Aug. 20, 1958
CRYSTAL AND MOLECULAR STRUCTURE OF TRIMETHYLINDIUJI
4141
[CONTRIBUTION No. 604 FROM THE INSTITUTEFOR ATOMICRESEARCH AND DEPARTMENT OF CHEMISTRY, IOWA STATE O F THE u. s. ATOMIC ENERGY COMMISSION] COLLEGE. WORK WAS PERFORMED I N THE AMESLABORATORY
Electron Deficient Compounds. VIII. The Crystal and Molecular Structure Trimethylindium
cjf
BY E. L. AMMAAND R. E. RUNDLE RECEIVEDMARCH13, 1958 Trimethylindium is a tetramer of symmetry 2 in the crystal, with methyl bridges of a new type. The tetramers appear to interact, though more weakly, through additional methyl bridges. Within each tetramer nearly trigonal In(CH3)3“monomers” with short, “normal,” In-CHS bonds are found. One In-CH3 bond of each monomer points almost directly normal to the trigonal plane of another “monomer” within the tetramer, forming a n unsymmetrical, linear In-CHZ.. , . . I n bridge. A second In-CHS bond of each monomer points approximately a t the trigonal plane of a monomer in a n adjacent tetramer to form another unsymmetrical CHa-In . . . . . CH3 bridge. There are then five methyl groups about each indium, foeming a somewhat distorted trigonaLbipyramid, with three short, 2.1 A. bonds in the trigonal plane, and two much longer bonds, 3.1 A. within the tetramer, 3.6 A . to the next tetramer, on either side of the plane. Steric reasons for expecting the sharp-angled M-CHI-M bridges, found in trimethylaluminum and dimethylberyllium, to be unstable when M is a larger metal atom have been noted previously. A qualitative MO description of the new type of bonding is given.
Introduction Trimethylaluminum dimerizes2 and dimethylberyllium polymerizes3 through methyl bridges with very sharp M-C-M bridge angles of 70 and GBO, respectively. Presumably the best overlap of one tetrahedral carbon orbital with orbitals from two different metal atoms t o form one delocalized, three-atom, molecular orbital requires such sharp bond angle^.^,^ I n higher members of the series, though excess metal orbitals still make such bond delocalization attractive, the larger metal atoms are expected to lead to metal-metal repulsion of a magnitude to render sharp bridge angles unstable.s Trimethylindium has been reported to be a tetramer in solution a t low temperature^,^ but a monomer a t higher temperatures and in the vapors5 Hence, there is reason to suppose that there is bond delocalization, but of a new type, in this compound. Experimental Procedure Preparation and Properties of Trimethy1indium.Trimethylindium was prepared by the method of Dennis, et ~ 1 . by , ~ Dr. B . Zaslow and furnished t o us in sealed tubes. The preparation consists of refluxing dimethylmercury with indium metal for several days and sublimation of the trimethylindium from the mercury and excess indium. The pure compound is a colorless solid, melting a t about 89”, and strongly birefringent. It is very reactive with water and air, so X-ray samples were prepared by sublimation into suitable capillaries on a vacuum line. Single crystals were grown by heating the capillaries in a waterbath and allowing the bath t o cool slowly. Many crystals were very unstable in the X-ray beam, perhaps due to small amounts of impurities, and decomposed after a few hours. However, acrystal \vas eventually obtained which was sufficiently stable in the beam t o yield three-dimensional diffraction data. As had been noted b e f ~ r ea, ~tetragonal and a less common, less stable, pseudohexagonal (more likely triclinic) form was found to exist. The pseudo-hexagonal type was not investigated in this research. X-Ray Data.-Timed-exposure precession photographs of the hkO, hhl and Okl reciprocal levels were obtained with Zrfiltered MoKa radiation. Equi-inclination Weissenberg photographs mere obtained for the reciprocal levels, hk0, (1) Presented before the American Chemical Society Convention, New York, h-.Y., September, 1957. This paper is based in part on the Ph.D. thesis presented by E. L. Amma to Iowa State College in partial fulfillment of the requirements for the Ph.D. degree, December, 1957. (2) P. H. Lewis and R. E. Rundle, J. Chem. P h y s . , 21, 986 (1953). (3) A. I. Snow and R . E. Rundle, Acta Crysl., 4, 348 (1951). (4) L. M. Dennis, R. W. Work, E. G. Rochow and E. M. Chamot, THISJOURNAL, 56, 1047 (1934). ( 5 ) (a) A. W. Laubengayer and W. F. Gilliam, ibid., 63, 477 (1941); (b) L. Pauling and A . W. Laubengayer, ibid., 480 (1941).
