Studies of coordinatively unsaturated metal carbonyls in the gas

J. Phys. Chem. 1987, 91, 3945-3953

3945

FEATURE ARTICLE Studies of Coordinatively Unsaturated Metal Carbonyls in the Gas Phase by Transient Infrared Spectroscopy Eric Weitz Department of Chemistry, Northwestern University, Euanston, Illinois 60201 (Received: November 6, 1986; In Final Form: February 17, 1987)

Progress is the investigation of coordinatively unsaturated metal carbonyls in the gas phase via transient spectroscopy is reviewed. The reasons for an emphasis on gas-phase systems with infrared detection is discussed. Recent results involving the spectroscopy, reaction kinetics, and photophysics of coordinatively unsaturated metal carbonyls are presented for a number of systems. Comparisons are made between the results obtained on different systems. Results are summarized in a series of propensity rules and potential areas for future work are delineated.

Introduction In his Nobel laureate address, Professor R. Hoffmann referred to coordinatively unsaturated organometallic compounds as “the building blocks” of organometallic chemistry.’ Coordinatively unsaturated organometallic species are also of great potential industrial importance since many coordinatively unsaturated organometallic species are potent catalysts.2-8 Yet an understanding of their catalytic activity has been hampered by the fact ,that in many systems the actual catalytic species have eluded detection, structural characterization, and the measurement of rate constants for chemical reactions. The purpose of this article is to discuss progress that has been made in the detection of coordinatively unsaturated organometallic species with a special emphasis on coordinatively unsaturated metal carbonyls detected in the gas phase via infrared spectroscopy with its inherent structural sensitivity. Why Infrared Detection. One of the earliest studies of coordinatively unsaturated organometallic species involved the gasphase photolysis of a metal carbonyl, Ni(C0)4 by Callear and Oldman9 Employing visible detection they provided evidence for the generation of Ni(C0)3 in the photolysis of Ni(CO),. In another study involving flash photolysis with visible detection they observed atoms produced in the photolysis of Fe(CO),.’O One of the earliest solution-phase studies of coordinatively unsaturated organometallic species also involved a metal carbonyl, Cr(C0)6. This work by Koerner von Gustorf et al. reported the observation of a broad transient which was assigned to Cr(CO)5.11 Subsequent work by Bonneau and Kelly’* and Peters et a1.I3 have cast some doubt on this assignment indicating it is more likely that the broad absorption is due to a coordinatively unsaturated species, (1) Hoffmann, R. Angew. Chem., Zntl. Ed. Engl. 1982, 21, 7 1 1 . ( 2 ) Poliakoff, M.; Weitz, E. Ado. Organomet. Chem. 1986, 25, 277. (3) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic: New York, 1979. (4) Seder, T.; Ouderkirk, A. J.; Church, S . P. Weitz, E. High Energy Processes in Organometallic Chemistry; American Chemical Society: Washington, DC, 1987; ACS Symp. Ser. No. 333, p 81. (5) Casey, C. P.; Cyr, C. R. J. Am. Chem. SOC.1973, 95, 2248. (6) (a) Whetten, R. G.; Fu, K. J.; Grant, E. R. J. Chem. Phys. 1982, 77, 3769. (b) Whetten, R. G.; Fu, K. J.; Grant, E. R. J. Am. Chem. SOC.1982, 104,4270. (7) Mitchener, J. C.; Wrighton, M. S. J. Am. Chem. SOC.1981, 103, 975. (8) Miller, M. C.; Grant, E. R. SPZE 1984, 458, 154. (9) Callear, A. B.; Oldman, R. J. Nature (London) 1966, (L) 210, 730. (10) Callear, A. B.; Oldman, R. J. Trans. Faraday SOC.1967, 63, 2888. (11) Kelly, J. M.; Bent, D. V.; Hermann, H.; Schulte-Frohlinde, D.; Koerner von Gustorf, E. J. Organomet. Chem. 1974, 69, 259. (12) Bonneau, R.; Kelly, J. M. J. Am. Chem. SOC.102, 1220 (1980). Kelly, J. M.; Long, C.; Bonneau, R. J. Phys. Chem. 1983, 87, 3344. (13) Welch, J. A.; Peters, K. S.; Vaida, V. J. Phys. Chem. 1982, 86, 1941.

