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Encyclopedia of Physical Science and Technology EN011H-551 July 25, 2001 18:33

Periodic Table (Chemistry) 693

LiH molecules can form a much more stable rhomboidal e symmetry and a total of two electrons. Hence, BH and
dimer.Hence,weshouldtransformthesimpleprototypical Fe(CO) 3 are said to be isolobal. The analogies can be ex-
equation above to the one shown below in reaction (3). The tended. All of the following fragments are isolobal, having
bonding mechanisms operative in reactants and products one a and two e orbitals containing two valence electrons:
+
+
+
are shown in Fig. 16. C, Si, HC , HB, Fe(CO) 3 , Os(CO) 3 , Co(CO) , Ir(CO) .
3 3
According to extended H¨uckel MO theory, replacing
H H----Li Li−→ Li H H Li (rhombic)
(3) any one of these groups by another one should cause no
T T FR I change in the architecture of the molecule unless there is
On the reactant side, the T-bond in H 2 is partly localized, some problem of orbital mismatching or steric repulsion.
while the I-bond is converted to an unfavorable T-bond in The notion of “isolobality” was developed by Hoffmann
Li 2 . These are the “promotions” required for forming an and his school, and it motivated much fruitful research,
intermolecular E-bond (due to induction/dispersion) in- most notably by Sone, Herrmann, and Huttner (1986).
dicated by a dashed line. One the product side, essential Nonetheless, our conclusions are at odds with the EHMO
abolition of the T-bond of H 2 and the I-bond of Li 2 is scenario: the shape of a molecule depends on the mech-
the price necessary for establishing a very strong I-bond. anism of bonding, i.e., whether T- or I-delocalization
The existing data suggests that the reaction is exother- predominates. This depends on the electronegativity of
mic, something well explained by the superiority of I- to the central atom. Thus, we expect that, for example, HC +
E-bonding. and HB will combine with a given fixed fragment in a way
which is stereochemically different despite the fact that
they are isoelectronic and isolobal. In this sense, bonding
VIII. BEYOND THE ISOELECTRONIC AND theory starts after symmetry considerations have ended.
ISOLOBAL ANALOGIES Here are some facts which make clear that it is the
“color” of atom electronegativity that determines the
Oxygen has one 2s, three 2p AOs, and six valence elec- bonding mechanism and, hence, their shapes and proper-
trons. Sulfur has one 3s, three 4p AOs, and six valence ties. Since a whole book can be written just on the differ-
electrons. Oxygen and sulfur are isoelectronic. This ex- ences of isoelectronic or isolobal species, we restrict our
plains the similarity of the shapes of HOH and HSH (both attention to the fragment series HC, HSi, N, P, (CO) 3 Co,
molecules are bent). By contrast, we say that the very sim- and (CO) 3 Ir.
ilarity hides a fundamental difference in bonding mecha-
nism due to the fact that S is more electropositive than O. 1. Nitrogen forms a strong triple bond to a second
As a result, HOH lies toward the T-delocalization limit, nitrogen, while phosphorus shows a preference to
while HSH lies toward the I-delocalization limit. We ex- form rings, cages, and even deltahedral clusters
pect that, in two other isoelectronic systems, the difference (e.g., P 4 ).
in bonding mechanisms will cause them to have radically 2. C 2 H 2 is linear with a carbon-carbon triple bond,
different shapes. while Si 2 H 2 was computed to be “butterfly” by
BH has one radial n and two tangential p AOs of a and Lischka and Koehler (1983).
3. HC CH is a stable organic molecule, but
e symmetry (in C 3v ) and two valence electrons. The C 3v
Fe(CO) 3 fragment also has three frontier MOs of a and (CO) 3 Co Co(CO) 3 has not yet been made.

FIGURE 16 The expected energy minima of the Li 2 H 2 hypersurface. A solid line connecting two dots implies a T-bond
when overlap is substantial. For the “peculiar” bonding of the rhombus, consult Epiotis (1996).