X-Ray Diffraction Lynne
L. Merritt,
Jr., and William E. Streib, Department of Chemistry, Indiana University, Bloomington, lnd.
T
follows the same outline as the two previous reviews (122) presenting general, analytical and industrial applications of x-ray diffraction in the first part and structural applications in the second part. The period covered is January 1966 through December 1967 measured primarily by the appearance of an abstract within this span of time, although several of the most important journals were independently scanned by us. Furthermore, this review is not intended to be comprehensive; rather it is hoped that it will give an idea of the directions in which developments are proceeding in the field and will flag significant advances or changes for the reader’s attention. HIS RCYIGW
GENERAL
A series of review articles by Reuben Rudman appearing regularly in the Journal of Chemical Education (148) have recently discussed various aspects of instrumentation for x-ray diffraction including generators and safety, tubes, monochromators, detectors, cameras, etc. Several new books dealing with various aspects of the theory and general practice of x-ray diffraction have recently appeared, among these are “XRays and Seutron Diffraction,” by G. E. Bacon (12), “Single Crystal Diffractometry,” by Arndt and Willis (8),“Theory of X-Ray and Thermal Neutron Scattering by Real Crystals,” by Krivoglaa (106), “hlathematical Theory of X-Ray Powder Diffractometry,” by Wilson ( I % ) , “Optique des Rayons X et Microanalyse,” by Castaing et al. ( S T ) , “X-Ray Technique,” by Bajza, et al. ( I S ) , “Local Atomic Arrangements Studied by X-Ray Diffraction,” edited by Cohen and Hilliard (44) and volume 2 of Brill and Nason’s, “bdvances in Structure Research by Diffraction hlethods,’’ (30). -4 collection of 24 papers published by the staff of the Philips Laboratories has been edited by Parrish (136). One of the most productive sources for the chemist who is interested in new and modern techniques and apparatus in x-ray diffraction is the series “Advances in X-Ray Analysis” which reports the proceedings of anliual conferences on Applications of X-Ray Analysis held in Denver, Colorado; volume 9, edited by Mallett, Fay, and Mueller (117) and volume 10, edited by Newkirk and Mallett (131) appeared during the past two years. Mention should also be made of the chapter on
molecular structure determinations by R . F. Bryan in the series, “Advances in Analytical Chemistry and Instrumentation,” edited by Reilley and hIcLafferty (146) and a second edition of volume 5 of Wyckoff’s “Crystal Structures” (178). Several books dealing with the applications of x-ray diffraction in specific fields or industries have appeared. Among these are “Diffraction Xethods in Materials Science,” by Cohen ( 4 3 ) ; “Applications of X-Ray Diffraction in the Atomic Energy Field,” by Jolley and Clark (93); and two books in Russian dealing with reinforced concrete quality control (107) and the analysis of mineral raw materials (156). Vainshtein’s “Diffraction of X-Rays by Chain Molecules,” has been translated from Russian into English (169). Two books dealing with proteins and nucleic acids have appeared (88, 177). Useful tables of wavelengths, 2 e angles, etc., are represented by two new books (6, 60) and a book of tables of mass attenuation coefficients (159) is also available. More attention to instruction in xray diffraction methods seems to be developing. Nuffield’s book, “X-Ray Diffraction Methods,” (134) and Brown’s book, “X-Rays and Their i2pplications,” (51) are both designed for college courses. An undergraduate course a t the University of Louisville is described by Cooke (47). A cooling apparatus and laboratory experiments therewith are proposed by Rudman (149) and a simulator to teach students the principles of Bragg’s law has been patented by Meiners (121). Along the same line is a hard-sphere model to simulate alloy thin films which has been devised by Xowick and Mader (135). Improvements in x-ray diffraction apparatus continue unabated. A review of recent developments by Kishi and Igarashi (102) was noted. A device for automated measurement of powder diffraction patterns giving punched card output and a computer program to convert the output to dspacings and peak intensities has been described by Frevel (65). An x-ray microdiffraction unit with a microanalyzer was developed by Jchinokawa (90) from an electron probe microanalyzer. Activity in the high-temperature, x-ray diffraction field continues a t an ever increasing pace and higher and higher temperatures are being attained. A review of high-temperature studies (150) is noted as well as several
new or improved cameras for work a t elevated temperatures, some reaching or slightly exceeding 2500 K (14, 16, 34, 57, 78, 184). High-temperature cameras for special purposes, such as the examination of catalysts in the course of a reaction (174), the examination of ceramic systems (156), high-polymer studies (115) and transmission topography (28) have been described. Likewise, we noted an apparatus combining the heating microscope and the DebyeScherrer camera (139) and another which permitted simultaneous recording of x-ray diagrams, thermograms, and electrical resistance curves (26). Precise temperature control with a gas stream has advantages described by Young (179) and the precise measurement of high-temperatures in x-ray cameras by measuring lattice parameters of an internal standard i s described by Merryman and Kempter (125). The measurement of surface temperatures is considered by Baker, et al. (15) who point out that surfaces are sometimes up to 150’ C cooler than the bulk of the sample. A few cameras for low temperatures have also been described in the literature; one operating from -180’ to 0’ C was developed by Gervais, Sella, and Spritzer (71), a Sieman-Bohlin camera was modified for liquid nitrogen temperatures by Mascarenhas and Mascarenhas (118) and another diffractometer for use in the same range was described by Xkolaenko and Karpukhin (132). An apparatus for simultaneous x-ray diffraction and differential thermal analysis in the range -180’ to 600’ C appears in the Russian literature (145). Apparatus which permits observations on solids a t high pressures is also being developed. Among such devices are ones described by Przedmojski (I.@), Class, ef al. (41), Bassett, et al. (18) and Francois, et al. (63). Some of these devices permit simultaneous optical measurements and some go to 300 kilobars pressure. Two diffractometers for remote analysis have been developed, the first, described by Schenk, et al. (152) is for the investigation of radioactive substances and the second, described by Schnopper, et al. (163) is for the remote analysis of the moon although it may have uses in an earthly laboratory. Progress is reported in the development of apparatus for x-ray diffraction topography by several authors (7, 36, 42, 114). Other notable pieces of new VOL. 40, NO. 5, APRIL 1968
