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Periodic Table (Chemistry) 691

that interatomic CT specified by the two solid arrows is TABLE III Thermochemistry of Bond Interchange (in
complemented by dispersion, i.e., intra-atomic CT, indi- kcal/mol)
cated by the two hatched arrows. This is the concept of a. Pauling trend
overlap dispersion, the crucial underpinning of the I-bond
which sends a clear message: without explicit recognition F F + H H −→ H F + H F
E =−129
of electron-electron repulsion, one cannot arrive at a real- HO OH + H H −→ H OH + H OH
E =−83
istic theory of chemical bonding. Without the assistance H 2 N NH 2 + H H −→ H NH 2 + H NH 2
E =−45
of dispersion, (charge generating) I-bonding could never H 3 C CH 3 + H H −→ H CH 3 + H CH 3
E =−11
compete with (charge conserving) T-bonding. Using high- b. HSAB trend
level ab initio computations, Davidson and co-workers COF 2 + HgBr 2 −→ COBr 2 + HgF 2
E =+85
(1993)discoveredthatthistypeofbondingmechanism,la- CO + PbS −→ CS + PbO
E =+71
beled “dynamic shielding of extramolecular correlation,” H 2 O + CaS −→ H 2 S + CaO
E =+37
is crucial in transition metal complexes, e.g., Cr(CO) 6 . HF + NaI −→ HI + NaF
E =+32
The physical picture of overlap-dispersion I-bonding is c. Metal trend
electrons delocalizing without running into each other. Li 2 + H 2 −→ LIH + LiH
E =+13
Clearly, the sigma bond of BF does not belong to the Na 2 + H 2 −→ NaH + NaH
E =+26
catalog of conventional theory. Rather, it is a strong bond Cs 2 + H 2 −→ CsH + CsH
E =+30
due to CT which is assisted by its environment, the latter
being the pi space. The situation is expected to be the same
in oxoboron molecules. Hawthorne commented in a 1998 metal ions bind most strongly with first row donors (N,
UCLA faculty research lecture: “Boron especially likes to O, F), while “class b” ions form complexes of high sta-
combine with oxygen. The nice thing about (boranes) is bility in combination with donors from lower rows of the
that when they burn they release large quantities of heat.” periodic table. In the early 1960s, Pearson (1969) made
So, could it be that the superiority of I-bonding points the a related important contribution. He recognized that, if
way to rocket fuels? At the moment, we cannot dismiss neutral metals and metal ions were classified as “hard”
the alternative viewpoint of T-bound BF 3 , the stability and “soft,” the available thermochemical data implies that
of which can be explained by Pauling’s notion of polar “softlikestobindsoftandhardlikestobindhard.”Further-
covalence (by vide infra). more, Pearson recognized that the Hard Soft Acid-Base
The proposal of a new bonding pantheon and a col- (HSAB) concept led to thermochemical predictions which
ored periodic table is a beginning, not an end. We wish were diametrically opposite to those of the Pauling “po-
to emphasize this by giving explicit examples of our own lar covalence” concept. Typical examples of the “HSAB
uncertainty as to how things really are. Three simple il- Trend” are given in part (b) of Table III.
lustrations are shown in Fig. 15. The HSAB concept marks a revolution. Selectivity of
atom combination implies the existence of more than one
mechanism of chemical bonding. The common denomi-
VII. THE HARD/SOFT ACID-BASE nator of all reactions in parts (b) and (c) of Table III is the
(HSAB) CONCEPT existence of products which are combinations of atoms
of different color: green/blue or red/blue combinations.
Many years ago, Pauling (1960) pointed out that “polar For example, consider the red/blue Li H. Ideally, Li has
covalence” is responsible for the fact that many recombi- affinity for I-bonding and H has an affinity for T-bonding.
nation reactions of the type shown below in reaction 1 are Their combination is permissive of E-bonding provided
exothermic. that their electronegativity difference is large. This failing
to be the case, we can conclude that Li Hisa frustrated
AA + ZZ −→ 2 AZ (1)
molecule: none of the three mechanisms of chemical
This trend applies when A and Z are nonmetals hav- bonding, T, I, or E, is appropriate for this atom pair.
ing different electronegativities. Examples of the “Paul- CS is a blue/green diatomic. It can exhibit bimodal
ing Trend” are given in part (a) of Table III. Note that in T/I-bonding to the extent that the formal triple bond can
section a, all atoms have the same color—blue. be “2I + 1T.” The problem lies in the fact that I-bonding
In 1958, Ahrland, Chatt, and Davies made a contribu- couples overlap and dispersion, and a nonpolarizable blue
tion of great significance. They pointed out that metal ions atom is unable to support dispersion. Hence, blue/green
(Lewis acids) can be divided roughly into two classes de- diatomics or triatomics, like those on the product sides of
pending on their ability to coordinate with specific donor the equations in part (b) of Table III, are said to be frus-
atoms or groups (Lewis bases). Specifically, “class a” trated. A bonding principle emerges: atoms prefer to bind