J. Phys. Chem. 1994,98, 414-411
414
Electron Affinity of Co(C0)zNO Measured by Negative-Ion Photoelectron Spectroscopy N. J. Turner, A. A. Martel, and I. M. Waller' Department of Chemistry, 2036 Main Mall, University of British Columbia, Vancouver, BC V6T lZ1,Canada Received: August 24, 1993; In Final Form: October 26, 1993'
In this work, we present the fixed-frequency (at 355 nm) negative-ion photoelectron spectrum of Co(C0)2NO-. Analysis of the spectrum yields the electron affinity of Co(C0)2NO; EA[Co(CO)zNO] = 1.73 f 0.03 eV. The lack of extensive vibrational progressions in the spectrum is consistent with the neutral species and the negativeion having similar equilibrium geometries.
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1. Introduction Negative-ion photoelectron spectroscopy is a useful technique for the study of radicals and reactive complexes since it combines the ability to isolate the species of interest with a direct spectroscopic probe. To accomplish this, mass-selection techniques isolate the corresponding negative ion of the species of interest. The output from a fixed-frequency laser induces a transition from the anion to the neutral molecule. This method probes states that are accessible by a one-electron transition from the anion. The photodetached electron carries away the excess energy between the photon energy and the energy deposited in the neutral species. By energy analyzing the kinetic energy of the detached electrons, one can infer information about states in the neutral molecule. Unsaturated transition-metal complexes are intermediates in many catalytic reactions' and often have been viewed as prototypes for molecules adsorbed on surfaces.2 However, these species are very reactive, and thus a proper characterization of the electronic and geometric structure of many of these compounds is lacking. Negative-ion photoelectron spectroscopy of unsaturated transition-metal complexes yields the gas-phase spectra, free from perturbations due to surrounding solvent or matrix molecules. The measurement of electronic spacing, vibrational structure, and electron affinities adds significant information to the characterization of these reactive complexes and provides stringent tests for the theoretical models used to describe these systems. Researchers have used negative-ion photoelectron spectroscopy for studies of metal carbonyl complexes,34 metal hydride metal dihydride complexes? and very recently a metal-cluster hydride complex.10 We add to this body of knowledge by presenting, in this work, the spectrum of a metal nitrosyl complex. In particular, we present the fixed-frequency negative-ion photoelectron spectrum of CO(CO)~NO-.
2. Experimental Section Figure 1 shows a schematic of the experimental apparatus. The design is similar to that described by Neumark and coworkers.11 It consists of a triply differentially pumped time-offlight mass spectrometer and a time-of-flight electron energy spectrometer. The ion source region floats at -1 kV with respect to ground, and a IO-" aperture separates the source from the first differential region. A 5-mm aperture separates the first and second differential regions, and a second 5-mm aperture separates the second differential region from the detector region. Three diffusion pumps (Edwards Diffstak 250, 160, and 100) maintain the operating pressures of 8 X lO-5,7 X 10-5, and 7 X 10-7 Torr in the source and in the first and second differential regions, respectively. A 400 L/s turbomolecular pump (Leybold TMPAbstract published in
Advance ACS Absrracrs, December 15, 1993.
0022-3654/94/2098-0474%04.50/0
\12
Figure 1. Schematic of the fixed-frequency negative-ion photoelectron spectrometer: (1) pulsed-molecular beam valve, (2) electron gun, (3) IO-mm aperture, (4) acceleration plates, (5) beam-modulation plates, ( 6 ) einzel lens, (7) deflectors,(8) temporal focus, (9) 5-mm defining slit, (10) laser inter action region, (11) ion detector, (12) electron detector.