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equipment are-an apparatus with diffraction and spectrometer tubes mounted on a common housing giving a diffraction range of -45’ to +165’ and intensities of 95-100% of maximum (lO)-a single-axis, two-crystal instrument capable of measuring diffraction angles to the order of 0.1” (54)-a diffraction goniometer with stationary specimen and movable source and detector (11)-a dynamic diffractometer based on stroboscopic techniques for observations of patterns a t a particular phase of a periodic deformation of a specimen (98)-a focussed double chamber for fine structure studies (96)-a symmetrical back-reflection focussing camera with a microfocus tube for precise lattice measurements (17‘) and special tubes for portable field apparatuses ( I ) . The intensification of images is considered by Kennedy (100) and the electron-optical recording of images by Mokul’skaya and Mokul’skii (166). Improved slits for diffractometers are described by Parrish (137) who has also developed a ray-proof slit mount (138). A protractor for measuring diffraction patterns is reported in the Russian literature (55). An investigation of pulse amplitude shifts in gas proportional detectors was carried out by Burkhalter, et al. (33) who also note that more work is needed to determine the causes of large peak shifts in proportional counters. Bearden (22) has continued his investigation into x-ray wavelengths and again recommends WKal wavelength as the s t y d a r d with a value of 0.2090100 A =t 5 ppm on an absolute scale. Four lines of Ag, Rlo, Cu, and Cr are taken as secondary standards and 61 additional lines of various elements have been used as reference lines. Two additional publications in the National Bureau of Standards series by Swanson, et al. (163, 164) on “Standard X-Ray Diffraction Powder Patterns” have appeared with standard patterns for 103 and 80 substances, respectively. Precision lattice parameter measurements by interferences from lattice sources (Kossel lines) and divergent beam x-ray diffraction (pseudo-Kossel lines) in the back reflection region are discussed by Ullrich (167’). Kunze (109) points out that the normal Bragg law is not always satisfactory in solid state physics and he has derived a diffraction law on the basis of kinematic interference theory for the case of asymmetric back reflection by plane objects. Much work is appearing on methods of indexing of powder diffraction patterns. A detailed review of graphical and analytical methods and a new graphical method is described by Gattow and Piotter (69). The application of the I t o method to orthorhombic substances is proposed by Gilli and Pulidori (72’) and a graphic method for
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ANALYTICAL CHEMISTRY
hexagonal and tetragonal substances by Nedoma (130). A new graphical method for all crystal symmetries, developed a t Los Alamos, is described by Coleman and IUcInteer (46). Computer methods for indexing have been developed by numerous authors including Battelle Memorial Institute (ZO), Goebel and Wilson (74) and Hoff and Kitchingman (86). Matthews and Thompson (119) describe the use of KWIC, a permuted title program, to the orderly arrangement of lists of diffraction data. A computer program to develop control tapes for the Ferranti Mark I1 automatic diffractometer has been written by Curry (51). Computers have been used to correct x-ray powder line profiles by use of the Stokes’ numerical Fourier analysis method (85). Other correction procedures noted in the literature arenomograms for correction of integrated intensities as measured on a photometer record of lines on a Guinier film (161)-least -squar es weighting schemes for diffractometer data (101) and an iterative method for slit-correcting small-angle x-ray data (112). X-ray diffraction is used, as everyone knows, for a wide variety of purposes. I n what follows of this General Section we will only give a selection of applications in an attempt to indicate the magnitude of the variety and possible new uses. Keely (99) has developed a line-broadening technique for obtaining rapidly calculated surface area of pure substances and of the various components of a multicomponent mixture. Numerous articles have appeared which deal with the x-ray determination of stress and of crystallite size. A review with 151 references was prepared by Macherauch (116) and another dealing with iron and steel, specifically, by Hauk (83); industrial uses and the effect of surface conditions on the measured results are described by Aida and Yamamoso ( 3 ) while an evaluation of various techniques has been made by Fukura and Fujiwara (67‘). A new type apparatus, especially designed to measure the stress in large immovable samples, is described by Hashimoto, et al. (82) and one to measure microstrains in large, spherical, machine parts by Epshtein, et al. (59). Krauth (104) points out that line-broadening is usually used for $rystal-size measurements up to 200 A but, by use of suitable correction factqrs, the limit can be extended to 600 A or higher. The propagation of systematic errors in line profile analysis is considered by Young, et al. (180) while Berry (24) has studied uncertainties in crystal size computed from the standard deviation of line breadth. Muro, et al. (126) conclude that it is enough to make merely the background correction when measuring the residual stress of hard-
