Heat shrinks neW heteropolytungstates reversibly Carbonyl group reflects double-bond character of metal-metal bond
0
C
\ /
Nontransition metal inserts itself between two transition metal atoms
Co2(CO)8 + SnCl 2
? Co(co)4 Sn
a
/
Co(Co)4
Patmore, J. A. J. Thompson, and W. }etz at Alberta have studied mixedmetal bonds between main-group and transition-group metals. Infrared spectra of compounds of the type R 3 M - M ' (CO),, (where M is germanium, tin, or lead, and M' is manganese or rhenium) have been useful in this study, Dr. Graham says. Using a technique developed by Dr. Cotton, he calculates carbonyl stretching force constants from infrared carbonyl stretching frequencies. The nontransition metal M and the carbonyl group trans to it undergo piinteractions with the same partially filled d-orbitals of the transition metal M'. In valence bond terms, the two extreme contributing forms are M— M ' = C = 0 and M = M ' - C = 0 . The more the second form contributes, the greater should be the carbonyl stretching force constant. Values for the constant are high for the trans carbonyl group in compounds such as C l 3 S n - M n ( C O ) 5 . The implied piinteraction probably involves overlap of occupied 3c/-orbitals of manganese with vacant 5c/-orbitals of tin. Nuclear magnetic resonance spectra also provide information on the nature of the M—3VT bond. In methyltin derivatives, tin-proton coupling constants are smaller than they are in tetramethyltin. This means, Dr. Graham believes, that the hybrid tin orbitals bonding to the transition metal have more s-character than do the tin orbitals bonding to methyl groups. 44 C&EN MAY 30, 1966
Crystals of salts of a new structural class of heteropolytungstates shrink reversibly and continuously when heated or dried [/. Am. Chem. Soc, 88, 2329 (1966)]. Chemists at Georgetown University, Washington, D.C., and Boston (Mass.) University believe that this crystal shrinking results from a "reversible thermal disorganization" of hydrogen bonds formed by water molecules of hydration (and sometimes by ammonium ions). These molecules lie in channels and octahedral pockets and separate the large heteropoly anions in the crystal. The hydrogen-bond disorganization would allow the polyanions to move continuously closer together as the crystal is heated. Earlier work by the GeorgetownBoston group indicated that many heteropoly anions, unlike other types of oxy anions, would form only very weak hydrogen bonds with surrounding water molecules. Georgetown's Dr. Louis C. W. Baker says that the weakness of the attachments between water molecules and the large complexes results from the way the exterior oxygen atoms of the heteropoly complexes are tightly bonded to the tungsten (VI) atoms. He and his coworkers believe this is an essential factor in making the shrinking effect possible. Heteropoly electrolytes make up a large fundamental class of compounds. But relatively few studies of these compounds have been made using modern techniques such as x-ray diffraction. A heteropoly electrolyte is the free acid or salt of an anion that contains oxygen atoms and at least two different kinds of atoms in positive oxidation
states. One example is ammonium 12-molybdophosphate (NH 4 ) 3 [ P 0 4 Mo 1 2 0 3 6 ] , aq., which is used in volumetric phosphate and molybdenum determinations. Because of their large surface area, heteropoly electrolytes are used as industrial catalysts. For example, Standard Oil Co. (Ohio) has used bismuth, tin, or antimony salts of molybdophosphoric acid in a fluidized catalyst bed in its process for making acrylonitrile from propylene, ammonia, and oxygen. Compounds belonging to the new structural category all contain large heteropoly anions (more than 10 A. in diameter). The anion structure is a modification of the "Keggin" structure, Dr. Baker explains. In 1934, Dr. J. F. Keggin of England's University of Manchester proposed the correct structure for the 12-tungstophosphate anion
[PO4W12O3GP-.
