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

832 Inorganic Exotic Molecules

D. Carbon Monoxide as a Ligand Following the same structural motif but going to the
other extreme—short instead of long bridges—results in
Carbon monoxide is a ligand of central importance in
a single carbon atom surrounded by two metal atoms.
organometallic chemistry. Most transition metals form ho-
A complex of this kind has been synthesized having a
moleptic carbonyl complexes (CO as the only ligand), sev-
(TPP)Fe fragment on one side and an Re 2 (CO) 9 fragment
eral of which can be isolated under ambient conditions.
on the other (TPP = tetraphenylporphyrinato). The Fe C
The reason for the success of CO is its ability to act as
˚
bond length in (TPP)FeCRe 2 (CO) 9 is only 1.60 A, rep-
a good σ-donor/π-acceptor ligand. This electronic flex-
resenting the shortest metal–carbon bond known, not that
ibility was the basis for some unexpected discoveries in
much larger than the C C bonds found in the porphyrin
recent years. The transition metals chromium and irid-
assemblage or phenyl substituents.
ium are commonly found in positive oxidation states. Sur-
rounded by carbonyl ligands, however, negative oxidation
states such as −IV in [Cr(CO) 4 ] 4− and −III in [Ir(CO) 3 ] 3−
F. Sandwich Compounds
were observed. These “superreduced” carbonyl metallates
can be considered modern extensions of the well known In the early 1950s ferrocene (18), the first sandwich com-
[Fe(CO) 4 ] , the alkyl derivatives of which have found plex, was one of the most exotic inorganic molecules.
2−
5
applications in organic synthesis. The variability of CO as Soon after, a variety of η -cyclopentadienyl complexes
a ligand is highlighted by the synthesis of carbonyl cations were discovered and ferrocene slowly come to be consid-
such as [Fe(CO) 6 ] 2+ and [Ir(CO) 6 ] 3+ as well as of the cal- ered “normal.” In 1965 it was shown that the open-cage
cium and uranium mixed cyclopentadienyl/carbonyl com- carborane [C 2 B 9 H 12 ] 2− (“dicarbollide”) can be used in-
−
plexes [C 5 (CH 3 ) 5 ] 2 Ca(CO) and [C 5 (CH 3 ) 4 H] 3 U(CO). stead of [C 5 H 5 ] (“cyclopentadienide”) (19) (see Fig. 7),
Remarkably, the ν(CO) stretching bands in the infrared where in a formal way, we may say that the former con-
spectra of the iridium carbonyls [Ir(CO) 3 ] 3− and tains 6-coordinated carbon. Since then, the coordination
−1
[Ir(CO) 6 ] 3+ differ by more than 600 cm . chemistry of carboranes has also flourished. An interest-
ing feature of carborane ligands is that they are especially
well suited to construct structurally unique compounds:
E. Metals Bridged by Unsaturated Elemental the multidecker complexes. These rodlike molecules can
Carbon Chains be considered expanded sandwich complexes. So far, up
to five metal ions have been staggered between slices of
Dinuclear complexes in which the two metal atoms are
heteroaromatic ring systems. The hexadecker complex 20
bridged by polyyne, unsaturated elemental carbon chains
(also shown in Fig. 7) was structurally characterized and
(15–17), have received considerable attention due to their III
contains four Co centers (with two associated hydrido
unique properties. For example, upon oxidation, these IV
ligands) and, remarkably, one Co center.
complexes form metallacumulenes of the general formula
Textbooks on organic chemistry depict molecules with
[L n M (C C) p ML n ] , where L includes pentamethyl-
2+
single, double, and triple bonds between elements. For
cyclopentadienyl, [Cp ,C 5 (CH 3 ) 5 ], triarylphosphine,
∗
a certain class of dinuclear transition metal complexes
[Ar 3 P, Ar = Ph, C 6 H 5 , and Tol, p-CH 3 C 6 H 4 ], aryl, CO,
this description is not sufficient and a quadruple bond
and NO. With the fragment [C 5 (CH 3 ) 5 ]Re(NO)[P
between the two metal centers has to be introduced.
(C 6 H 5 ) 3 ] as the capping, complex molecules with chains
The fourth bond is due to overlapping d orbitals with
containing up to 20 carbon atoms have been synthesized.
δ symmetry. Especially interesting are Cr 2 L 4 complexes
For compound 17 a rhenium–rhenium distance of 2.87 nm
(L = bidentate, anionic ligand), some of which show ex-
was estimated. These complexes can be considered the ˚
tremely short metal–metal bonds (<2.00 A). For [Cr 2
first step toward metal-capped, one-dimensional carbon
(5-CH 3 2-CH 3 OC 6 H 3 ) 4 ](21) a chromium–chromium
allotropes.
˚
distance of only 1.83 A was determined, the shortest
metal–metal bond known.