RESEARCH
Phosphines form pi bonds with tungsten Nuclear spin-spin coupling measurements give new evidence of phosphine's ability to accept pi electrons Phosphine ligands can form pi bonds as well as sigma bonds with tungsten in tungsten carbonyl complexes. The relative ability of the phosphine to ac cept pi electrons from the metal can be correlated to the magnitude of the phosphorus-tungsten nuclear spin-spin coupling. Dr. Samuel O. Grim, Dr. David A. Wheatland, and Dr. William McFarlane conclude this from studies they performed on monophosphine pentacarbonyl tungsten complexes at the University of Maryland [/. Am. Chem. Soc, 89, 5573 (1967)]. Much previous work in the field of metal carbonyl chemistry has centered on correlating the carbonyl stretching frequency with the mechanism of bonding between phosphorus and the central metal atom. Dr. Grim points out, however, that the spin-spin cou pling is directly related to the phos phorus-tungsten interaction, and hence should be more sensitive to this inter action than is the carbonyl stretch. The Maryland chemists examined a series of eight octahedral complexes of the type (R K Ph 3 _„P)W(CO) 5 , where R is alkyl, Ph is phenyl, and η
is 0, 1, 2, or 3. They found a linear re lation between the ΐ83\γ_3ΐρ S p m . spin coupling constant and the car bonyl stretching frequency for the se ries of compounds. Both properties decrease in numerical value as alky Is are substituted for phenyls in the terti ary phosphine. Homologous complexes of chromium and molybdenum were also prepared, but spin-spin coupling measurements were not possible in these cases, since neither species has natural isotopes of nuclear spin 1 / 2 . In these complexes, phosphorus, tungsten, carbon, and oxygen lie on a line perpendicular to a plane contain ing tungsten and the remaining four carbonyls. Six sigma bonds, involving d 2 sp 3 hybridization of the tungsten orbitals, form between tungsten and the six ligands. Each of these six sigma bonds can be accounted for by donation of electron pairs from the ligands. This leaves six valence elec trons on tungsten, with three remain ing d orbitals to accommodate them. Two of these tungsten d orbitals over lap with two empty d orbitals on phos phorus and with two pi* (antibond-
ing) orbitals on the carbonyl. Any pi bonding between tungsten and the lig ands will involve these orbitals and the six electrons. If any of these tungsten electrons go into pi* orbitals of the carbonyl, this will weaken the carbon-oxygen bond of the carbonyl, and a decrease in the carbonyl stretching frequency will re sult. If tungsten electrons go into the empty d orbitals on the phosphorus, this would give pi character to the tungsten-phosphorus bond, strengthen it, bring the nuclei closer together, and increase their nuclear spin-spin inter action. Experimentally, both the carbonyl stretching frequency and the 183 W— 31 P spin-spin coupling constant are greatest for the triphenylphosphine, decreasing monotonically with alkyl substitution to a minimum for the tributylphosphine. Thus Dr. Grim and his colleagues conclude that pielectron acceptance by the phosphine is a factor in the bonding of these com plexes. An explanation which accounts for the observed carbonyl stretches is that
How do tungsten and phosphorus bond in (tert-phosphine)W(CO)s? • Carbonyl stretches alone indicate phosphine's sigma inductor ability © But trend in spin-spin coupling constants reflects its pi-acceptor ability
(0*Η 5 ) 3 Ρ
§1940 or
s (CHiMC^P (C*H5) (C4H9)2P
1935 (C 4 H 9 ),P
ARGUES. Maryland's Dr. S. O. Grim argues for pi bonding in tungsten penta carbonyl monophosphines
40 C&EN OCT. 30, 1967
1930 200
220
240
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mW-"P nuclear spin-spin coupling
280 (c.p.s.)
300
the phosphine ligand donates sigma electrons to tungsten, which passes them on to the pi* orbitals of the carbonyl. Phosphines with strong inductive ability, such as tributylphosphine, would thus weaken the carbonoxygen bond of the carbonyl. This interpretation, advanced by Dr. Robert Angelici of Iowa State and Dr. M. Bigorgne and Dr. R. Poilblanc of Ecole Nationale Supérieure de Chimie de Paris (ENSCP), is compatible with pi bonding if both effects act synergically, Dr. Grim says. In any case, the size of the i83\y_3ip spin-spin coupling constant appears to be a measure of the ability of the phosphine to accept pi electrons, irrespective of the coupling mechanism, he adds.
Ceramics used to store computer information Research workers at Sandia Laboratory, Albuquerque, N.M., have discovered that certain ferroelectric ceramics have unique properties which permit a new approach to information storage and retrieval, Cecil E. Land said at the 13th annual International Electron Devices meeting, in Washington, D.C. The meeting was sponsored by the Institute of Electrical and Electronic Engineers. Others at Sandia who worked on the ceramics were Ira McKinney and Dr. Gene H. Haertling (supervisor of the firm's active ceramics division and developer of the material ). These ceramics make it possible to produce reliable, high-density computer optical memories that have characteristics superior in many cases to those of conventional memories, Mr. Land says. Namely, they can store more information in a smaller area and in more states than most conventional memories. Sandia's memory elements are thin, polished plates of hot-pressed polycrystalline materials. They are produced by heating and pressurizing a mixture of finely crushed compounds (lead zirconate and lead titanate) for precise periods. When voltage is applied to a ferroelectric plate, internal changes occur which affect either the transparency or the nature of the light passing through the plate. In other words, the voltage aligns (in the direction of the electric field) the electrical charges in the ceramic molecules. Even when the voltage is removed, the charges in the ceramic plate remain aligned until switched back to the original state or some .other state. Individual crystals retain their polarization indefinitely. Electro-optical effects on ferroelectric crystals were first observed some
years ago. However, it was not until the Sandia work that a ceramic memory was developed which would retain its polarity indefinitely, Mr. Land notes. He described two kinds of memories : • A multistate memory—fine-grained materials of less than 2 microns nominal grain size depolarize transmitted light by 5 to 20%, but otherwise behave the same as biréfringent crystals. • A binary memory—coarse-grained materials of grain size greater than 2 microns almost completely depolarize transmitted light by scattering. For example, when a small area of a ceramic plate (a binary bit location) is poled at a right angle to the major surface of the ceramic plate, light striking the surface at the same angle is scattered in a narrow central beam. The area then appears transparent. When the same area is switched parallel to the major surface, light transmitted in the central beam is greatly reduced and the area appears opaque. This gives the element two s t a t e s transparent and opaque, or binary "0" and binary " 1 . " Mr. Land and his coworkers have constructed a prototype with a storage density of 5120 bits per square inch— a density of about five times that of conventional memory units. It seems theoretically possible to store a million bits per square inch by employing more sophisticated write-and-read techniques, Mr. Land says. The ceramics also are adaptable for other uses: information display screens and light modulators. Perhaps in the future, because they have high optical resolution, fine-grained ceramics could be adapted for television screens, Mr. Land feels. The polycrystalline ceramics and devices developed at Sandia are part of the laboratory's research and development work for the U.S. Atomic Energy Commission.
Sandia's Land Superior and adaptable
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