340) maintains the operating pressure in the detector region (4 X le9Torr). 2.1. Ion Source and Mass Spectrometer, The ion source and mass spectrometer are based on the design of Kitsopolous et aL12 Negative ions are generated using the pulsed molecular-beam/ electron gun configuration developed by Johnson et al.13 Dilute mixtures of Co(CO)3NO in helium expand through a pulsed molecular-beam valve (General Valve Corp., Series 9, 0.5" nozzle, 30-Hz repetition rate)14 and intersect a 1-keV, 250-pA electron beam (Tektronix electron gun). The energetic electrons ionize the gas, producing slow secondary electronsthat efficiently produce the anion complex Co(C0)zNO- by dissociative attachment of Co(C0)3NO. The initially produced ions may react with the parent complex to form cobalt cluster complexes. The ions, formed in the continuum flow region of the freejet expansion, cool as the expansion progresses. A similar source has produced SH- with a rotational temperature12 of 50-15 K and IHI- with a 100 K vibrational temperat~re.'~ The negative ions, accelerated to 1keV, are mass-selected using a beam-modulated time-of-flight mass spectrometer based on the design of Bakker.16 The spectrometer consists of a pair of flat plates (6 cm X 6 cm) separated by 1cm and a 5-mm defining slit located 1 m away. The potential on one of the plates is held at V / 2 while the other plate rapidly switches from V to 0 when the ion pulse is halfway along the plates. (Vis typically about 20-50 V, and the fall time is 10 ns.) Only the ions in a small volume midway through the plates when the voltage switches will 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 475
Electron Affinity of Co(CO)*NO CdCO)1NO
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200
300
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Figure2. Mass spectrum of negative ions produced from a Co(CO),NO in helium expansion. pass through the defining slit. The ion bunch in this small volume will separate in time according to their masses along the 1.4-m mass spectrometer. An einzel lens (=-500 V) focuses the ion bunch perpendicular to the ion beam axis, and a second lens system can temporally focus the ions parallel to this axis. The temporal focusing optics consist of two 7.6-cm-diameter plates with 1-cm apertures separated by 1 cm. The second plate is held at ground potential, while the first plate is switched from ground to -100 V (10 ns fall time) when the target ion is midway between the two plates. A pair of chevron-mounted microchannel plate detectors (Galileo MCP-O25N, active area diameter of 25 mm) monitors the negative ions. 2.2. Electron Spectrometer. The mass-selected negative ions cross, at 90°, the 355-nm (3.49-eV) output from a pulsed Nd: YAG laser (Spectra-Physics GCR 4-30 Hz) timed so that only irradiation of the ions of interest occurs. A small fraction of the photodetached electrons separate according to their energy along a second 1-m time-of-flightspectrometer, oriented perpendicular to the direction of both the ion flight and the laser beam. The electrons are detected at the end of a 1-m flight tube using a pair of chevron-mountedmicrochannel plate detectors (Galileo MCP040N, active area diameter of 42 mm). Two layers of 0.030-in. thick Conetic AA magnetic shielding (Magnetic Shield Corp.) surround the flight tube to minimize magnetic fields. The electron signal is preamplified (Ortec 9301), digitized by a 300-MHz discriminator (Philips 6904), and then recorded using a 200MHz transient digitizer (LeCroy TR8828D). The signal is averaged (LeCroy 6010 Magic controller) and periodically transferred to and stored on a 386 computer.
1.2
1.4
1.6
1.8
2.0
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2.4
Electron Kinetic Energy (eV) Figure 3. The 355-nm (3.49-eV) negative-ion photoelectronspectrumof Co(CO)2NO-. The spectrum represents the average of 506 OOO laser shots. mixture of freons wasco-expandedwith Co(CO),NO. This allows calibration of the mass spectrometer with the negative ions, 79Br, 8 1 B r , and 1Z7I-, produced by dissociative attachment of CF3Br and CF31. With this calibration, the two major mass peaks from the Co(CO)3NO sample occurred at 145 and 173 amu, corresponding to Co(C0)zNO- and Co(CO)3NO-, respectively. A similar freon co-expansion with Coz(C0)~(heated to 60 "C) yields a major mass peakat 171 amu corresponding to Co(CO),-. We are thus confident with our mass assignment. 3.2. Electron Spectrometer, Calibration of the electron spectrometer is achieved using the known electron affinities of 0 (1.461 122 f 0.000 003 eV)fl and 0 2 (0.451 i 0.007 eV)22 and the vibrational spacing in 02.23 The calculated relative energies of the photodetached electrons, E,I, from these species are converted to the lab energy, Etab, by applying a small centerof-mass correction factor:
Where me is the mass of an electron, M is the ion mass, and Em is the center-of-mass energy of the ion. The observed electron arrival times, t, are fit to the equation
to obtain the length of the flight tube, I , and the offset, to. With these parameters, the flight times of electrons from photodetachment of a new anion are converted to energy. In addition, it is important to include the Jacobian in the transformation
3. Results and Analysis 3.1. Mass Spectrometer. Figure 2 shows a mass spectrum of the negative ions generated from Co(CO)3NO. The mass spectrometer has a resolution of m / A m = 50 over the whole mass range of the spectrometer. Temporal focusing of one ion improves the resolution to 120-150. Electron impact of Co(CO)3NO in the gas phase has generated Co(C0)2NO- and Co(CO),- as well.17J8 The ultraviolet photolysis of Co(CO)3NO produces both nitrosyl-containing fragments and simple binary cobalt carbonyl complexes.lg In lowtemperature CO matrices, photolysis of Co(CO)3NO has generated C O ( C O ) ~ .It~ is ~ thus imperative that we correctly identify the anions produced under our source conditions. To obtain an internal standard for the mass spectrometer, a dilute
The resolution of the electron spectrometer was determined by observing the photodetachment of I-using 4.66-eV photon energy. The resolution is 83 meV at 1.60-eV electron kinetic energy, and this scales as E312. 3.3. Co(C0)2NCFPhotoelectronSpectra. Figure 3 shows the 355-nm negative ion photoelectron spectrum of Co(CO)zNO-. The spectrum consists of essentially one broad peak at an electron kinetic energy of 1.76 f 0.03 eV with a full-widthhalf-maximum (FWHM) of 187 meV (1 508 cm-1). The predicted instrumental resolution at this electron kinetic energy is 96 meV (774 cm-1). The peak position and width remained constant over a 4-fold change in the ion intensity, indicating that space charge effects
476
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994
Turner et al.