h k l , . . . hk6, also with Zr-filtered Mo KCY radiation using a combined multiple film and timed exposure method. -411 intensities were estimated visually with a calibrated standard set of intensities. Precession data were corrected for Lorentz-polarization effects b y means of a template.6 The Weissenberg data were corrected for Lorentz-polarization effects from a table computed from the function given by Cochran.’ Spot extension corrections for higher layer Weissenberg data were made from a Phillips chart.8 Preliminary scale factors and crystal temperature factors were obtained from log FJFo v s . sin20/X2plots for each reciprocal level using parameters from the two-dimensional analysis.
Structure Determination Unit Cell and Space Group.-Trimethylindium was found to be tetragonal with lattice constants : a = b = 13.24 + 0.01 8.,c = 6.44 0.01 8. The lattice parameter a was determined by back reflection methodsg and the lattice parameter, c , was determined from precession photographs on which a was used as an internal calibration. With 8 molecules per unit cell, Pcalcd = 1.88 g./cc., while Pobs = 1.57 g . / m 4 The Laue class was observed to be C l h - 4/m with the systematic extinctions
+ + +1
(hkO) reflections absent for h k = 2n (001) reflections absent for 2 = 2% 1
This uniquely fixed the space group as P42/n. Determination of Atomic Positions The diffraction data indicated that the metal atoms were in the general &fold set of positions =!= [x,y , 2 ; 1!2 - x, ”2 - y, 2 ; ”* - - y, x, ‘/z - 2 ; y, 1 / 2 - x, 1/2 ,].lo Metal atom parameters were determined from conventional two-dimensional Patterson projections and trial and error methods. Carbon atom positions were obtained by a combination of conventional heavy atom Fourier techniquesll (an indication of the resolution obtained from this very heavy metal-carbon system can be seen in Fig. l), difference Fourier syntheses and
+
(6) J. Waser, Rev. Sci. Instr., 22, 567 (1951). (7) W. Cochran, J. Sci. Instr., 25, 253 (19B). (8) D. C. Phillips, Acfo Crysl., 9, 819 (19561. (9) M . J. Buerger, “X-Ray Crystallography,” John Wiley and Sons, Inc., New York, N. Y., 1942, Chapter 20. (10) “International Tables for X - R a y Crystallography,” Vol. I , The Kynoch Press, Birmingham, England, 1952, p. 176. Rote: the origin used here is not a t the center of symmetry given in the tables a t 3/4. 1/4, r 4 from on p. 176. (11) H. Lipson and W. Cochran, “The Determination of Crystal Structures,” G. Bell and Sons, London, England, 1953, Chapter 7.
E. L. AMMAAND K. E. RUNDLE
4142
VOl.
so
omic distances were computed on the I. B If.630 with a program from Dr. Templeton, and bond angles were computed manually. Standard deviations of bond distances were computed with conventional formulas.14 Final results after four least square cycles are shown in Table I. Boric1 distances and bond angles are given in Table 11.
