Page 368 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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

Periodic Table (Chemistry) 687

T-antibond, indicated by the hashed line. Trans-bending
lowers the energy of Z(bc). Hence, trans-bending in heavy
congener derivatives of ethylene and acetylene can be
taken to be the fingerprint of predominant I-bonding. The
strange, nonplanar, trans-bent geometries of disilenes (see
theworkof West,Fink,andMichl,1981),digermenes,and
distannenes are already a topic discussed in recent texts of
inorganic chemistry (see Shriver, Langford, and Atkins,
1999). Noting the diversity of proposed explanations, the
one most often cited is the “double singlet carbene” model
of Pauling (1960). A barrage of reports in recent issues of
Chemical and Engineering News (July 97, March 98, Jan-
FIGURE 9 The effects of “executing” the different possible arrow
uary 99) on the synthesis and characterization of heavy
pairs. In the top figure, the ab exchange conserves the charge of
p-block multiple bonds is indicative of widespread in-
each atom, as in a T-bond. On the other hand, if the bc arrow pair
terest in this topic. The well-known Cotton (Cotton and
is executed, as in an I-bond, one arrives at a configuration which
confines two pairs of electrons to a single AO, causing severe Walton, 1982) transition metal multiple bonds now await
electron-electron repulsion within the atoms. Relief of exchange reinterpretation.
repulsion (i.e., T-antibonding) can be affected by transbending as We are going one step further: the trans-bending of
indicated by the dashed arrows.
“heavy ethylene” has exactly the same origin as the
square-to-rhombus deformation of the Li tetramer where
I-bonding is expected to be predominant. This is illus-
where J 11 is the self-repulsion bielectronic Coulomb in-
trated (Fig. 11) by reference to the Z-type configuration
tegral, IP is the orbital ionization potential, and EA is the
produced by an I-type 2 − e hop starting from the low-
orbital electron affinity. This is only tolerable when the
est energy R-type tetraradical reference configuration of
AOs are large in size or, saying the same thing in different
Li 4 .
language, if the atom has low IP, i.e., low electronegativity.
The following quotation is attributed to E. Schr¨odinger,
Hence, we expect that replacing C by a heavier congener,
a pioneer of quantum mechanics: “Suppression of de-
such as Si, Ge, etc., will change the character of the double
tails may yield results more interesting than a full treat-
bond from T to I.
ment. More importantly, it may suggest new concepts.
Two arrows make up either one bond or one antibond,
Pure quantum mechanics alone, in all its details, can-
and all depends on spin and electron count. Examples of
not supply a definition of, e.g., an acid or a base or a
T-antibonds are shown below in Fig. 10. We make a prac-
double bond.” This is the philosophy espoused in this
tice not to show T-antibonds in our arrow formulae so as
article.
not to clutter the picture.
With this in mind, let us now reassess the consequences
of the configuration interaction (CI) characteristic of VI. THE COLORED PERIODIC TABLE
T- and I-bonding which were illustrated above in Fig. 9.
It is clear that the bc arrow combination generates a VB We now set out to construct a two-dimensional map-
configuration Z(bc) which features a “diagonal” double ping of molecules that reveals two things: the mechanism

FIGURE 10 The different types of T-antibonds. These are left out FIGURE 11 The origin of the deformation of square to rhombic
of our formulae for clarity. Li 4 . Compare with the Z(bc) formula of Fig. 9.