are minimal. In addition, a small intensity peak occurs at an electron kinetic energy of 1.46 f 0.03 eV.
at an electron kinetic energy of 1.46 f 0.03 eV may be evidence of a transition to the u = 1 state of the C-O symmetric stretch. If this assignment is correct, we can set an upper bound of 20% and a lower bound of 5% for the ratio of the intensity of this 4. Discussion transition as compared to the intensity of the origin transition. 4.1. Co(C0)flcand Co(CO)zNOCeometriesandElectronic However, this assignment is very tentative at this point. Structure. The electronic structure of metal nitrosyl complexes The abrupt fall of the signal at high electron kinetic energy has not received the extensive treatment that metal carbonyl indicates that transitions from vibrationally excited states do not complexes have enjoyed. Many of the studies have dealt with contribute significantly to the spectrum. Assuming that the understandingthe linear to bent geometry change in the M-N-O expansion cools all vibrational modes in the anion uniformly to moiety as the NO ligand changes from being ~ 3 to- ~ I - b o u n d . ~ ’ ~ ~ 100 K, modes that have frequencies I160 cm-I will have excited Enemark and Felthamz4suggested a method for electron counting state populations 210%. Comparison with the known frquenin metal nitrosyl complexes. They designated the complex as cies3’ of CO(CO)~NO shows that this would include modes such (MNOP, where n is the sum of the number of d-electrons in the as the symmetric C-Co-C (U6) and C-Co-N (~13)deformations metal plus one for the electron occupying the ?r* orbital of each (80.7 and 85.2 cm-I, respectively) and the C-Co-C (Y14) NO ligand. Co(C0)2NO is a 16-electron complex with a deformation (64.1 cm-I). The total vibrationally excited state configuration designated as (MN0)’O by the preceding method. populations,at 100 K, in these modes would correspond to roughly It is isoelectronic with Ni(C0)s. 31%, 29%, and 39%, respectively. Very little information is available about the geometry of Co4.2. Comparison of Spectrumwith Those for Metal Carbonyls. (C0)2NO. Crichton and Rest30 generated Co(C0)2NO from We can compare our results with the spectra of other metal photolysis of Co(CO)3NO in low-temperature matrices. They carbonyl complexes. Metal carbonyl anions generally display observed the CO AI mode at 2071 cm-I(2064 cm-I) and the CO CO stretching vibrations at lower frequencies than the correBI mode at 1997 cm-I (2064 cm-I) in an argon (methane) matrix. sponding neutral complexes.39The differencein C-O symmetric Interaction with the argon matrix split the NO AI band into a stretch frequencies between negative ions and their corresponding doublet (1785, 1783 cm-l) and into a triplet with the methane neutral complexes ranges from 110 to 124 cm-I for V(C0)c matrix (1785,1776,1774 cm-I). The number of bands and the Cr(C0)5, Mn(CO)S, Fe(C0)4, and Co(C0)4. For Ni(CO)3this intensities were consistent with the Co(C0)2NO having a planar mode is reduced by 158 cm-I in the anion. This large change in geometry. The electronic structure of Co(C0)2NO may be the frequency ( and correspondingly in the force constant) results thought of as a perturbation of the isoelectronic and isostructural in the strong progression in this mode in the Ni(CO)i negativeNi(CO)3 by transferring charge from the nickel to the carbon. ion photoelectron ~pectrum.~The photoelectron spectrum of Thesymmetry changes from D3h in Ni(CO)3” to C , in Co(C0)zNi(C0)3- displayed a progression in the C-O symmetric stretch NO. Note that this structure is consistent with Burdett’s32 mode with the intensity of the transitions to the (01 = O):(ul = prediction of a D3h geometry for dlo M(CO)3 complexes. 