化学专业英语之卤族元素及其化合物

1、化学专业英语之卤族元素及其化合物INTERHALoGEN AND NoBLE GAS CoMPoUNDSIn terhaloge n and n ObIe gas ComPo UndS ComPriSe a relatively Iimited family of highly reactive and UnStabIe molecules whose Primary importance is their role in testing ChemiCaI bonding1theory . At first it may Seem rather Strange to treat the Che

2、miStry of the halogens and the noble gases, two groups that represent the extremes of ChemiCaI activityand inertia,in theSameSeCti on. The SUPerfiCiaIdiffere ncesbetwee n the haloge nsand the noble gases are much reduced, however, if We focus our atte nti on on the COmPariS on of halide ions (PartiC

3、UIarly F-)With the isoelectr onic n oble gas atoms and the no ble gas compo UndS With haloge n atoms or the haloge ns in their higher POSitiVe-OXidati on states.Noble gases are exceptional in their reluctance to either gain or lose an electr on. HaIide ions because of their excessnegative charge, re

4、lative to the isoelectronicnoble gas atomshave both a lower ioni Zati on en ergy and a IeSSer electr on affini ty. On the other hand, n oble gas Cati ons have greater electron affinities and greater ioniZation energies than do isoelectr onic haloge n atoms. From SUCh con SideratiO ns, it is obvious

5、that inert gases should be less reactive than are halide ions, but their compo UndS should show eve n higher reactivity than the halogens. The big question remaining is : Are there any ChemiCally SignificantconditionsUnder WhiCh noble gases Canbe PerSUaded to yield electrons SUffiCiently to PrOdUCe

6、StabIe compoUnds? The anSWer is definitely, yes! (The Same questionCan be asked of haloge n atoms, WhiCh have ioni Zati on en ergies COmParabIe to those of the inert gases.)Another obvious point of SimiIaritybetween halogen and noblegas compo UndS is the characteristically large nu mber of electro n

7、s that must be accommodated in the vale nce shell. For a noble gas atom bonded to any number of other atoms, the octet rule must be exceeded; for a haloge n atom to be bon ded to more than one other atom, the Same must be true. It is a CUriOUS historical fact that the mythical inertiaof a closed she

8、ll didmuch to diminish the en ergy expe nded in the SearCh for n oble gas ComPo Un ds, l Ong after nu merous examples of SuPeroCtet VaIe nce shells Were known, particularly among in terhaloge n compo Un ds.We may roughly CIaSSify the in terhaloge n compo UndSin to two categories: those in OXidati on

9、 State zero (the bi nary an alogs2of the eleme ntary diatomics ) and those in WhiCh one of the haloge ns is in a formally POSitiVe OXidati on state.HeterOdiatOmiC haloge ns are gen erally formed readily on mixing the required Pair of halogens in a 1:1 ratio. The bond energies are always higher in th

10、e heteropolar molecules tha n are the average bond energies of the two conStitUentsand in SOmeCaSeShigher than either. It is this factor that drives the reactions.2All heterodiatomics are more or less StabIe Un der ambie ntcon diti ons except for BrF, WhiCh SPOntan eously disproportionates to BrF3 a

11、nd Br 2. The bonding in the halogendiatomics Can be attributed to a Sin glea bond, formed byoverlap of P orbitals. In the heterodiatomics, the Prin cipal new features are the poorer OrbitaI overlaps that are POSSibIe between atoms of WideIy differentPrincipalquantum number(n),the polarity arising fr

12、om the difference in electronegativity, the contribution of ionic terms to inCreaSe bond energy, and the relief in in terelectro nic repulsi on in the fluorides.relative to difluori ne.Dihalogens (except for F2) usually react by dissociation into atoms or by heterolyticdissociati onUn der the in flu

13、e nce of anattacki ng reage nt.Thus, reacti on of Cl 2 With hydroxide may beVieWed as displacement of Cl - from Cl 2 by OH-:CI2+OH HOCl + Cl -The tendency to Un dergo heterolytic fissi on in CreaSeS on descending the group, and the 12 molecule Can actually be CIeaVed to two StabIe species:I2 + 2C 5H

14、5N+AgNQ- C 5H5N)2INO3 + AgIThe in CreaSed homolytic bond en ergies of the heterodiatomic halogens decrease the tendency toward homolytic reactions, but the in CreaSed polarity in CreaSeS the tendency toward heterolytic reactions. Thus, ICl is a muchbetter electrophilic iodi nati ngage nt tha n is 12

15、 and un Iike 12 eve n iod in ates aromaticcompo Un ds.In the interhalogen compounds, one of the halogen atoms may be assigned a positive oxidation state. As may be expected, the general trends reflect the increasing difficulty of withdrawing electrons from the central atom on ascending the group and

16、 with increasing oxidation state. As might be anticipated, all known stable compounds are fully electron-paired, and the series IF, IF3, IF 5, and IF 7 give usa homogeneoussequence of molecules exemplifying all possible odd-coordination numbers and their associated geometries (there is no known nine

17、-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 configur

18、ations 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 electro