ened steel. Cucka (50) points out that failure to take account of the nonuniformity of stress near a surface may lead to gross errors when stress measurements are made on crystallites of one phase dispersed in a matrix of another phase. Removal of successive layers of material alters underlying stress fields and corrections must be applied to get the original fields as is pointed out by Sikarskie (157). A comparison of the Schoening and the Warren-Auerback methods for particle size and stress determinations has been made by Sen Gupta (154); other methods are described by Rao and Anantharaman (144, Wilke and Hoseman ( l 7 5 ) , Harrison (81), and Halder and Wagner (79). Two methods were developed by Wagner (172) for the analysis of the broadening and changes in position of peaks in x-ray powder patterns. ; i mathematical expression for Fourier profile coefficients in lines of x-ray diffraction patterns for crystallites of various forms and sizes was worked out by Kachamin (95) and computer programs to evaluate line broadening are given by 1)eAngelis (53) and Uerman and Katz (23)). -11)plication of particle size determinations to aluminum specimens are described by Ebel and Effenberger (56) and to polycrystalline graphites by Fitzer, et al. (62). X-ray methods for the detection of dislocations in crystals are reviewed by Brunimer (32). Observations of locally diffused regions in silicon wafers by the Berg-Harret method are described by Juleff and Wolfson (94) and a study of magnetic domains in terbium iron garnet n a s made by Basterfield and Prescott (19). High resolution topographic methods were used by Lang (113) to study lattice imperfections in many natural and man-made crystals and lattice imperfections in single crystals were observed directly by means of their contrast effects on an image of the crystal formed by a diffracted x-ray beam by Wallace (17 3 ) . The meaning of crystallinity measurements in polymers is discussed by Stratton (162) and Bodor (29) considers factors causing line broadening in the diffraction patterns of high polymers. An absolute method for crystallinity determination in certain polymers was described by Corradini, et al. (48) and a wide-angle method for degree of crystallinity measurements of linear polyethylene mas developed by Gopalan and Mandelkern (75). A low-angle niethod which permits the investigation of molecular dimensions in bulk polymers was worked out by Krigbaum and Godwin (105). The method involves tagging the polymer chains with end-groups of high-electron density and dispersing these in a solvent of untagged polymers. Molecular
weight determinations on macromolecules are described by Feigin (61). Milberg (184) points out that the difference pattern can reveal the presence of oriented, ordered material whose conventional diffraction patterns would be masked by that of unoriented, disordered material. A review of x-ray diffraction and possible applications of optical transforms in studying imperfectly or partially crystalline material, such as fibers, has been published by Taylor (165). X-ray diffraction has been used to study teeth and bone and related structures (25, 64, 73). I t has also been used by Zosi (183) for the measurement of the thickness of thin amorphous or crystalline layers on crystalline substrates. Characterization of nonionic surfactants by x-ray methods, among others, is reviewed by Sadeau and Siggia (187). A review of x-ray methods of analysis including theory, equipment and applications to the identification of minerals, crystal-size and orientation determinations, thermal expansion measurements, quantitative analysis, sample preparation, and safety is given by Croft (49). Quantitative methods have been considered by several authors. Karlak and Burnett (97) developed a general equation for quantitative analyses of mised crystalline phases from fundamental considerations and the addition method was generalized by Alegre (4). Use of more than one detector for the simultaneous qualitative and quantitative determination of two or more phases is described in a Bayer (21) patent application. h data card index for quantitative determinations is proposed by Naray-Szabo and Peter (129). Calibration curves for the quantitative analysis of antimony oxides are given by Rajkovic (149). Errors have been investigated by Il’in, et al. (91) and the effect of trace impurities by Chikawa and Newkirk (40). The effects of orientation of powder specimens has been considered by Bezjak and Jelenic (27), Gacesa and Jelenic (68),and by Otsu (135). Kittrick and Hope (103) have worked out a simple procedure for the identification of small crystals. Chemical analysis of objects less than 1 micron in size can be performed on an apparatus described by Vasichev (170). The instrument is a combination of an electron microscope with two x-ray diffraction spectrometers. Simultaneous phase analysis of different layers with intact sequence in stratified fine crystalline specimens is described by Przybora (141). An investigation of molten alloys was made by Steeb and Hezel (160) and of SaC1-type solid solutions of silver halides by Asada (9). ilpplication of diffraction methods to
clay minerals is the subject of articles by Chernyakhovskii and Ryazin (39), Stoerr (161), Taylor and Norrish (166), and Zkhus (182). Silicate minerals are considered by Smolczyk (158). ilpplications to the cement industry are the subject of articles by Alegre and Debray (5) and Mehta and Shah (120). The quantitative determination of phases in aluminum-silicon alloys is described by Zav’yalova, et al. (181), in copper-nickel alloys by Raichenko (146) and of austenite in steel by Geru (70). Isherwood and Quinn (92) show that an unconventional glancing-angle technique enables analyses to be made of thin oxide films without removing them from their substrate. Diffraction methods used to control a wire mill are outlined by Custer (59). Other applications of x-ray diffraction techniques which might be mentioned as examples of the wide diversity and usefulness of such techniques include the control of the sintering process (go), the investigation of glasses and amorphous bodies (168),the behavior of rust under paint systems (68),the identification of minerals in coal (89, 147), the determination of dialkyldimethylammonium-urea adduct in urea (38), the quantitative determination of active components in ointments (110, I l l ) , the analysis of household detergents (d), diffraction from oilseeds (I%?), the determination of the purity and crystallinity of molecular sieves (171), detection of partial-burning of RDXT N T mixtures (77), and applications to Irish soil science (35). A report on extraterrestial applications by Herglotz (84) should be noted as well as applications in the ceramic industry (46, 7 6 ) , the determination of the chemical composition of antique ceramics (87), investigation of white lead in art paintings (108), and studies on fossil and modern resins (66). STRUCTURE DETERMINATION
