P.A. Cox - Inorganic chemistry (793955), страница 15
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Diborane is an example. The structure (16) as often drawn appears to have eight bondsand would therefore seem to need 16 valence electrons. In fact, there are only 12 and the molecule is sometimesdescribed as electron deficient. Two pairs of electrons form three-center bonds each linking two boron atomsand a bridging hydrogen, as illustrated in the preferable way of drawing the valence structure in 16′.
Transition metalchemistry (see Section H) is another area where bonding often cannot be described in terms of localized electron pairs.C1—ELECTRON PAIR BONDS59The resonance concept is one way of overcoming some of the limitations of the localized electron pair model, butsuch cases are treated more naturally by molecular orbital theory, which is not limited to bonds involving two atoms(see Topics C6 and C7).Section C—Structure and bonding in moleculesC2MOLECULAR SHAPES: VSEPRKey NotesVSEPR principlesUsing VSEPRExtensions, difficultiesand exceptionsRelated topicsThe valence shell electron pair repulsion (VSEPR) model is based onthe observation that the geometrical arrangement of bonds around anatom is influenced by nonbonding electrons present. It is assumedthat electron pairs—whether bonding or nonbonding—repel eachother and adopt a geometrical arrangement that maximizes thedistances between them.It is first necessary to decide which atoms are bonded together.Drawing a valence structure gives the total number of electron pairsaround an atom, sometimes known as its steric number.
The basicVSEPR geometry is then used to assign positions for bonding andnonbonding electrons.In spite of a lack of firm theoretical foundation the VSEPR model iswidely applicable to molecular geometries and even to some solids.Occasionally it fails to predict the correct structure.Electron pair bonds (C1)Molecularorbitals:Molecular symmetry andpolyatomics (C6)point groups (C3)VSEPR principlesStereochemical arrangement becomes an issue whenever an atom is bonded to two or more others. Thus triatomicspecies AB2 can be linear (e.g. CO2,) or bent (e.g. H2O 1,).
It is observed that when a central atom has nononbonding electrons, the surrounding atoms are usually arranged in a regular way that spaces them as far apart aspossible. When nonbonding electron pairs are present in the valence structure, however, less regular arrangements ofbonds are often found. The valence shell electron pair repulsion (VSEPR) model is based on the idea that bothbonding and nonbonding electron pairs in the valence shell of an atom ‘repel’ each other. This idea is useful but can bemisleading if taken too literally. Detailed calculations show that the shape of a molecule is determined by a combinationof factors, of which the electrostatic repulsion between electrons is not the most important.
Furthermore, the realelectron distribution in a molecule is much more evenly spread out than the localized pictures used in VSEPR (1, 2, …)suggest. It is best to think of ‘repulsion’ as coming primarily from the exclusion principle (see Topic A3), whichforces electron pairs to occupy orbitals in different regions of space.SECTION C—STRUCTURE AND BONDING IN MOLECULES61The basic principles of the model are as follows.(i) Valence electron pairs round an atom (whether bonding or nonbonding) adopt a geometry that maximizes thedistance between them.
The basic geometries usually observed with 2–7 pairs are shown in Fig. 1.(ii) Nonbonding electron pairs are closer to the central atom than bonding pairs and have larger repulsions: in fact, theorder of interactions is(iii) If double (or triple) bonds are present the four (or six) electrons involved behave as if they were a single pair,although they exert more repulsion than do the two electrons of a single bond(iv) As the terminal atoms become more electronegative relative to the central one, bonding electron pairs are drawnaway from the central atom and so repel less.Using VSEPRBefore applying VSEPR to a molecule or complex ion it is necessary to know the connectivity, that is, which atomsare bonded together.
With a species of formula AXn this often gives no problem, especially if X is a monovalent atom orgroup (e.g. H, F, CH3). Sometimes it is not so obvious, and a useful rule (which does not apply to hydrides or organicgroups) is that the central atom is usually the least electronegative one. For example, in N2O one of thenitrogen atoms is central (see Topic C1, Structure 12). Drawing a valence structure including nonbonding pairs on thecentral atom then gives the total number of ‘pairs’ (multiple bonds counted as a single ‘pair’).
This is sometimes calledFig: 1. Basic VSEPR geometries with 2–7 electron pairs.the steric number (SN) of the central atom A, equal to the number (n) of bonded atoms, plus the number ofnonbonding electron pairs. It may generally be assumed that one electron from A is used in each bond formed to X.62C2—MOLECULAR SHAPES: VSEPRThus in SF4 four electrons are used in single S—F bonds, leaving two electrons (i.e. one pair) nonbonding, so thatSN=5.
