CHAPTER 1
Part I
ELECTRON DIFFRACTION
Introduction to Part I: Electron Diffraction
BY L. E. SUTTON
In this Volume there are three Chapters relating to electron diffraction studies of molecular structure.
The first is the usual comprehensive survey of recent work, again written by Dr D. W. H. Rankin, covering papers published up to the end of August 1975. Dr Rankin had considerable difficulty in obtaining copies of certain journals, because libraries are cutting down their subscription lists; so the need for such a review is evidently becoming even greater. He remarks that although the fall in output of papers continues, the quality has advanced markedly. There are numerous studies of structural detail in large and complex molecules, often by the now well-established techniques for combining diffraction and spectroscopic data. There are also measurements of notable accuracy on small molecules. Of unique interest are the investigations on substances that are very difficult to volatilize, which have been the special province of Russian workers.
The mass of data for bond lengths is now enormous: one can say, in current parlance, that there is a data mountain. This means that it is becoming something of an embarrassment. Our contributors have noted the relevance of many of the observations that they have reported to the concepts which have been used in rationalizing bond lengths and their variations, such as covalent radius, bond order, hybridization, bond polarity, co-ordination number, and van der Waals radii or potential functions between non-bonded atoms; but they have only been able to do this in passing. There is no lack of bases for rationalization; there may be a surfeit. A fresh, systematic, and critical review of their adequacy, usefulness, and reliability now seems due. This Senior Reporter will not be volunteering for the task,
Dr H. Oberhammer has contributed a second Chapter on Apparatus Developments, to complete the review begun in the previous Volume by R. L. Hilderbrandt. This one deals with advances made in Europe including the U.S.S.R. In view of the interest of the high-temperature work it is appropriate and timely that Dr Oberhammer should be able to include an authoritative description of the equipment used for it. He also describes the first commercially-built camera.
The third Chapter, by Dr A. G. Robiette, is on the study of large-amplitude vibrations in molecules. Investigators are increasingly concerned to elucidate the 'molecular mechanics'. This includes establishing, and if possible explaining, the potential energy functions that govern the relative motions of atoms in a molecule. These functions can become highly anharmonic and may have more than one minimum, It is with the subtleties of these difficult but very interesting problems that Dr Robiette is especially concerned.
The volume in the Landolt-Bornstein series of Tables, on interatomic distances, which was mentioned in the Introduction to Volume 1, is due to appear in September or October of this year (1976). It will list data for molecules, obtained from vapour-phase measurements by spectroscopy, electron diffraction, or the two combined; but it will not include data for diatomic molecules. The deadline for publications to be included was June 1974. Its appearance will be most welcome; and all concerned with its production are to be thanked and congratulated on completing their task.
It remains for me to thank the contributors to this Volume for their promptness and their general helpfulness, which I do with great pleasure.
1 Electron Diffraction Determination of Gas-phase Molecular Structures
BY D. W. H. RANKIN
1 Introduction
During the year from September 1974 to August 1975, about 85 papers were published reporting the results of structure determinations by electron diffraction. Thus the trend of a decreasing volume of output, noted a year ago, has continued. A crude extrapolation suggests that the last electron diffraction paper of all time will appear in 1984, and this series of chapters of results will end with Volume 13!
This decline in quantity has fortunately been accompanied by a marked advance in quality. To an ever-increasing extent, information derived from electron diffraction experiments has been combined with data from other sources, to provide solutions to problems that proved to be insoluble using one technique in isolation. At one extreme of scale, very small molecules are being studied with great care and precision, using microwave and diffraction data. By such means it has been possible to determine the effect of substituting 37Cl for 35Clo n the average V — Cl bond length in vanadium(v) oxychloride, even though the change is as small as 6 x 10-5 Å. At the other extreme, the structures of some large or complicated molecules have been determined, with the aid of rotational constants or theoretical determinations of stability of conformers, or by using amplitudes of vibration calculated from spectroscopically observed vibration frequencies or standard force constants.
In addition to these fairly sophisticated studies, there are still plenty of examples of electron diffraction being used to answer questions raised by the preparative chemists. Advances in high-temperature technology have enabled useful work on the gas-phase structures of salts to be done, while more conventional techniques suffice for a variety of new and intriguing volatile compounds.
In general, the results given in this Chapter are in the same form as in the original papers. Any comments may be taken to be those of the original authors, unless specifically stated otherwise, and references to earlier work, quoted for comparison, will not be given here.
Distances are rα, values, unless otherwise stated, and errors quoted (in brackets) are estimated standard deviations, expressed in terms of the least significant digit given.
2 Main-group Inorganic Compounds
Two reviews of structural work on inorganic compounds have appeared in the past year. One is concerned with structures of organometallic molecules, an area in which there has been much solid-state work, but until very recently, few gas-phase studies. The other is a review of the stereochemistry of compounds containing bonds between Si, P, S, Cl, and N or O. This is a welcome collation of results from a field that has attracted many workers, and it should prove to be an invaluable reference work.