Much of what was said in our previous review (162) with regard to the rapid increase in the number of structure determinations published, can be reiterated here. Primarily, larger faster computers are becoming available to more and more crystallographers. A second factor has been the development and production of highly automated precision diffractometers by all the major manufacturers of x-ray diffraction equipment. Both of these factors not only decrease the time it takes to solve a structure, but they greatly improve the accuracy with which a structure can be determined. Finally, important advances continue to be made in finding methods of solving crystal structures. Previously (126) we specifically mentioned the direct method of phase determination and how successfully it
had been demonstrated. This technique is now well established (256, 303, 362, 363, 366, 371). Many of the centrosymmetric structures reviewed during the current two-year period were in fact solved in an almost routine manner using direct method computer programs. Of even greater significance is the extension of the method (364, 369, 370) to noncentrosymmetric crystals and the demonstration (365, 368) of its utility in this more general case. Because this is far from being exploited, as the development of automated equipment is in its infancy, and because computers will certainly continue to improve, one can predict with considerable confidence the continued very rapid growth of the field of x-ray crystallography and of the number of structure determinations in particular. The above factors have, of course, resulted in a large increase in the amount of literature to be reviewed and we must again remind the reader that the references actually cited below are only a small sampling to indicate the structural interests during the past two years. With few exceptions, all of the papers do represent complete three-dimensional work. A recent review by Sim (456) is recommended for more complete coverage of the structural work on medium sized (four- to eight-membered) rings, hydrogen bonding, and metal carbonyls. It seems appropriate t o begin with a few of the structures that are specifically of interest to the analytical chemist. Abrahamsson, et al. (185) report on rhodanine derivatives which are mesoionic, referring to compounds having a negative charge on a group which is covalently bonded to a heteroaromatic ring having a positive charge, and for which no satisfactory structure can be written. I n particular they have studied the 3-amino, 5-phenyl, 5methylated derivative, and from their bond lengths conclude the molecule has a “betaine” structure. A crystal structure determination of nitrilotriacetic acid (459), which is used in complexometric titrations, has proven it to be a zwitterion. Dibenzoylmethane, an analytical reagent for the determination of uranium has been shown (491) to exist in the enol form. The structure of the benzaldehyde-potassium bisulfite addition product (385) and a number of xanthate salts (326, 348, 349) have also been reported. Some attempt has been made to group the structure determinations below into categories; however, considerable overlap occurs in many cases. Inorganic Structures. GENERAL INORGANIC. A number of interesting studies involving the elements were reported. For example, Hittorf (red) phosphorus has been shown (468) to contain two units, P8 and Ps, which are VOL. 40, NO. 5, APRIL 1968
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linked by two additional phosphorus atoms to form an infinite tubelike structure of pentagonal cross section. The complexity of the structure is used to explain why phosphorus tends to form a glass and crystallizes only on long annealing. The crystal structure of a cyclododeca form of sulfur has been determined (386) in which the S12 molecules form a crown-like ring having 3-fold symmetry. -4 hexagonal polymorph of diamond having the wurtzite structure, long anticipatcd because of the known polymorphs of other group IV elements, has been found in meteorites (287, 309, 330) and has also been synthesized (246). The naturally occurring hexagonal diamond can properly be considered a mineral and the name Lonsdaleite has been suggested (309). h’umerous other structure determinations of minerals were, in general, very complex structures whose description is beyond the scope of this review. Typical are the structures of glauberite (378), CaNa,(SO&, which consists of a network of Ca octahedra and SO4 tetrahedra, and the continuing studies on the polymorphs of mica (465). Also, several impressive studies on zeolites (400, 480) have appeared, such as the zeolite Paulingite (319) which is cubic with a = 35.10 and contains a framework made up of 2016 atoms. The work of Lipscomb and coworkers (284, 286, 306, 335, 417, 434, 481) continues to dominate the boron hydride and carborane literature, this work including for example an interesting series of studies on dicarbadodecaborane derivatives (910, 431-433). Sequeira and Hamilton (447) have shown that iodine substitution for the low melting monoiododecaborane is a t the 1-position. A dicarbanonaborane with nonvicinal carbon atoms has been reported (475). hletal coordinated carboranes include a dicarballyl cuprate (492) in which the metal coordinates primarily with boron atoms only and a rhenium carbonyl complex of a dicarbannnane (494) in which the metal coordinates with both boron and carbon. The latter compound is quite analogous to a n iron carbonyl (495) derivative cited in our previous review. Interest has been developing in the special type of boron derivatives referred to as “boron-nitrogen compounds,” the B-T\T bonds, of course, being isoelectronic to C-C bonds. The interest appears to be very wide, including amine-boranes (19.4, 337), a trisaminoborane (244) and a triborylamine (245), a substituted BK analog of cyclooctatetraene (258), which was shown to have the normal tub configuration, and a borazine (195) “B-trimethylborazole. I f Many studies have been devoted to elucidating the structures of phases in binary and ternary systems. Unfortu432 R