In the new anions, an octahedrally coordinated metal ion (such as Co 2 + , Co;^ + , or Ga H +) replaces one of the 12 octahedral tungsten atoms of the conventional Keggin structure. An ion such as Si4 + , Co 2 + , or Co H + occupies the Keggin unit's central tetrahedral cavity. In some anions, H2L>+ occupies the cavity. The number of hydrogen atoms in the anion which do not have this function seems to depend on the metal replacing tungsten. Apparently, Dr. Baker says, these hydrogens are firmly linked to exterior oxygen atoms of the complex, probably to those surrounding the metal. Eight chemists contributed to studies on the new heteropoly electrolytes. In addition to Dr. Baker, the group includes his wife, Dr. Violet Simmons Baker, Dr. Klaas Eriks of Boston Uni-
SHRINKAGE. Dr. Violet S. Baker, Dr. M. T. Pope, Dr. L. C. W. Baker, and Dr. O. W. Rollins (left to right) and their coworkers believe that the reversible shrinkage in the new heteropolytu ngstate crystals stems from disruption of hydrogen bonds formed by water of hydration in the crystal cavities
versity, Georgetown's Dr. Michael T. Pope, Dr. Muraji Shibata (now at Ibaragi University, Mito City, Japan), Dr. Orville W. Rollins (at the U.S. Naval Academy, Annapolis, Md. ), Dr. Jen H. Fang (now at Southern Illinois University in Carbondale), and Dr. Lip L. Koh (now at Nanyang Univer sity, Singapore). The Georgetown-Boston chemists have analyzed more than 15 of these heteropolytungstate salts. So far, they have identified five distinct anions in which a tungsten atom is replaced by Co 2 + , Co 3 + , or Ga 3 + . They have preliminary evidence for several other anions. Reversible shrinking. After the salts are heated or dried, x-ray diffrac tion studies show that a cubic unit cell (containing eight anions) has a mark edly shorter edge than it has at low temperature (15° C.) and high hu midity. An example of a compound showing this shrinking behavior is (NH4)7Na2[GaOeH204W1103o]·15H 2 0. An air-dried sample of this compound (first prepared by Dr. Rol lins at the U.S. Naval Academy) has a unit cell edge that shrinks rapidly, reversibly, and continuously with heat ing (without loss of water). At 10° C., the unit cell edge is 22.17 A. At 47° C., it decreases to 21.84 Α., cor responding to a 4.6% volume shrink age. When cooled, the cell edge im mediately returns, reproducibly, to a given value for each temperature, the Georgetown-Boston chemists find. The reversible, continuous shrinking of the heteropoly electrolyte crystal with heating bears a rough analogy to the much less dramatic shrinkage of liquid water when heated from 0° to 4° C. There is another possible explana tion for the crystal shrinkage, Dr. Baker says. The water molecules may move reversibly from the channels be tween the complexes into vacancies in the large octahedral pockets (in the case of a partly dried crystal). Dr. Baker's group has a more thorough study of the shrinkage phenomenon under way.
Magnetic.
Some of the heteropoly
anions show unusual magnetic proper ties. For example, the anion in which Co 2 + is the octahedral replacement and in which Co 3 + occupies the cen tral tetrahedral cavity has a paramag netic susceptibility that drops only 13% when the temperature rises from - 1 4 3 ° to + 2 3 ° C. The GeorgetownBoston group knows no parallel for this marked departure from the CurieWeiss Law, which relates the paramag netic susceptibility to the absolute temperature. They say that the de parture can be explained by strong intraionic paramagnetic interactions be tween the Co 2 + and Co 3 + .
Time-of-flight technique detects astatine compounds Scientists at Argonne National Labora tory have carried out the first direct study of the chemistry of astatine [Inorg. Chem., 5, 766 (1966)]. Us ing a time-of-flight mass spectrometer, Dr. Ε. Η. Appleman, Ε. Ν. Sloth, and Dr. Μ. Η. Studier have confirmed that this highly radioactive halogen behaves chemically very much like other halo gens, particularly iodine. For exam ple, astatine forms interhalogen com pounds (Atl, AtBr, and AtCl). How ever, the Argonne scientists did not detect any diatomic astatine molecules. More significant than the chemical studies of astatine, perhaps, is the method used. This is the first time that such a spectrometer has been used in studying the chemistry of a rare, synthetic element. In the Argonne work, volatile astatine compounds were passed into the time-of-flight spec trometer (Bendix Model 12). Here the compounds are ionized and accel erated down a 100-cm. flight tube to measure their speeds and thus their masses. The heavier the ion, the longer it takes to get down the tube. All earlier work on astatine, and much of the work on the chemistry of other such radioactive elements, has been done by indirect tracer methods. These methods are useful, but they cannot unambiguously identify chemi cal species. However, the Argonne approach gives masses, which identify species with certainty. The Argonne group prepares asta tine by bombarding a bismuth target with 29-m.e.v. alpha particles from the laboratory's 60-inch cyclotron. An eight-hour bombardment produces 0.05 microgram of astatine-211, which has a half-life of about seven hours. After bombardment, the astatine is distilled from the bismuth at 800° C. and collected on a water-cooled plati num plate. This plate is placed in an all-glass apparatus directly attached to the spectrometer. Introducing other halogens into the apparatus with the astatine causes Atl, AtBr, and AtCl to form. The Argonne scientists also detected small amounts of hydrogen astatide (HAt) and methyl astatide (CH 3 At). These result from the reaction of asta tine with traces of organic impurities in the spectrometer, the Argonne sci entists say. Iodine behaves in much the same way. The spectrometer used in the Ar gonne work is a modification that Dr. Studier designed several years ago. The modified instrument produces ions continuously instead of in pulses. This increases sensitivity ultimately to about 40 atoms per cc.
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