1) states in the neutral complex being approximately 2:l. CZ,Co(C0)2NO has 15 fundamental vibrations consisting of In the Cr(CO)3- photoelectron spectrum, the ratio of intensities 6A,+ A2 3B,+ 5B2. Only the frequencies of the three modes of the first two states in the C-O symmetric stretch mode was listed above have been measured for Co(CO)*NO, and there approximately 9: 1.6 Leopold and co-workers’ Franck-Condon have not been any measurements of the frequenciesin the negative analysis indicated that the C-0 bond length changed by 0.015 ion. A between the Cr(CO)3- and the neutral Cr(C0)p. The anion, Co(C0)2NO-, is isoelectronic with Ni(CO)3- and Cu(CO)3. Ni(C0)3- and Cu(CO)3 both have planar D3h Our results are typical of the trends observed in metal carbonyl g e o m e t r i e ~ . ~It~is.very ~ ~ reasonable that Co(C0)2NO- is planar complexes. The photoelectron spectrum of Co(C0)2NO- indiand therefore of C , symmetry. cates that the intensity of transitions in the CO symmetricstretch is in the same range as that observed for other metal carbonyl The Franck-Condon factors between the anion and the neutral complexes. The lack of extensive vibrational progressions in the species govern the intensities in negative-ion photoelectron spectrum suggests that Co(C0)2NO- and Co(C0)2NO have spectra.35 If Co(C0)2NO and Co(C0)zNO- both have similar similar C-0 bond lengths and similar equilibrium geometries. CZ, geometries, then extended vibrational progressions in the negative-ionphotoelectron spectrum are not expected. From the 4.3. Electron Affinity of Co(C0)tNO. In the experiment, a symmetric ground state of the anion, Franck-Condon factors photon induces a transition from the negative ion to the neutral will allow transitions to all of the vibrational levels of the totally species: symmetric (AI) modes but only to those of the even vibrational levels of the antisymmetric modes. Furthermore, for a transition Co(CO),NOhv Co(CO),NO ebetween an anion and a neutral molecule, both of C , symmetry, The electron carries away (as kinetic energy, eKE) the excess the intensity in the overtone transitions of the antisymmetric energy between the photon energy and the energy deposited in modes will be very small with respect to the intensity of the the neutral complex. Energy balance determines the energy in fundamentalband. This will be true even if the two species exhibit the neutral complex, E ~ ~ ( c o ) 2 N by0 1 , a large change in the bond length. Therefore, the antisymmetric modes will not contribute significantly to the spectrum.36 eKE = hu - EA[Co(CO),NO] - E/:O(Co)2N01 + The appearance of the negativaion photoelectron spectrum is ~/ny(CO)d’JO-I consistent with this assignment. The width of the main peak in the spectrum can be explained as arising from transitions to the where, EA[Co(C0)2NO] is the electron affinity of Co(C0)2NO low- and medium-frequency symmetric modes in CO(CO)~NO. and E!wCo’2N@1 mt is the internal energy in the negative ion. In These frequencies will range from 80 to 600 cm-1 on comparison the absence of hot band transitions, the peaks in the spectrum with the known frequencies for C O ( C O ) ~ N O .These ~ ~ modes with the highest electron kinetic energies correspond to states in correspond to the Co-N stretch, the symmetric C-0 bend, the the neutral complex of lowest internal energy. symmetric COX stretch, and the symmetric C-Co-N deform a t i ~ n . ~The * transition to the u = 1 state of the symmetric N-O The spectrum displays maximum intensityat an electron kinetic stretch is expected to occur within the envelope of the main peak energy of 1.76 f 0.03 eV for photodetachment with a 3.49-eV at an electron kinetic energy of about 0.22 eV to the low kinetic photon. As a first approximation, we can assign this peak as energy side of the origin transition. The small peak that occurs corresponding to the ground state to ground state transition, Co-