ANALYTICAL CHEMISTRY
nately, as in the case of the minerals above, these are frequently very complex structures. A good example is the M phase, Nb48Ni3+4113,which was shown (449) to contain atoms exhibiting coordination of 12, 14, 15, and 16. I n the Ce~Mg42structure (358), Ce atoms were found to be surrounded by a 20 atom polyhedron of &Ig atoms. Jeitschko and Parthe (356) describe an unusual Rh17Ge22 structure, unique in that the relative valence electron contributions could be determined by measurement of diffraction line positions. The structure of NaTl has been determined (331) and a molecular orbital bonding scheme is presented which relates this structure to numerous other binary systems. hletallic binary and ternary systems such as those above are an obvious source of information concerning metalmetal interactions. Many other studies have been devoted to observing more isolated metal-metal bonds, particularly in coordination complexes. Hansen and Jacobson (332) reported the first x-ray structure of a complex with a metal-metal bond between different transition metals in their determination of r-C6HsFe(CO)2Mn(Co)5. Since then, numerous other studies have appeared describing complexes containing CoH g bonds (413), Be-Ge bonds (2471, Sn-hln bond (240, 241, 485), Sn-Fe bonds (221, 242, 416), and a n Sb-Fe bond (486). A number of very unusual metal-metal bonding arrangements are found in metal carbonyl coordination structures. For example in Fe3(C0)1zJ the iron atoms bond to form an equilateral triangle (Car), in the [Fer(CO)13]2- anion (275) they bond to form a tetrahedron as does Co in Co4(CO)12 (487), and in the Sn[Fe(CO)rId structure (390), tin is tetrahedrally bonded to the four iron atoms. These compounds are also of interest in that they are metal carbonyls, an area which has also received a great deal of attention-e.g., the series of studies by Carter, McPhail, and Sim (239,251-254). COORDINATION COMPOUNDS. As in the previous two-year period crystallographers have probably devoted more attention to coordination compounds than any other area, certainly so in the field of inorganic structure determinations. The metal carbonyls mentioned above could have been included here as could the *-complexes discussed below. We shall, however, limit the following section to those structures containing unusual ligands and those containing atoms having unusual coordination number and valency, in that order. Although sulfur containing ligands such as dithio- and diselenocarbamates (191, 229, 260, 311, 379, 427) are far from unknown, there did seem to be a greatly enhanced interest in these com-
pounds recently, resulting in the discovery of a number of interesting and unusual coordination compounds. A good example is the use of dimethylsulfoxide (DMSO), whose structure has also been determined recently (467) and found to have the expected pyramidal configuration. DAIS0 is particularly interesting in that it can presumably coordinate via either sulfur or oxygen (214). I n fact, both situations have been demonstrated. Structures have been determined in which DMSO is sulfur-bonded to Pd(I1) (216, 388) and Ir(II1) (399) and in which it is oxygen-bonded to Fe(II1) ($15). Some rather surprising results were obtained in the studies of Pd (373) and P t (207) complexes containing a carbon disulfide ligand. The CS2 was found to be a bidentate ligand coordinating with the carbon and one sulfur, and was found to be quite nonlinear, the S-C-S angle being 140’ and 138’, respectively, for the two studies. X rhodium complex (267) containing a thiocarbonyl ligand has been reported, the thiocarbonyl coordinating via the carbon atom as in other metal carbonyls. The structure determinations of cis- and trans-
bis- (methylthiohydrosamate)nickel(II) (441, 442) are believed to be the first examples of thiohydroxamate ligands to be studied by x-ray methods. Also the structure of [CsHJlo0]2S? (461) is the first instance in which tlvo sulfur atoms bridge two metal atoms, and the structure of [ S C O ~ ( C O ) ~(462) ] ~ S ~is the first instance in which two sulfur atoms bridge four metal atoms. The latter structure also contains an interesting SCo subunit which forms a tetrahedron. A novel C U ~ [ S C ( X H ~ ) ~ ] ~ (strucNO~)~ ture is reported (482) which contains five distinctly different metal-sulfur bonds. A number of papers were found on the relatively rare peroxo-bridged complexes -e.g., Grandjean and Weiss (320-324) report a series of studies on oxofluoroand oxoperoxofluoro-complexes of ;\lo(V) and (VI). Schaefer and Marsh (444) have determined the structure of (SH3)5C O O ~ C O ( N H ~ ) & ~ O ~ ( Hand S O ~ )find ~ the O2 group skewed to the Co-Co axis, but clearly not perpendicular to it, contrary to a n earlier study (479) on the very similar compound [(SH3)&00&0(SH3)5](N03)s. A number of papers were also found describing structures of the relatively unusual chlorine bridged complexes (407, 408, 411). The complex Co3(C0)&CH8 was found to have a most unusual structure (464), containing a metal coordinated triply bridging aliphatic carbon atom, the c03c group assuming a tetrahedronlike configuration. Finally, the structure of [ R u I I ( S H , ) ~ Y ~ ] C is ~reported ~ ( B O ) , which contains molecular nitrogen as a ligand. Although the structure is disordered, residuals in a differ-