Salts of Caesium, Thallium, and Barium. — The structures of a number of compounds of electropositive metals, normally regarded as ionic salts, have been determined, and show the presence of discrete molecules in the gas phase. Caesium sulphate, molybdate, and tungstate, and thallium sulphate all have D2d symmetry, with bond lengths and angles as shown in Table 1. Amplitudes of vibration show that the XO4 groups are fairly rigid, but that the outer metal atoms move considerably. The short Cs — O distance in the sulphate, compared with the distances in the molybdate and tungstate, should be noted.
It is suggested that the thallium atom in thallium(I) perrhenate moves fairly freely over the surface of the ReO4 sphere, as the Tl — O vibrational amplitude is 0.22(6)Å, whereas that for Tl···Re is only 0.18(2) Å. In the favoured structure, the thallium is bonded to two oxygen atoms, with r(Tl — O) 2.46(7)Å, r(Re — O) 1.72(1) ÅA, [angle] OReO 98(6)°, and [angle] OTlO 64(3)°. Barium tungstate has a similar C2υ symmetry structure, with r(W — O) 1.82 Å, r(Ba — O) 2.18 Å, [angle] OWO 83° and [angle] OBaO 67°. The WO4 unit is said not to be a regular tetrahedron.
Three structures (1a, 1b, and 1c) for thallium(1) nitrate have been considered.8 The C2[upsion] form (1 b) fits the experimental data best of the three, although a combination of 71% (1b) + 18% (1c) + 11 % (1a) is slightly better than any one form alone. The Tl — O distance is 2.30(3)Å, and the mean N — O distance is 1.40(2) Å.
Covalent Compounds of Groups II and III. — The chemical properties of dicyclopentadienyl magnesium suggest that it is more ionic than ferrocene, but electron diffraction results9 imply that it is covalent, with the cyclopentadienyl rings parallel, and equidistant from the magnesium atom [r(Mg — C) 2.339(4) Å]. The ring C — C distance [1.423(2)Å] is much less than that in ferrocene [1.440(2)Å], reflecting the interaction of iron d orbitals with antibonding ring π-orbitals.
The non-bonded I···I distance in boron tri-iodide has been found to be 3.662(8) Å (rg), and from this, with due allowance for shrinkage, the B — I distance has been determined as 2.1 18(5) Å. The structures of all the boron trihalides are now known. Their bond lengths agree well with those predicted using the Schomaker-Stevenson rule, for the chloride, bromide, and iodide, but the B — F bonds in boron trifluoride are about 0.1 Å shorter than this simple rule leads one to expect.
Trivinylborane probably has a planar average structure, but using a static C3 symmetry model, the vinyl groups are found to be twisted 31(1)° out of the plane. However, the non-planarity may be genuine, as a planar structure would involve non-bonded H···H contacts of only 2.04Å. The fairly long C=C bonds [1.370(6) Å] and short B — C bonds [1.558(3) Å compared with 1.576 Å in trimethylboron] may indicate some π-delocalization into the boron 2p orbital.
Tris(methylseleno)borane almost certainly has a planar skeleton, the optimum twist angle found being only 2°. The B-Se bond is 1.936(2) Å long, compared with 1.99 Å calculated from the B-C distance in trimethylboron, and standard covalent radii for C and Se. It is suggested that this shortening, and the overall planarity, indicate some π-bonding between B and Se. Other important parameters are r(Se — C) 1.954(4) Å, and [angle] BSeC, 102.5(5)°.
Several authors have described structures of derivatives of higher boranes. Pentaborane(9) substituted in the 1 or 2 position by a methyl or silyl group [see structure (211 shows little variation in the structure of the boron cage, but parameters involving the substituent are significant (Table 2). The conclusions, in agreement with Hückel calculations, are that the methyl compounds have comparable stability, and roughly equal B — C bond lengths, whereas the 1-silyl derivative is more stable than the 2-silyl compound by 0.40 eV, because of overlap of silicon 3d and 3p orbitals with cage orbitals, this being shown by a difference between the two Si — B bond lengths of 0.025 Å.
1,2-Dicarba-closo-hexaborane(6), structure (3), has normal bond lengths: C — C 1.535(2); C — B(5) 1.621(4); C — B(4) 1.618(14); B(3) — B(4) 1.745(10) and B(3) — B(5) 1.723(8) Å (all rg). In contrast, carbahexaborane(7), structure (4), has some surprising features. This molecule is derived from CB5H6-, by addition of a proton to one of the BBB faces. The bonds around that face are then exceptionally long [B(2) — B(3) 1.921(8) and B(2) — B(6) 1.909(11) Å], while those around the opposite face [B(4) — B(5) 1.756(9) and C(1) — B(4) 1.659(6) Å] are also somewhat longer than the remaining bonds [B(3) — B(4) 1.685(18); B(4) — B(6) 1.689(8) and C(l) — B(2) 1.602(6) A, all rg]. There is thus a 'pseudo-three-fold axis' through the bridging hydrogen atom and two faces of the octahedron.
1,6-Dicarba-closo-hexaborane(6), structure (3, has D4h symmetry, with r(B — C) 1.635(4) Å and r(B-B) 1.725(12) Å. The latter distance is compared with the basal B — B distance in B5H9, 1.80 Å, and an analogy is made with tetrachlorodiboron, which has a B — B distance of 1.702 Å, and diborane, in which the two boron atoms, with two bridging hydrogen atoms between them, are separated by 1.775 Å.