In XeOF4 the Xe=O double bond uses two electrons from xenon, and again there is one nonbonding pairmaking SN=6. In complex ions account must be taken of the charge. Thus inwe can include the charge on theion and assign eight valence electrons to chlorine. Six are involved in bonding, so that there is one nonbonding pair, andSN=4.Steric numbers 2–4The shapes shown in Fig. 1 are simple and the rules generally easy to apply. Examples without nonbonding electronsare:• linear species (SN=2): BeH2 (gas phase only; see Topic F3), HgCl2, CO2 and ions isoelectronic to it such asand NCO−;• trigonal planar species (SN=3): BF3,and• tetrahedral species (SN=4): CH4,SiCl4, POCl3 and.AX2 species with SN=3 or 4 are bent, with the nonbonding pairs occupying positions of the trigonal plane ortetrahedron, respectively (e.g.
water, 1). As predicted by rules (ii) and (iv) the XAX bond angles are less than the idealvalues of 120° (SN =3) or 109.5° (SN=4), and tend to decrease as the electronegativity difference between A and Xincreases. Some examples with their bond angles are:• SN=3:ClNO (113°);• SN=4: H2O (104.5°), H2S (92°), F2O (102°).In AX3 with SN=4 the nonbonding pair forces the bonds to be pyramidal (see ammonia, 2). Examples with their bondangles are:Steric number 5The normal shape adopted by five groups is the trigonal bipyramid, as with PF5. There are now two inequivalenttypes of position, two axial (top and bottom in Fig.
1) and three equatorial. It appears that the equatorial positionsallow more space than axial ones. Thus bulkier groups (e.g. Cl in PF4Cl) tend to be found in these positions, as dononbonding pairs when these are present. With successively one, two and three nonbonding pairs, the molecular shapesare as follows.• AX4 is often described as a ‘see-saw’ with two axial and two equatorial X positions, the former being slightly bentout of the ideal linear configuration by the lone-pair repulsion. Examples are SF4 (3) and XeO2F2 (where O inpreference to F occupies the equatorial position; see rule iii).• AX3 gives a T-shape, as in ClF3 (4).• AX2 is linear as the bonded atoms are axial.
Examples are XeF2 (5) and .SECTION C—STRUCTURE AND BONDING IN MOLECULES63Steric number 6The basic shape is octahedral and is found with SF6 and. All positions are equivalent and with one nonbonding pairAX5 adopts a square pyramidal structure (e.g. BrF5 6, and XeOF4, where repulsion between the double bond andthe lone-pair is minimized by putting these trans to each other). When two non-nonbonding pairs are present theyminimize their repulsion (rule ii) by adopting the trans configuration, giving a square planar molecule (e.g.
XeF4, 7and).Steric number 7The only simple examples are the pentagonal bipyramidal IF7 (see Fig. 1. and the ionplanar, having two lone-pairs occupying the axial positions (for XeF6 see below).which is pentagonalExtensions, difficulties and exceptionsOne of the problems with VSEPR is that its rules appear somewhat arbitrary and hard to justify in a rigorous quantummechanical formulation. The interpretation of small variations in bond angle is often considered to be particularlydubious. In spite of this (and of the exceptions noted later) the model is surprisingly useful.
Although the discussion hasconcentrated on cases where single atoms are bound to a central one, VSEPR should be able to predict the geometryaround any atom in a complex molecule, where main-group atoms are involved. (It cannot be generally applied totransition metals; see Topics H2 and H6.) For example, in hydroxylamine, H2NOH, the bonds around the nitrogen arepyramidal, those around the oxygen bent as expected.
The model is even useful in interpreting solid-state structurescontaining ions such as Sn2+ where nonbonding electrons appear to have a stereochemical influence (see Topic G6).One type of exception to VSEPR arises when apparently nonbonding electrons are really involved to some extent inbonding. For example, the geometry around nitrogen is planar when bonded to carbonyl groups in the peptide linkage(-NHCO-) in proteins, and in trisilylamine, (SiH3)3N (8, only one of three equivalent resonance structures shown).
Inboth cases the ‘nonbonding’ pair on nitrogen is used to form partial double bonds. In 8 this requires valence expansionby the silicon, and contrasts with pyramidal trimethylamine (CH3)3N, where the carbon cannot accommodate extraelectrons (see Topics C1 and F4).64C2—MOLECULAR SHAPES: VSEPRAX5 species with no lone-pairs are occasionally square pyramidal rather than the normal trigonal pyramid of Fig. 1 (see,e.g.
Topic F6, Structure 2). Other difficulties arise with AX6 where there is one nonbonding pair. This is the case withXeF6, which, as predicted, is not regularly octahedral. A unique shape cannot be determined in the gas phase, however,as the molecule appears to be highly fluxional and converts rapidly between different distorted configurations.By contrast, the ions [SeCl6]2− and [TeCl6]2− are regularly octahedral in spite of having a nonbonding pair.
There is nosimple explanation, although the comparatively large size of the chloride ion could be a factor.Other notable exceptions are some of the group 2 dihalides such as BaF2, which in the gas phase are bent, not linearas VSEPR predicts.
(In their normal solid-state forms they have different structures; see Topics D3 and G3.) Twofactors that are thought to contribute are (i) the use of valence s and d orbitals for bonding (rather than s and p as is normalin later main groups, and (ii) the possibility that core polarization could lower the energy of the bent form.Section C—Structure and bonding in moleculesC3MOLECULAR SYMMETRY AND POINT GROUPSKey NotesSymmetry operationsand symmetry elementsPoint groupsUses and limitationsRelated topicsThe symmetry of a molecule can be specified by identifying thesymmetry operations and the symmetry elements corresponding tothem. Possible symmetry elements are rotation axes, reflectionplanes, an inversion center, and rotation reflection axes.The full set of symmetry operations of a molecule is known as a pointgroup.
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