eiice map clearly indicate a linear Ru-N-1; group, implying that XZ is bonding like a carbonyl. The structures containing atoms of unusual coordination number and valency will be described in order of increasing coordination number. Zero valent metal complexes are of interest because of the unusual properties they exhibit, including abnormally low coordination nuniber. X good example is the structure of tris(tripheny1phosphino)platinuni, Pt(PPh3)3 (288), in which Pt is tricoordinate, the three 1%-P bonds being trigonal and just slightly noncoplanar. Similar situations are observed in AgC(CS)3 (380) with =ig(I) coordinated in a flat pyramid to three nitrogens, and in carbonyl piperidine-S-carbonitrile riickel(O), Si(C5H1JCS)C0 (382), which is trimeric and in which each nickel is ubonded to a carbonyl carbon and a nitrile nitrogen and is =-bonded to a nitrile bond. h n interesting one sided four-coordinated arsenic atom is found in the structure of K [ X S ( C ~ H ~ O & ] (466'), the arsenic being a t the apex of a t'etragonal pyramid. Foss and coworkers have reported an extensive series of tellurium complexes (296-301). These appear to be the world's most consistently square planar complexes. Even when T e is only three-coordinated, the complex assumes a square planar configuration, with one position vacant (297, 300). I n the past, peiitacoordinate complexes were rather unusual. 111 fact many of the papers concerned with pentacoordination would emphasize this in the introductory re. This situation has obviously st,imulated a great deal of interest, for during the two-year period covered, there were reports 011 five coordinated atoms from one end of the periodic table to the other. .1 partial listing of these includes : aluminum (228),silicon (437), phos1)horus (329, 458), titaniuin (438), niaiigaiiese (286), iron (238, 318), cobalt (220, 276, 435, 490), nickel (418, 420, &0), copper (218, 219, 354), zinc (302. 41 9), arsenic (209), molybdenum (31Z ) , palladium (224), silver (477), tin (283), antimony (4$9), and iridium (36.5, 480). The vast majorit,y of these have a trigonal bipyramid configuration, although instances of square pyramidal configuration with the coordinating atom in the base are also reported (224, 318). The crystal structure of 1,lO-phenanthrolinedimethylthalliumperchlorate (223) revealed a n interesting case of six coordination in which the configuration is a pentagonal bipyramid with one equatorial position vacant'. Pentagonal bipyramidal coordination is in fact quite rare although a few such st'ructures mere found for iron (859, 293), protactinium (274), and uranium (327). In one of these (293) Fleischer and Hawkinson report a
the metal coordinates with just one double bond in that the metal is symmetrically located above the plane of the ring, I n the studies cited, the arene complexes all contained a chromiumcarbonyl moiety. The orientation of the moiety about a n axis perpendicular to the ring showed a strong dependence on the ring substituents; however, the aromatic ring showed little or no effect from the presence of the moiety. An interesting case in which metal coordination has a very striking effect on ring configuration is found in the structure of tris-cyclooctatetraenedititanium (272). Two of the three cyclooctatetraene rings, each coordinated to one of the titanium atoms, are completely planar giving the structure an arene-like complex appearance. The Organo-Metallic Structures. ACOMPLEXCOORDINATION COMPOUNDS. third cyclooctatetraene ring is coordinated to both titanium atoms and apStructures containing a-cyclopentapears to be in a twist configuration. A dienyl groups continue to receive much somewhat less extreme case of ring attention. Trotter and coworkers have distortion is found in the structure noted that the five-membered rings in of cyclooctatetraenemolybdenumtricarferrocene are staggered, but that the bony1 (397). rings are eclipsed in ruthenocene and its Other a-complexes involving delocalderivatives, and they have raised the ized a-electrons include the more question as to whether the difference in familiar *-allyl ligands in complexes orientation is due to the difference in such as a-allylirontricarbonyl (406), metal atoms, the different intramolecua - allyl - - cyclopentadienylpalladium lar forces, or the different intermolecular (405), and a substituted a-allylnickelforces. Their work on diferrocenylbromide (257). The conjugated bonds ketone (478) and 2-biphenylferrocene in cycloocta-2,4-diene (COD) and in (474) suggests a strong dependence on cyclooctatetraene (COT) apparently intermolecular forces. Other work on can act as an allyl group in the comsubstituted ferrocenes and ruthenocenes (292, 360, 484) also s h o w a consistent plexes (COD)Pd(CH3COCHCOCH3) (255) and (COT)Fe,(CO)S (294). The pattern of eclipsed ring orientation. Xills and coworkers have described allyl complexes are of considerable interest for it is by no means obvious some very unusual rhodium a-comunder what conditions the a-electrons plexes. The structure of trimeric a-cyclopentadienylcarbonyl rhodium (402) are delocalized. For example in compounds very similar to those described was shoivn to contain an equilateral above, a niethylallylpalladium complex triangle of R h atoms, each atom eo(395), and a cyclohexa-1,3-dieneiron ordinating a a-cyclopentadiene ring and complex (251), the molecular parameters each pair of R h atoms being bridged by a carbonyl. The structure of hydridoindicate distinct U- and a-bonds, imtetracyclopentadienyltrirhodium (289) plying localized double bonds. h similar situation exists in the complexes bewas shown to also contain an equilateral triangle of R h atoms, again with each tween platinum and di(cyc1opentaR h atom coordinating a cyclopentadiene diene)-yl (489) and between nickel and ring, but in this case, with a fourth cyclooctene-yl (402). Other structures cyclopentadiene ring parallel to the having coordinated localized double triangle and shared by all three rhodium bonds include tlie complex between atoms. h novel tetramer of cyclofumaric acid and irontetracarbonyl peiitadienylironsulfide has been re(425), the dimeric complex between ported (488) which can be described as 1,3-butadiene and