The complex Ni[S2C2(CF3)2]2 reacts with light olefins, including ethylene and propylene, selectively and reversibly. The reaction is not poisoned by hydrogen gas, carbon monoxide, acetylene, or hydrogen sulfide, which are commonly present in olefin streams, presumably because olefin binding occurs through the sulfur ligand rather than the metal center. The reversible reaction of olefins with Ni[S2C2(CN)2]2n (n = 0, -1, -2) can be controlled electrochemically, where the oxidation state-dependent binding and release of olefins are fast on the electrochemical time scale. The observed tolerance to poisons and controllable electrochemical reactivity present an alternative approach to the separation of olefins from complex streams.

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tions is the same. However, off axis (8, > 0), they are substantially different. To make a true omnidirectional dielectric mirror. one wants to have a broad reflection band that survives for all angles and for both polarizations. The p-polarized band was thought to always have a hole or "window" at Brewster's angle, OB, where p-polarized light propagates from an nl to an n2 layer, and from an n2 to an n1 layer without any reflection (3) Once light gets into the multilayer film at OB-regardless of the frequency-it will propagate unimpeded: there is a hole in the reflection band at that angle. It is somewhat ironic that even Joannopoulos himself once implied that it was impossible to make a one-dimensional (ID) omnidirectional reflector in his own book with Meade and Winn (4) Their proof actually showed that there is no omnidirectional photonic band gap in an infinite ID, periodic, layered structure. However, all physical 1D structures are finite, and this offers the way out.The real reflection coefficient R12 at a n, to n7 interface is identicallv zero for Brewster's condition, O I B = arctan n2/nl. Now in the figure, we see that the overall incident angle O0 has a full domain [0, n/2]. However, the corresponding range of the refracted angle O1 is restricted by Snell's law (no sin O0 = n1 sin 01) to the interval [0, Olillax], where 011"" = arcsin nO/nl. If Olmhx < elB, then incident light from the outside can never couple to the Brewster window. That is the trick, and it is easy to do by making no/nl sufficiently large. Now we make sure that 11, and n2 are just right so that we run out of angle Oo before we run out of bandwidth, and voila, we have the

consistent with a formal oxidation state of 4+ as proposed from Mossbauer spectra.1,9,10 The platinum 4f?/2 binding energies for both Pt8Sn2 cluster compounds fall well within the range of 72.4-73.9 eV reported by Riggs8 for 18 Pt compounds of oxidation state 2+. (This range does not overlap those of Pt complexes with oxidation states of 0 and 4+.) Thus, the platinum atoms in these clusters should be regarded as divalent rather than zerovalent if an assignment of oxidation state must be made. The assignment is quite arbitrary; a molecular orbital treatment of the cluster provides a better representation.11The platinum 4f?/2 binding energy of 73.2 eV for the [Pt(SnCl8)o]3~a nion12 likewise indicates an oxidation state of 2+ for the platinum and suggests that there is little accumulation of negative charge on the central metal atom. The similarity of Pt environments in this complex and in the cyclooctadiene complex is consistent with the postulate18 that the SnCl8 ligand is like an olefin in being able to accept electron density from a metal through x-bonds.Acknowledgment.-The author is deeply indebted to Dr. W. M. Riggs for determination of the spectra and for permission to quote his results prior to publication.(9) As pointed out by a reviewer, a correlation between Mossbauer chemical shifts and 4d binding energies has previously been noted.10 It is quite reasonable that a similar correlation with the 3d energies should exist.

