyne, which was stabilized in C 3 B 4 .
the coordination sphere of an organometallic zirconium
complex (8; (Me = CH 3 ,Cp = C 5 H 5 ).

B. Planar Tetracoordinate Carbon
In 1874 J. H. van’t Hoff and J. A. Le Bel proposed in-
dependently a tetrahedral geometry for tetracoordinated
carbon atoms. Despite heavy initial criticism, this con-
cept became one of the fundamental principles of modern
organic chemistry. In organometallic chemistry, however,
the situation can be different. Albeit still a rarity, a tetraco-
ordinated carbon atom can adopt a planar geometry when