cobaltcarbonyl (869), an elongated tetrahedron or sphenoid and several silver complexes with novel with a sulfur atom above each triangular unsaturated ring compounds such as face and Rith a cyclopentadienyl ring bullvalene (396), a cyclorioiiatriene coordinated to each corner. (353), and nonbornadiene (204). A Cyclopentadienyl-metal complexes number of complexes containing the have of course been widely studied for highly unsaturated acetylene and cumusome time. This has not been the case lene groups have also been studied with the somewhat analogous arene (222, 284,266,268). The interaction of complexes, although a few structures of the metal is very obvious in these comthis type have been solved recently pounds, for the normally linear un(250, 884, 409). They are distinct saturated groups invariably become from other metal ion-aromatic ring very nonlinear in the complex. complexes, such as the copper and silver General Organometallic Struccomplexes of benzene (476,477) in which tures. Aliphatic organometallic com-
macrocyclic pentadelitate ligand resembling a porphyrin which fills the 5 equatorial positions. Bergman and Cotton ($2 7 ) have determined the structure of tetraphenylarsonium tetranitratocobaltate(II), which is believed to be the first example of eight coordinated cobalt. Eight coordinated zirconium with square antiprism configuration rvas revealed in the studies on Cu3Zr2FI4 16 H20 (291) and Cu2ZrFs.12Hz0 (290). A few examples of high coordination are nine coordinated uranium in LiUFs (243), ten coordinated strontium in Sr(MnO&.3H20 (288), and eight, nine, and eleven coordinated thorium in its complexes with formate, sulfate, and nitrate hydrates, respectively (199, 200, 478). e
VOL 40, NO. 5, APRIL 1968
0
433 R
pounds are among the more difficult compounds t o obtain as stable crystalline solids; however a few such structures have been reported recently. Methylzincmethoxide has been shown (448) to exist as a tetramer with the zinc and oxygen atoms forming a novel cubane-like structure and with the methyl groups projecting from the corners of the cube. The cube is only slightly distorted, the Zn-0-Zn angles being approximately 94' and the 0-Zn-0 angles being approximately 83". The crystal structure of trans-p-chlorovinyl mercuric chloride (426) revealed a C-Hg-Cl angle of 167' which represents an unusually large deviation from linearity. This structure is also somewhat unusual crystallographically in that the compound crystallizes in space group P i with 6 molecules per unit cell. I n continuing studies on cyanides, Schlemper and Britton have determined the structure of trimethyltincyanide (445) and trimethylcyanogermane (446). They find the (CH3)3Sn group to be planar with disordered cyanides symmetrically disposed above and below to give Sn trigonal bipyramidal coordination; however, (CHJ3GeCX is tetrahedrally bonded much as one would expect for isolated molecules. Vranka and Amma (483) have redetermined the interesting dimeric trimethylaluminum structure which contains not only normal carbonmetal bonds, but also bridging methyl groups. The crystal structure of C~H~C=CCUP(CHZ)~ (263) is similar in that it is a tetramer and it also has bridging carbon atoms; however, the structure of the tetramer is much more complicated, involving two different copper environments as well as the two different acetylide carbon environments. Acetylides of silver (262), gold (264), and nickel (266) have also been reported, all of them containing collinear metalacetylide bonds. A number of aromatic organometallic compounds have also been determined. There seems to be a striking analogy between these and the aliphatic organometallics discussed above. For example, a diphenylmercury derivative (383) is found to have (slightly) noncollinear CHg-C bonds, and even more striking, dimeric triphenylaluminum (393) is completely analogous to dimeric trimethylaluminum, with four of the phenyl groups in terminal positions and two of them bridging. Finally, Avoyan, Chapovskii, and Struchkov (202) report the structure of (C6H&P (n-C6H5)C6H5-FeC0 which contains a rare example of a Fe-C o-bond. Organic Structures. As techniques, equipment and computing facilities improve one would expect t h a t i t should be possible t o obtain more and more detailed structural information from x-ray diffraction studies. Investigators have always been concerned 434 R
ANALYTICAL CHEMISTRY
with the problems of accuracy in structure determinations, and several have been concerned with the specific goal of obtaining accurate electron density maps. Very accurate recent work on small organic molecules has shown that goal may indeed be obtainable in the foreseeable future. I n his studies on a substituted norcaradiene, which contains a three-membered ring, Fritchie (307) has shown that the experimental data are best matched with a bent bond cyclopropane model. A structure determination of cis-1,2,3-tricyanocyclopropane has revealed residual electron densities of 0.05 e/Aa displaced 0.32 .k from a line between carbon centers which Hartman and Hirshfield (356) believe is attributable to bonding density. Coppens (261) has combined x-ray diffraction and neutron diffraction data collected at moderately low temperature in a study of s-triazine in which 0.08 electrons were found in the region between carbon and nitrogen in the C3X3ring. Our previous review mentioned a few rare examples of organic ions. A number of additional examples of carbanions (193, 269, 423, &O), carbonium ions (190, 226, 428, 472)) dicarbonium ions (203, 398), ion radicals (308, 470, ,$?I), and carbene complexes (403, 404) have since appeared. I n general, the carbonium ions and carbenes have coplanar bonds and the carbanions have significantly noncoplanar bonds as one might expect. The central carbon-carbon bond in the dication [tetra-p-anisyl ethyleneI2+is twisted 41' which is consistent with MO calculations (203). Aliphatic acids studied recently include dimethylmalonic acid (325) and aminomalonic acid (361), which was shown to exist as a zwitterion, and two crystalline forms of fumaric acid (211, 236). Oxalic acid was studied in the deuterated form by both x-ray (352)and