I5 60
Inl-CZ In2-CS
Inl-Cl In-C2 In&, Inl-Inl 60
60
Fig. 1.-Fourier projections onto (001): (A, top) Direct Fourier. Heavy peak is indium, lighter peaks are two of the three unique carbon atoms. (B, bottom) Partial difference Fourier. 2rfetal atom only has been subtracted out. X is indium position. Third carbon appears near X.
three-dimensional Fourier blocks. l 2 The structure was refined on an I. B.M. 650 by a least squares method with individual but isotropic atomic temperature factors.I3 The least squares program (kindly furnished to us by Dr. D. H. Templeton of the University of California) was originally designed for only orthorhombic symmetry, but i t was modified for monoclinic symmetry. Trimethylindium was reduced to monoclinic symmetry by doubling the parameters and including those reflections in the refinement dependent by tetragonal symmetry but independent by monoclinic symmetry. Interat-
Inl-In~
Ini-Inr CS-C~ In-C-In CI-In-C3 C2-In-CJ CI--Iri-Cz
TABLE I1 BONDDISTSNCES (.A,) AND ANGLES 2 . 1 5 zk 0 . 0 4 Short bridge bond within tetramer 3.11 zk .04 Long bridge bond within tetramer 2.12 .04 Short bridge bond to another tetramer 2.06 zt .04 Unbridged In-C distance 3 . 5 9 i 04 Long bridge bond to m o t h e r tetramer 5 . 2 3 5 =k ,004 Closest In-In distance within tetramer 5 . 6 6 5 Z!C ,004 Closest In-In distance betw eeii tetramers 6 . 7 9 zk ,004 Distance across tetramer 3.78 zk .05 Bridge methyl contact distances within tetramer Bridge angle within tetramer is linear within exptl. error Angle = 122" Angle = 117' "Monomer" Angle = 119'
Discussion Description of Structure.-Trimethylindiuni is a tetramer or, more exactly, a pseudo-tetramer in the shape of a tetrahedron flattened along one fourfold screw axis. T h s nearest metal atoms within the tetramer (3.24 A,, Inl-Ins), Table I1 and Fig. 2, are held together by linear, asymmetric, electron
TABLE I" I:ISAL
PARAUETERS,
STANDARD
DEVIATIONSAND ATOM
TEMPERATURE FACTORS Atom
In Cl Cp CI
x
0,2140 ,1420
,1710 ,3422
Y Z us uy ua 0.0038 0 . 4 1 2 4 0,0002 0.0002 0.0005 ,0070 ,0027 ,0027 ,2884 ,1282 ,0028 ,0027 .0067 ,9620 ,7086 ,0071 ,0028 ,0028 ,2788 ,9282
Flt~~ii TI
B/X2 10.3 9.29 8.94 9.37
tFcli/
R, =
= 11.8%
Foii
Calculated and observed structure factors may be obtained by ordering Document 5554 from the American Documentation Institute, Auxiliary Publication Project, Photoduplication Service, Library of Congress, Washington 25, D . C., remitting $1.25 for 35" microfilm or $1.23 for photocopies by check or money order payable to Chief of Photoduplication Service, Library of Congress. (12) This technique was developed by D. R. Fitzwater of o u r laboratory. T h e blocks consist of a 4 X 4 X 4 grid, where the grid spacing is 1/80 of the unit cell. T h e program is such that the function is calculated for each of the 84 points in the block in one computation. T h e hlock can he arbitrarily located anywhere in the unit cell. ( 1 3 ) This routine was programmed for the I. B. M. 6.50 by Drs. M. Senko and D. H. Templeton. It can be used with a constant or variable weighting factor; we used the variable weighting factor, which is more consistent with error theory.