+
+
-
+
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 477
Electron Affinity of Co(C0)2NO
+
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(C0)2NO eCo(C0)2NO-, taking note that this is not corrected for a shift in the peak maximum due to overtone transitions. This assignment yields EA[Co(C0)2NO] = 1.73 f 0.03 eV. McDonald and Schell observed electron transfer from Co(C0)2NO- to tetracyanoethylene (TCNE, (NC)2C=C(CN)~),suggestingthat EA[Co(C0)2NO] < EA[TCNE] = 3.17 f 0.2 eV." They did not observe electron transfer between Co(C0)zNO-and (CF3)2c==O. Thissuggeststhat EA[CO(CO)~NO] > EA[(CF&C--O]. The EA[(CF&C=O] is not known. However, McDonald and Chowdhury41 observed that electron transfer was a minor channel in the reaction of (CF3)2C=O with the phenylnitrene anion. This suggests that EA[(CF3)2C=O] 1 EA[phenylnitrene]. Drzaic and Brauman42measuredthe EA[phenylnitrene] = 1.46f 0.01 eV by photodetachment in an ion cyclotron resonance spectrometer. These experiments bracket theelectron affinity of CO(CO)~NO:1.46eV IEA[Co(CO)zNO] I3.17 eV. Our results are consistent with these bracketing experimentsand yield a more precise value for the electron affinity of Co(C0)2NO. 4.4. Comparison with Electron Affinities of Other Cobalt Complexes. McElvany and Allison were able to order the electron affinitiesof threecobalt complexes by investigating thegas-phase reactions of the anion complexes with several different reagents.8 They concluded that EA[Cr(CO)3] < EA[Co(C0)2] < EA[Co(CO)(NO)] < EA[Co(CO)3] < EA[Fe(CO)3]. Leopold and -workers6 used negativeion photoelectron spectroscopy (NIPES) to measure EA[Cr(CO)3] = 1.349 0.006 eV. Engelking and Lineberger3measuredEA[Fe(CO)3]= 1.8f 0.2eVusingNIPES. We expect that EA[Co(C0)2NO] > EA[Co(CO)J because of the greater electron affinity of the N O ligand as compared to the CO ligand. Our result of EA[Co(C0)2NO] = 1.73 f 0.03 eV decreases the upper bound for the electron affinities in this series of cobalt complexes. Combining this information yields an order for the electron affinities of 1.35 eV < EA[CO(CO)~]< EA[Co(CO)(NO)] < EA[Co(CO)3] < EA[Co(C0)2NO] = 1.73 eV.
*
5. Conclusiom We have determined the electron affinity of Co(C0)2NO to be 1.73 f 0.03eV by fixed-frequency negative-ion photoelectron spectroscopy. With our instrumental resolution, the electron spectrum consists of one main peak. The width of the peak is modeled by invoking only weakvibrational activity. This spectrum is consistent with Co(C0)2NO and its anion, Co(C0)2NO-, both having similar, planar C, equilibrium geometries. Acknowledgment. This work is supported by funding from NSERC (Canada) and UBC start-up funds. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and theNSERC Center of Excellence in Molecular and Interfacial Dynamics for the partial support of this research. I.M.W. thanks Prof. D. M. Neumark for the data collection program and Prof. T. B. McMahon for many helpful discussions.