neutron (351)diffraction and was shown to have an unusually large isotope effect. It crystallizes in the same space group as the hydrogenated form, but is not isomorphous, the difference being in the manner in which it is hydrogen bonded. The crystal structure of tetraacetylethane (443) has shown the molecule to exist as a dienol with the planes of the central carbon bonds being oriented almost exactly perpendicular to one another. A study (213) of the calcium salt of the unusually strong dibasic organic acid hexacyanobutylene, comparable in strength to sulfuric acid, has shown the doubly ionized molecule [C(C(CN)2)3]2-to exist in a propeller shaped configuration rather than the previously assumed planar configuration which would afford maximum r-overlap. The structures of a number of fundamental amides such as hydroxyurea (389), ethylcarbamate (231), adipamide (342), and suberamide (341) have been reported, as well as a n exhaustive series
of studies on substituted quinones such as the work of Gaultier and Hauw (313, 314, 315) and Breton (232,233) on naphthoquinones. The structure determinations of p-cresol (227), catechol (237), and terephthalic acid (205) are typical of the numerous studies on other fundamental aromatic compounds. An interesting series of studies have been concerned with the conformation of the cyclobutane ring which can be either planar or puckered depending on the ring substituents. Thus it has been found that trans-l,3-cyclobutane dicarboxylic acid is planar (394) whereas the cis isomer (186), and both the cis and trans isomers of 1,2 dibrom0-1~2-dicarbomethoxy cyclobutane (367)) are puckered with a dihedral angle of approximately 150'. Many additional studies have been concerned with the factors that determine configuration in medium sized nonaromatic ring systems. The work of Dunitz and coworkers (272, 273, 277-282, 346) has been most impressive in this area. Overcrowding in aromatic rings has also received considerable attention in studies varying from just slightly crowded molecules such as o-fluorobenzoic acid (381) in which the carboxyl group is twisted 21' out of the plane of the ring, to highly substituted rings such as hexanitrobenzene (187) in which the nitro groups are twisted 53" out of the plane of the ring, to compounds such as 10-dicyanomethylene anthrone (451) where crowding is so severe as to cause distortions in the anthracene skeleton and in the substituents. Several multiple ring structures have been elucidated, some of the more unusual including a Ci0 homocubane (416), a pentacyclononane dicarboxylic acid (347),and a dimeric product resembling two twistane molecules with a ring in common (192). Also a number of large ring systems have been reported--e.g., all - cis - 1,6 - dichlorocyclodecatetraene (377) and 1,8-bis-dehydro-(14)annulene (206),the latter being a planar molecule. The bond length distribution in the structure of trans-15,16-diethyldihydropyrene (3%) has confirmed the aromatic character of the outer ring. I n continuing studies, Housty, Hospital, and coworkers (338, 343-346, 455) report a number of new long chain acids and amides. An increasing amount of work is being devoted to the crystal structure of polymeric materials such as polythiomethylene (249), poly(m-xylyleneadipamide) (421), and polyselenomethylene (248). Also a, number of monomers such as e-aminocaproic acid (225) and oligomers such as 2,4-dicyanopentane (189) and ~entane-2~4-dioldiacetate(469) have been investigated. Acrylamide is of interest because it will polymerize in the solid state. A crystal structure determination (350) revealed that indeed, the
ethylenic bonds of neighboring molecules were in very close prosimity to one another.
Natural Products and Molecules of Biological Interest. T h e structure determination of natural products has been a major area of crystallographic research. Studies on antibiotics such as siccanin (340) and gliotoxin (212), steroids such as androsterone (339) and substituted cholestaiies (316, 317 ) ,alkaloids such as tomatidine (376), vertaline (328), and hodgkinsine (305), and toxins such as anisatin (439), and telecidin 13 (334) appear in almost every issue of many journals. Equally impressive is the work being done on fundamental molecules of interest in biological systems. There have been an increasing number of studies on intermolecular complexes between purines and pyrimidines (208, 387, 414, 457) such as the complex 9-ethyladenine : 1- methyl - 5 - bromo - uracil (37‘4). These complexes are of interest because the synthesis of all proteins is believed to proceed through the interaction of purine and pyrimidine bases. Some of the amino acid structures reported recently include I-alanine (454), I-valine (424), I-leucine (463), I-proline (375), and I-tryptophan (466). Finally, there are the studies on very large molecules, for which much of the above work is subsidiary. Sobbs, Katson, and Kendrew (412) report further results on their study of myoglobin, and Perutz and coworkers (410, 426, 453) report on their continuing work on hemoglobin. Progress continues to be made on the structure of carbonic anhydrase (304). The structure of carboxypeptidase A (391, 392) reported underway in ou,r last review is now being studied at 2.8 A resolution. The struetureoof ribonuclease has been solved at 3.5 A resolution through the efforts of three independent research groups (201, 872, 493). Other large molecules under study, some of them only just begun, that we can espect to hear more about in the future include a-chymotrypsin (450), double-helical RNA (196-1 98, ferricytochrome C (270),lactic dehydrogenase (436), and triosephosphate isomerase (357). LITERATURE CITED
General (1) Ab, E. A., Andrianova, G. AI., Plotnikov, R. I., Khutsishvili, L. A., GeofLz. Priborostr. (Leningrad: Nedra) Sb.. ?rTo. 22. 81 (19651. (2) Abe, K.,’Tobari, XI., Yukagaku 15 (12), 629 (1967). ( 3 ) Aida, H., Yamamoso, ll., Zairyo 13 (135), 970 (1964). ( 4 ) Alegre, R., Bull. SOC.Franc. Mineral. Crisl. 88 (4), 569 (1965). ( 5 ) Alegre, lt., Debray, L., Bull. SOC.Fr. Ceram. No. 71,53 (1966). ( 6 ) Amer. Soc. for Testing and Materials,
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