Fig. 2.-The trimethylindium tetramer. IVeak liridge bonds t o neighboring tetramers are iiidicatecl by dashed lines. (14) F. R. Ahmed and D. W. J . Cruickshank, A d a C v y s l . . 6, :iFR (! 0 3 5 ) ,
Aug. 20, 1958
CRYSTAL AND MOLECULAR STRUCTURE
OF
deficient
bridges with a short In-C distance of 2.15 A. (Inl-C3), and a long In-C distance of 3.11 A., (Inz-Ca). The metal atoms of one tetramer are bonded to metal atoms of adjoining tetramers by other nearly linear, electron deficient methyl bridges with short In-C distances of 2.12 A. (InlCl), slightly shorter than reported for the gaseous monomer (2.16 A.)6band long In-C distances of 3.59 A. (In3-C1). These long In-C distances are 0.48 A. longer than similar distances within the tetramer; hence the molecules appear to be weakly linked and are not true tetramers. This arrangement makes the crystal a three-dimensional network of methyl bridges, but this three-dimensional network is in reality composed of two mutually exclusive three-dimensional networks which are interlaced by means of the fourfold screw axis. The three methyl groups bonded to a particular metal atom by more .or less “normal” covalent distances (Inl-C!l, 2.12 A , ; Inl-Ct, 2.06 A.; In1CB, 2.15 8.)are chemically as well as crystallographically distinct. C1 and C3 participate in inter- and intratetrameric bridge bonding, respectively, but Cz is non-bridged, a free methyl group. The three closest methyl groups about an indium atom form a slightly distorted trigonal monomer, weakly linked into tetramers and still more weakly linked to other tetramers. The local configuration of methyl groups about a
TRIMETHYLINDIUM
4143
H
ti
00 C
q.@ D
In d z z
E
Fig. 4.-Schematic and idealized representation of long bonds in trimethylindium : (A) presumed flattened metal groups with small back-side lobe for long bond; (B) small back lobe is represented by c-orbitals; (C) bonding MO; (D) non-bonding MO; (E) possibly significant MO using indium d,*-orbital.
metal atom is that of a distorted trigonal bipyramid, Fig. 3. The “normal” bonds extend from the metal atom in the center to the carbon atoms a t the vertices of an approximate equilateral triangle. The In-C intratetrameric long bridge bond (InzC3, 3.11 extends from the center to a carbon atom a t one apex and the intertetrameric long bridge bond (In3-C1, 3.59 A.) extends to the other apex. The two long bridge bonds are inclined a t an angle of 163’ to one another; hence the trigonal bipyramid is distorted. However, the plane of the “monomer” is, within experimental accuracy, perpendicular to the intratetrameric long bridge boyd. Although the intertetrameric bridge (3.59 A,) bond is rather long in comparison with normal covalent bonds, it is sufficiently shorter than the sum of the van der Waals radii of I n and methyl (2.2 and 2.0 re~pectively)’~ to be indicative of an interaction over and above van der Waals forces. It may be argued that the van der Waals radius of the metal atom (2.2 is not reliably known, but it is noteworthy that the nearest unbridged methylindium distance is 4.60 8. Also, the methyl group of the intertetraperic long bridge bond is abnormally close, 3.59 A., to the free methyl group on the metal participating in the long bridge bond. This shortening by 0.4 A. of the usual methyl-methyl distance of 4.0 A.supports the contention that both methyl groups are bonded to the same atom. Other abnormally short methyl-methyl distances are between the intratetrameric bridge methyl and the free methyl group on the metal atom partici-
A.)
A.,
A.)