References and Notes (1) Wender, 1.; Pino, P. Organic Synthesis via Meral Carbonyls, Vol. I,2; John Wiley and Sons: New York, 1977. (2) Plummer, E. W.; Salancck, W.R.; Miller, J. S . Phys. Reu B 1978, 18,1673. Muetterties,E.L.;Rhodin,T.N.;Band,E.;Bruckcr,C.F.;Retzer, W. R. Chem. Rev. 1979, 79,91. (3) Engelking, P. C.; Lineberger, W. C. J . Am. Chem. Soc. 1979,101, 5569. (4) Stevens, A. E.; Feigerle, C. S.;Lineberger, W. C. J . Am. Chem. Soc. 1982, 104, 5026. (5) Villalta, P. W.; Leopold, D. G. J. Chem. Phys. 1993, 98, 7730. (6) Bengali, A. A.; Casey, S. M.; Cheng, C.-L.; Dick, J. P.; Fenn, P. T.; Villalta, P. W.; Leopold, D. G. J. Am. Chem. Soc. 1992, 114, 5257. (7) Stevens, A. E.; Feigerle, C. S.;Lineberger, W. C. J. Chem. Phys. 1983, 78, 5420. (8) Stevens Miller, A. E.; Feigerle. C. S.;Lineberger, W. C. J . Chem. Phys. 1987, 87, 1549. (9) Miller, A. E. S.;Feigerle, C. S.;Lineberger, W. C. J . Chem. Phys. 1986,84,4127. (10) Casey, S. M.; Leopold, D. G. Chem. Phys. Lett. 1993, 201, 205. (11) Metz, R. B.; Weaver, A.; Bradforth, S. E.; Kitsopoulos, T. N.; Neumark, D. M. J . Phys. Chem. 1990,94, 1377-88. (12) Kitsopolous. T. N.; Wallet, I. M.; Loeser, J. G.; Neumark, D. M. Chem. Phvs. Lett. 1989. 159. 300. (13) Johnson, M. A.; Alexander, M. L.; Lineberger, W.C. Chem. Phys. Phvs. Lett. 1984. 112. 285. 114) The IOTA ONE pulsed valve driver was modified to allow thevalve to float at -1 kV by Dan W. Gibson of General Valve Cow. (15) Waller, I. M.; Kitsopoulos, T. N.; Neumark, D. M. J . Phys. Chem. 1990, 94,2240. (16) Bakker, J. M. B. J . Phys. E 1973.6, 785; 1974, 7, 364. (1 7) Kiser, R. W. In Recent Developments in Mass Spectroscopy; Ogata, K., Hayakawa, T., Eds.;University Park Press: Baltimore, 1970; pp 84+847. (18) McElvany, S. W.; Allison, J. Organometallics 1986, 5, 416. (19) Rayner, D. M.; Nazran, A. S.;Drouin, M.; Hackett, P. A. J. Phys. Chem. 1986, 90, 2882. (20) Crichton, 0.;Poliakoff, M.; Rest, A. J.; Turner, J. J. J. Chem. Soc., Dalton Tram. 1973, 1321. (21) Neumark, D. M.; Lykke, K. R.; Andersen, T.; Linebcrger, W.C. Phys. Rev. A 1985, 32, 1890. (22) Travers, M. J.; Cowles, D. C.; Ellison, G. B. Chem. Phys. k t r . 1989, 164, 449. (23) Herzberg, G. H. Molecular Spectra and Molecular Structure I. Spectra of Diutomic Molecules; Van Nostrand Reinhold: New York, 1950. (24) Enemark, J. H.; Feltham, R. D. J. Am. Chem. Soc. 1974,96,5002. (25) Enemark, J. H.; Feltham, R. D. J . Am. Chem. Soc. 1974,96,5004. (26) Hoffmann, R.; Chen, M. M. L.; Elian, M.; Rossi, A. R.; Mingos, D. M. P. Inorg. Chem. 1974,13, 2666. (27) Pierpont, C. G.; Eisenberg, R J. Am. Chem. Soc. 1971,93,4905. (28) Mingos, D. M. P. Inorg. Chem. 1973,12, 1209. (29) Mingos, D. M. P.; Ibers, J. A. Inorg. Chem. 1971, 10, 1479. (30) Crichton, 0.;Rest, A. J. J . Chem. Soc. Dalton Tram. 1977, 536. (31) DeKock, R. L. Inorg. Chcm. 1971, IO, 1205. (32) Burdett, J. K. J. Chem. Soc., Faraday Trans. 1974, 270, 1599. (33) Breeze, P. A.; Burdett, J. K.; Turner, J. J. Inorg. Chem. 1981, 20, 3369.
(34) Huber, H.; Kundig, E. P.; Moskovits, M.; Ozin, G. A. J. Am. Chem. SOC.1975, 97, 2097.
(35) Sieael. M. W.: Celotta.. R. J.: Hall. J. L.: Levine.. J.:. Bennett. R. A. Phvs. Rev. 2 1972.6. 607. i36) Novick,S. E.;Engelking, P. C.; Jones, P. L.; Futrell, J. H.; Lineberger, W. C. J. Chem. Phys. 1979, 70, 2652. (37) Jones, L. H.; McDowell. R. S.; Swanson, B. I. J. Chem. Phys. 1973, -7R-, -1151 .- . . (38) McDowell, R. S.; Horrocks, W. D., Jr.; Yates, J. T. J. Chem. Phys. 1961, 34, 530. (39) Breeze, P. A.; Burdett, J. K.; Turner, J. J. Inorg. Chem. 1981, 20, 3369. (40) McDonald, R. N.; Schell, P. L. Organometallics 1988, 7, 1806. (41) McDonald, R. N.; Chowdhury, A. K. J. Am. Chem. Soc. 1983,105, 198. (42) Drzaic, P. S.; Brauman, J. I. J . Am. Chem. Soc. 1984, 106, 3443.