2,4-Dicarba-closo-heptaborane(7), structure (6), has a pentagonal bipyramidal structure, with the carbon atoms equatorial, but not adjacent. Bond lengths are: B(3) — C 1.537(8)Å; B(5) — C 1.558(7) Å; B(5) — B(6) 1.659(10) Å; B(1)B(3) — 1.852(11) Å; B(1) — B(5) 1.772(11) Å; and B(1) — C 1.717(5) Å. The angles in the equatorial plane are: C(2)B(3)C(4) 100.5(10)°; B(3)C(4)B(5) 116.7(10)°, and C(4)B(5)B(6) 103.0(20)°. The possibility of two equatorial atoms being puckered out of the plane was considered, but the planar ring was found to be favoured, with the displacement from the plane being less than 0.02 Å.
The structures of the adducts of aluminium trichloride and gallium trichloride with ammonia have been determined, the assumption of a staggered C3υ conformation being justified by CNDO/2 calculations. The AlCl3, group has a structure intermediate between those of free AlCl3 and AIC14-, with r(Al — Cl) 2.100(5) Å and [angle] ClAlCl 116.3(4)°. The Al — N bond is 1.996(19) Å long. The gallium compound has a similar structure, with r(Ga — C1) 3.142(5)Å, r(Ga — N) 2.057(11) Å, and [angle] ClGaCl 116.4(3)°.
The dimer of dimethylaminodichloroalane has D2h symmetry, the four-membered ring having angles at nitrogen of 92.5(4)° and Al — N bond lengths of 1.961(6) Å. This distance, the same as that in the corresponding dimethylaluminium dimer, is rather surprising, as the inductive effect of the chlorine atoms is expected to lead to shortening of the bonds. Other parameters [r(Al — Cl) 2.106(4), r(N — C) 1,479(4) Å; [angle] ClAlCl 118.2(15)°] are unexceptional.
Two other dimeric aluminium species have also been studied. Dimethylaluminium thiomethoxide (7) has C2h symmetry, with the S — C bonds bent 65.9(20)° out of the Al2S2 plane, and trans to each other. Important parameters are: r(Al — S) 2.370(3) Å; r(Al — C) 1.945(4) Å; r(S — C) 1.811(10) Å; [angle] AlSAl 94.5(6)°, and [angle] CalC 128.6(25)°. Crowding of the methyl groups on aluminium and sulphur is relieved by a twist of the Al2C4 plane, 6.9(4)° away from the plane perpendicular to the ring. In dimethylaluminium t-butoxide (8) the crowding is so great that the bonds to the butyl groups lie in the Al2O2 plane, even though ab initio calculations show the OH bonds in (H2AlOH)2 to lie 25° out of the plane. The crowding is also thought to account for the existence of this compound as a dimer, unlike dimethylaluminium methoxide, which is preferentially trimeric. Other important parameters are : r(Al — O) 1.864(6)Å; r(Al — C) 1.962(15)Å; r(O — C) 1.419(12) Å; r(C — C) 1.533(5)Å; [angle]AlOAl 98.1(7)°, and [angle]CAlC 121.7(17)°. It should be noted that the wide AlOC angle possible in a dimer means that the AlOAl angle is narrow, 98.1° compared with 125.8(4)° in (Me2AlOMe)2, thus making the Al···A1 distance (2.814 A) shorter even than the interatomic distance in metallic aluminium (2.864 Å).
At 2300 K, aluminium suboxide has a bond length of 1.73(1) Å, and an apparent AlOAl angle of 141(5)°. Estimates of the bending frequency vary considerably, and the average angle could lie anywhere between 141 and 180°.
Trimethylindium has an In — C bond length (rg) of 2.093(6) Å. The observed C···C shrinkage, assuming a planar skeleton, is 0.004 Å, compared with 0.003 Å calculated from spectroscopic data.
Group IV. — Chlorofluorothiocarbonyl has been studied with great precision by electron diffraction, and the derived r°α structure agrees within experimental error with the microwave rz structure. Bond lengths (rg) are: C-Cl 1.7178(9)Å; C=S 1.5931(8) Å, and C — F 1.3387(14) Å. SCCl and SCF angles are 127.28(9) and 123.58(12)°, respectively.
There have been several studies of simple halogenated derivatives of silicon and germanium. Perfluorodisilane has an Si — F bond length of 1.569(2) Å, and the Si — Si bond is 2.324(6) Å long. This is only marginally shorter than the Si — Si bonds in Si2H6 and Si2Me6, and it is thought that lowering of the Si — Si bonding orbital energy by the fluorine atoms is matched by direct F···F repulsions. The angle SiSiF is 110.6(3)°.
Phenyltrifluorosilane, has a regular ring structure, with parameters exactly as would be expected: r(C — C) 1.402(8) Å, r(Si — F) 1.572(6) Å, r(Si-C) 1.822(31) Å, [angle] FSiF 105.0(21)°.
