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

694 Periodic Table (Chemistry)

4. Two acetylenes are more stable than one tetrahedrane. atom. Modern electronic theory has attempted to ratio-
Similarly, tetrahedral N 4 , irrespective of whether it is nalize this experimental observable. For example, from
a secondary minimum or not, lies much higher than the electronic configuration of phosphorus (a 3s pair and
two N 2 . By contrast, tetrahedral P 4 is more stable three 3p dots), we can expect that this atom will be ca-
than two P 2 by 55 kcal/mol. pable of forming a maximum of three covalent bonds to
5. Similarly, [Co(CO) 3 ] 4 is tetrahedral involving three univalent ligands. On such a basis, one expects that
phosphorus is primarily trivalent. The fact that quadri- as
carbonyls as bridging groups. By contrast, [Ir(CO) 3 ] 4
is a tetrahedron with no bridging carbonyls. well as penta-valent phosphorus compounds are known
is more conveniently explained by the concept of formal
One of the major impulses toward the new VB interpre- oxidation state. This is equal to the number of electrons
tation was the observation that isoelectronic or isolobal that can be transferred to a set of more electronegative
species are not necessarily isostructural. For example, C ligands. The maximum (positive) oxidation state is equal
and Si are isoelectronic and both are isolobal to Fe(CO) 3 . to the number of valence electrons, i.e., five in the case of
Yet the structures of C-containing molecules are differ- phosphorus.
ent from those of isoelectronic Si-containing molecules. The colored periodic table stands on very different the-
By contrast, replacing Si by Fe(CO) 3 does not cause a oretical grounds. In its ground electronic configuration,
gross change of molecular architecture. It was concluded phosphorus contains two types of elements: one pair and
that the difference between C and Si and between C and three dots. But we have already seen that each atomic el-
Fe(CO) 3 must be due to a difference in binding mech- ement (pair, dot, or hole) can receive a maximum of two
anism: C supports T-bonding, but the more electroposi- arrows emanating or terminating at some ligand. Because
tive Si and Fe support I-bonding. On the other hand, Si each valence AO of an atom can be classified only as a
and Fe(CO) 3 must be essentially interchangeable within a pair, a dot, or a hole, it follows that the maximum valence
molecule without causing a change of shape. We say that of an atom equals twice the number of valence AOs. This
these two groups are isosynaptic. According to Epiotis means that the maximum coordination number of phos-
(1989), “two groups are isosynaptic if they are isolobal phorus within a molecule is eight, rather than three or five.
or isoelectronic and they have the same color.” Green and Thus, the concept of the I-bond revolutionizes the concept
red isolobal groups are regarded, to a first approximation, of valence. The synthetic chemist can now throw away the
as isosynaptic. For example, red Fe(CO) 3 is isolobal to inhibitory chains of conventional wisdom and attempt to
green Si. Hence, they should also be isosynaptic. If dis- make octavalent phosphorus (and high-valent green and
ilaacetylene is a butterfly, then Fe(CO) 3 organometallics red atoms) by judicious selection of ligands. A molecule
with butterfly structures must abound. Mere perusal of in which each green or red atom employs each valence
the monograph of Marko and Marko-Monostory (1981) AO to tie up two elements is associatively saturated.We
makes evident that this is indeed the case. believe that there are many such examples in the literature
We have seen that molecules in which atoms with the which are obscured by the fact that coordinative satura-
same number of valence electrons but different color can tion is different from associative saturation. For example,
end up having different shapes. The other side of the coin in the case of phosphorus, associative saturation requires
is that molecules in which atoms have different numbers of the presence of eight elements around phosphorus. But
valence electrons as well as different colors may have the these may be contained within fewer than eight ligands.
same shape provided that T-bonds in one are replaced by I- Indeed, it is commonplace that an electronegative ligand,
bonds in the other. For example, NH 3 , which features three such as a halogen atom, may act via a dot plus one or two
N H T-bonds, is pyramidal. The same thing is true of the pairs (three- or five-electron ligand) to provide more than
−1
−2
isoelectronic XeO 3 , ClO , and SO , in which the central one element, rather than acting as a pure (one-electron)
3 3
atom is connected to each oxygen atom by one I-bond. univalent ligand. The well-known PF 5 can be thought of
as a molecule in which phosphorus acts with one 3s pair
plus one 3p pair, and one 3p dot plus one 3p hole to tie up
IX. ATOMIC VALENCE AND five fluorine dots and three fluorine lone pairs with four I-
THE COLORED PERIODIC TABLE bonds (vide supra). Molecules containing phosphorus in a
coordination state higher than five are not oddities. In fact,
One important utilization of the periodic table has been Holmes (1998) has reported hexacoordinate phosphorus
the connection of atomic number to atomic valence or to be implicated in enzymatic reactions. Hepta- and octa-
atomic oxidation number. The term “valence” refers to coordinate phosphorus (and, especially, heavier cogners)
the number of ligands which can be attached on a central are expected to be realizable synthetic goals.