化学专业英语之卤族元素及其化合物
INTERHALOGEN AND NOBLE GAS COMPOUNDS
Interhalogen and noble gas compounds comprise a relatively limited family of highly reactive and unstable molecules whose primary importance is their role in testing chemical bonding theory1. At first it may seem rather strange to treat the chemistry of the halogens and the noble gases, two groups that represent the extremes of chemical activity and inertia, in the same section. The superficial differences between the halogens and the noble gases are much reduced, however, if we focus our attention on the comparison of halide ions (particularly F-) with the isoelectronic noble gas atoms and the noble gas compounds with halogen atoms or the halogens in their higher positive-oxidation states.
Noble gases are exceptional in their reluctance to either gain or lose an electron. Halide ions — because of their excess negative charge, relative to the isoelectronic noble gas atoms — have both a lower ionization energy and a lesser electron affinity. On the other hand, noble g
as cations have greater electron affinities and greater ionization energies than do isoelectronic halogen atoms. From such considerations, it is obvious that inert gases should be less reactive than are halide ions, but their compounds should show even higher reactivity than the halogens. The big question remaining is: Are there any chemically significant conditions under which noble gases can be persuaded to yield electrons sufficiently to produce stable compounds? The answer is definitely, yes! (The same question can be asked of halogen atoms, which have ionization energies comparable to those of the inert gases.)
Another obvious point of similarity between halogen and noble gas compounds is the characteristically large number of electrons that must be accommodated in the valence shell. For a noble gas atom bonded to any number of other atoms, the octet rule must be exceeded; for a halogen atom to be bonded to more than one other atom, the same must be true. It is a curious historical fact that the mythical inertia of a closed shell did much to diminish the energy expended in the search for noble gas compounds, long after numerous examples of superoctet valence shells were known, particularly among interhalogen compo
unds.reactive to
We may roughly classify the interhalogen compounds into two categories: those in oxidation state zero (the binary analogs of the elementary diatomics2) and those in which one of the halogens is in a formally positive oxidation state. Heterodiatomic halogens are generally formed readily on mixing the required pair of halogens in a 1:1 ratio. The bond energies are always higher in the heteropolar molecules than are the average bond energies of the two constituents and in some cases higher than either. It is this factor that drives the reactions. All heterodiatomics2 are more or less stable under ambient conditions except for BrF, which spontaneously disproportionates to BrF3 and Br2. The bonding in the halogen diatomics can be attributed to a single a bond, formed by overlap of p orbitals. In the heterodiatomics, the principal new features are the poorer orbital overlaps that are possible between atoms of widely different principal quantum number (n), the polarity arising from the difference in electronegativity, the contribution of ionic terms to increase bond energy, and the relief in interelectronic repulsion in the fluorides. relative to difluorine.
Dihalogens (except for F2) usually react by dissociation into atoms or by heterolytic dissociation under the influence of an attacking reagent. Thus, reaction of Cl2 with hydroxide may be viewed as displacement of Cl- from Cl2 by OH-:
Cl2+OH- -----→HOCl + Cl-
The tendency to undergo heterolytic fission increases on descending the group, and the I2 molecule can actually be cleaved to two stable species:
I2 + 2C5H5N+AgNO3-----→ [C5H5N)2I]NO3 + AgI
The increased homolytic bond energies of the heterodiatomic halogens decrease the tendency toward homolytic reactions, but the increased polarity increases the tendency toward heterolytic reactions. Thus, ICl is a much better electrophilic iodinating agent than is I2 and unlike I2 even iodinates aromatic compounds.
In the interhalogen compounds, one of the halogen atoms may be assigned a positive oxid
ation state. As may be expected, the general trends reflect the increasing difficulty of withdrawing electrons from the central atom on ascending the group and with increasing oxidation state. As might be anticipated, all known stable compounds are fully electron-paired, and the series IF, IF3, IF5, and IF7 give us a homogeneous sequence of molecules exemplifying all possible odd-coordination numbers and their associated geometries (there is no known nine-coordinate neutral binary molecule, although many nine-coordinate complexes are known ). We may complement this series with some of the fluorides discussed earlier and with the xenon fluorides to be discussed below, thereby completing the primary family of molecular structures and electron configurations for main-group elements. In each of the cases shown, the experimental evidence indicates that the molecule adopts the structure predicted by VSEPR theory. A valence-bond description of the molecules with more than eight-valence electrons requires the inclusion of d orbitals in the hybridization scheme. An MO scheme without the participation of d orbitals requires location of electrons in antibonding orbitals, and therefore bond order of less than one, for molecules with more than eight-valence electrons. The mixing of empty d orbitals into the s
cheme can lower the energy of the antibonding electrons (make them less antibonding) and thereby increase the bond strength.

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