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The dihydrogen reactions of nitrogenase are H2 evolution, H2 inhibition of N2 reduction, and HD production from H2/D2O or D2/H2O. The relationships among these dihydrogen reactions are studied to gain insight into the mechanism of N2 reduction. Detailed studies have probed (1) the formation of HD by nitrogenase as a function of partial pressures of N2, D2, and CO, (2) the formation of TOH from T2 under N2-fixing conditions, and (3) the reduction of hydrazine by nitrogenase. Experiments under T2 demonstrate that negligible tritium is incorporated into water compared to the HD produced under similar conditions. Studies of total electron flow, in the presence or absence of D2, establish a requirement of 1 mol of electrons/mol of HD formed. These findings show definitively that HD formation is not due to a simple H2O/D2 exchange mechanism. Kinetic analysis shows that HD is produced by two separate processes. In the minor process, the HD formed is proportional to the H2 evolved, electron requiring, and partially inhibited by 1% CO. In the major process, HD formation is dependent on N2 pressure, electron requiring, and completely inhibited by CO. A mechanism is proposed whereby HD from the N2-dependent process is formed from a bound, reduced dinitrogen intermediate. This mechanism is supported by studies using hydrazine as a substrate for nitrogenase and leads to the conclusion that H2 inhibition of nitrogen fixation and N2-dependent HD formation are manifestations of the same molecular process.

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2956~i m )~, a and Ni(N-isopr0pyl-3-methylsim)~] ; l8 for difference in coordination configuration is ignored, as tetrahedrally coordinated Ni(I1) the bite is 2.84 A in Figure 3, the acac bite distance of 2.584 A obtained [Ni(N-i~opropylsim)2];'~ and for octahedrally coordi-by extrapolation of r / Q to 0 in Figure 3 is in fair agreenated Ni (I1) the bite is 2.91 [Ni(~a1)2.2H~O],~~ 2.91 ment with the salicylic acid bite distance of 2.620 A [Ni(acac)z.2H20],4 and 3.05 A [Ni(acac)~].~~ If the around the hydrogen i o r a s Acknowledgment. This work was supported in (42) Abstract:The physical properties of the trigcis-stilbenedithiolato) complexes of Mo, W, and Re are described in detail. The three neutral M(SZC2Ph2)3 complexes exhibit MSe trigonal-prismatic coordination in the solid state and in solution. The V and Cr complexes in this series are isomorphous with the Mo complex and thus very probably are also trigonal prismatic in the solid state. The syntheses and properties of the tris(toluene-3,4dithiolato) and tris(benzene-l,2-dithiolato) complexes of Mo, W, and Re are reported. In particular, those properties which have been found to be characteristic of trigonal-prismatic coordination are discussed in detail. It is concluded that at least 12 different well-authenticated complexes, spanning five metals and four different ligands, exhibit a six-coordinate trigonal-prismatic structure. In addition, the existence of numerous other complexes, related to the above by reversible one-electron transfers, is established by polarography. The electronic spectra of all the neutral complexes are discussed in terms of a molecular orbital model of energy levels. The molecular orbital bonding scheme is found to be consistent with the considerable electronic spectral and polarographic data. ix-coordinate complexes of bidentate, unsaturated S sulfur-donor ligands have been the subject of extensive current investigations. 2-7 These studies have recently assumed a new and quite fundamental significance in transition metal chemistry. The structure of Re(SzC2Ph& (I) has been solved by X-ray diffraction analysiss and has been found to contain a near-perfect trigonal-prismatic array of sulfur-donor atoms. that SGPhZ forms bis planar complexes with Os, Ru, Cr, Mo, W, Re, and V. This has since been shown to be in error as these are all tris complexes. See ref 5, 6, and G. N. Schrauzer, H. W. Finck, and V. Mayweg, Angew. Chem., 76, 715 (1964). (b) The Mo(tdt)8 complex has apparently been prepared previously by T. W. Gilbert, Jr., and E. B. Sandell, J. A m . Chem. Soc., 82, 1087 (1960). However, an accurate analysis was not obtained and the compound was thus not fully characterized. (8) R. Eisenberg and J. A. Ibers, ibid., 87, 3776 (1965); Inorg. Chem., 5, 411 (1966). 91 109.(9) This is the first reported trigonal-prismatic molecular complex. Trigonal-prismatic coordination was first reported [R. G. Dickinson and L. Pauling, J. Am. Chem. Soc., 45, 1466 (1923)J for the infinitely extended lattices of molybdenite (MoSZ) and tungstite (WSz). ...