Fig. 3.-Configmation about indium. 9 , C2 and Ca are in a plane, with IO’Sof the carbons atoms with the termiin solution, but this seems a t least somewhat for- nal indium atoms shown in 41.) The antibonding t uitous. SIO, $,AB = alpz - b ’ ( q - az)will not be occupied in this 4-electron system, and is not shown. Of possible significance is the use of the outer (iri,?-orbital of indium which belongs to the same representation as + l o 3 M set.-' (at 4') and kinetics, for reaction 1 are >lo2 Msec >lo6 M sec.-l ( a t O O ) , the smaller numbers coming from the optical-active measurements, which are free from separationinduced exchange uncertainties. Colorimetric observations led to specific rate limits a t 0" of >3 X lo8 X sec. for the net reactions between Os(dipy),++ and Fe(phen)3++', between Os(dipy)a++ and Fe(dipy),+++, and between Fe(phen)a+" and Ru(dipg),+++and of > 5 X l o 4 M set.-* for the net reaction between Os(dipy),++ and Ru(dipy),++". No reduction in the rate of the Os(dipy),++-Fe(phen)3+++ reaction was observed when resolved rather than racemic reactants were used
Introduction
tween Mn04-- and MnOc-5 and between Fe(CN)$---- and Fe(CN)e---.6 However, since immeasurably large rates were found, we repeated the experiments of Dwyer and Gyarfas3 and applied a similar optical-active method to the F e ( ~ h e n ) ~ + + Fe (phen)3 +++ system. Experimental
The rate of electron exchange between Os(dipy)3++ and Os(dipy)3+++2 was investigated by Dwyer and Gyarfas3 by observing the decrease in optical activity with time after mixing &Os(dipy)3++ with Z-Os(dipy)3+++. They reported that the rate was large but indicated that it was Chemicals.-Mallinckrodt "analytical reagent" grade measurable, 9.5 sec. being required for complete chemicals were used without purification, with these esracemization a t 5' and 5 x 10-5 M reactant con- ceptions: G. F. Smith Co. sodium perchlorate, Eastman centrations. Kodak Co. camphorsulfonic acid, and Eimer and Amend p Eimer and Medalia4 used conventional isotopic- toluenesulfonic acid, while Eastman Kodak Co. Practical sulfate was redistilled a t atmospheric pressure. tracer methods to investigate the exchange reac- dimethyl Eimer and .ilmend C.P.nitromethane was purified b y the tion between the tris-(5,6-dimethyI-l,lO-phenan-method of Thompson, et ~ 2 1 . ~ Mallinckrodt U.S.P. potasthroline) complexes of iron(I1) and iron(II1). sium tartrate was recrystallized from H20. Stock solutions of Fe(phen), were prepared by dissolvThey found complete exchange in 15 sec., indicating equivalent amounts of FeS04.7H?O or Fe(NH4)2(SO& ing either that the rate in 0.5 f H2S04was immeas- 6HzO and l,l0-phenanthroline (G. F . Smith). The Feurably large (specific rate > l o 3 M set.-' a t 0') or (phen)s solutions were prepared immediately before each that complete exchange was induced by the sep- run by dilution of appropriate amounts of Fe(phen),++ with dilute sulfuric acid and oxidation with PbOl. Excess PbOz aration methods. the product PbSOa were removed by centrifugation. We undertook the measurement of the Os- andTris-(2,2'-dipyridyl)-osmium(II) chloride was prepared (dipy)3 ++-Os(dipy) 3 + + + and Fe(phen13++-Fe by the method of Burstall, Dwyer and Gparfas.* The com(phen)3+++ exchange rates by the isotopic-tracer pound was purified by recrystallization. Stock solutions method modified to include rapid mixing and of Os(dipy), were prepared from weighed amounts of the compound. The Os(dipy),++" solutions were prepared by quenching techniques, which had made possible PbOz oxidation of Os(dipy),++in dilute HzS04. measurements of the large rates of exchange be++
+++
++
(1) This work was supported by the National Science Foundation. The paper was abstracted from the Ph.D. thesis of Eugene Eichler, Washington University, 1955. (2) dipy = 2,Z'-dipyridyl; phen = 1,lO-phenanthroline: en = ethylenediamine. ( 3 ) F. P. Dwyer and E. C Gyarfas, Nature, 166,481 (1950). 141 L.Eimer and A. 1. Medalia, THIS JOURXAL,74, 1592 (1952).
(5) J. C. Sheppard and A. C. Wahl, ibid, 79, 1020 (1957); 711, 5133 (1953). (6) A. C. Wahl and C . P. Deck, i b i d . , 76,4054 (1954). (7) C. J. Thompson, H. J. Coleman and R. V. Holm, ibid., 76, 3445 (1954). (8) F.H.Burstall, F. P. Dwyer and E. C Gyarfas. J . Chem. Soc., 953 (1950).