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most likely Cr(CO),, coordinated with solvent. More recently, Breckenridge and co-workers, also employing visible detection following laser photolysis, have looked at species produced following the dissociation and reaction of Cr(CO)6.14 Though all of these studies provided valuable information, they also illustrate some of the problems involved in visible detection of coordinatively unsaturated organometallics and in particular metal carbonyls. Typically, the visible or UV absorptions of either gas- or solution-phase organometallic compounds are broad, featureless, and relatively insensitive to the structure of the c ~ m p o u n d .This ~ can be contrasted to matrix studies where the infrared absorptions of matrix-isolated coordinatively unsaturated species are often very narrow with many structure-specific feature^.^^^ Indeed, it is precisely these properties of matrix spectra that have made them so valuable in the determination of structures of matrix-isolated coordinatively unsaturated organometallic compounds. Though infrared absorptions in either room temperature solutions or in the gas phase would not be expected to be as narrow as those in low-temperature matrices, infrared spectra in either of these phases would be expected to contain far more structural information than UV or visible spectra. Nonoverlapping, structurally specific spectra are important in studies of coordinatively unsaturated metal carbonyls in the gas phase, where, as we will see, the generation of multiple coordinatively unsaturated fragments via UV photolysis is the rule rather than the exception. To identify and characterize these fragments, many of which have never been produced in condensed phase and some of which can only be produced with difficulty and low yield in a matrix, it is imperative to be able to interrogate the system with a highly structurally sensitive probe. This points to the need for infrared spectroscopy. However, infrared transient absorption studies have historically been plagued by low sensitivity, slow detectors. Thus it is obvious that to perform infrared spectroscopy of highly reactive species in the gas phase on an appropriate time scale (a gas kinetic collision takes place approximately ten times per microsecond for a molecule at a pressure of 1 Torr) places a significant experimental demands on an envisioned infrared transient spectroscopy apparatus. In the preceding paragraphs, an argument has been made regarding the desirability of obtaining structurally specific spectral information which could be used to identify different coordinatively unsaturated species and to assign their structures. This description immediately evokes the thought of infrared or Raman spectroscopy. Though Raman spectroscopy has great power to obtain (14) Breckenridge, W. H.; Sinai, N. J. Phys. Chem. 1981, 85, 3557. Breckenridge, W. H.; Stewart, G. M. J. Am. Chem. SOC.1986, 108, 364.

0 1987 American Chemical Society

3946 The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 highly structure specific i n f ~ r m a t i o n its ’ ~ application to organometallic species in general and specifically metal carbonyls is fraught with potential problems. These problems arise from the high degree of photolability of the parent compounds and the coordinatively unsaturated photofragments.2 Thus use of Raman spectroscopy could lead to further dissociation of initially produced photofragments and consequently a power- and frequency-dependent mix of photoproducts which could be very difficult to identify. Nevertheless, due to the great potential power of the technique and the fact that Raman spectroscopy has been used to study other photosensitive molecules,*6it would be desirable to attempt studies of coordinatively unsaturated organometallic species using Raman probes. The aforementioned high degree of photolability arises from the broad dissociative absorptions in both the visible and UV that are generally exhibited by organometallic compounds and metal carbonyls in particular.2 In addition, compounds with similar chromophores typically exhibit similar visible and UV spectra. These characteristics make it very difficult to obtain structural information by methods such as laser-induced fluorescence and multiphoton ionization which have been so successful when applied to other types of systems. In fact, it may even be difficult to identify the presence and nature of multiple similar coordinatively unsaturated compounds by using these techniques. Why the Gas Phase. Since most practical chemistry occurs in solution it might seem that the solution phase would be the obvious medium of choice for the study of coordinatively unsaturated species. However, a convincing argument can be made that there are significant advantages to the study of coordinatively unsaturated species in the gas phase. This statement is made with the full awareness that solution phase studies have been very important in the development of transient spectroscopy of organometallic species and with the expectation that they will remain important and continue to contribute valuable information to our understanding of the reactivity and structure of coordinatively unsaturated specie^."-^^-"-'^ Nevertheless, there are problems in solution studies which do not exist in the gas phase. One problem with the solution phase is illustrated by the study of Peters et al. in which they observe coordination of Cr(CO)Swith solvent on a picosecond time scale.I3 Thus, unless experiments are performed in highly uncoordinating solvents, the species being probed on anything longer than a picosecond time scale will be a coordinated molecule rather than the ”naked”, coordinatively unsaturated, species. This can have significant ramifications for both kinetic and spectroscopic studies. In kinetic studies, reactions in solution will, in general, be displacement reactions rather than addition reactions. Thus, these reactions may not probe the rate of reaction of the “naked” species with the added reactant. Coordination of solvent can also affect the structure of the species being observed since the coordinated solvent molecule(s) can distort the structure of the “naked”, coordinatively unsaturated, species. The insidious feature of this effect is that it is not clear how large an effect solvent coordination will have on the structure and it is difficult to be sure that any solvent is totally uncoordinating. Neither of these problems exist in the gas phase where the “naked” species can be studied in the absence of solvent. Thus addition reactions can be cleanly probed and spectra, once determined, will be those of the “naked’! species. Studies in the gas phase have another advantage. Though many solution phase photochemical processes have been well charac(15) See for example: Time Resolved Vibrational Spectra, Atkinson, G. H., Ed.; Academic: New York, 1983. (16) Meyers, A. B.; Mathies, R. A. J . Chem. Phys. 1984, 81, 1552. (17) Moore, B. D.; Simpson, M. B.; Poliakoff, M.; Turner, J. J. J . Chem. SOC.,Chem. Commun. 1984, 972. (18) Hermann, H.; Grevels, F.-W.; Henne, A.; Schaffner, K. J . Phys. Chem. 1982, 86, 5151. Church, S. P.; Grevels, F.-W.; Hermann, H.; Schaffner, K. Inorg. Chem. 1985, 108, 418. Church, S. P.: Hermann, H.; Grevels, F.; Schaffner, K. J . Chem. SOC., Chem. Commun. 1984, 785. (19) Davies, B.; McNeisch, A,; Poliakoff, M.; Turner, J. J. J . Am. Chem. SOC.1977, 99, 7573. Poliakoff, M.: Breedon, N . ; Davies, B.; McNeisch, A,; Turner, J. J . Chem. Phys. Lett. 1978, 56, 474.

Weitz terized,) solution-phase photochemistry of metal carbonyls is dominated by events that result in the cleavage of a single bond. This typically results in a single photoproduct or, as in the case of Mn2(CO),owhere there is overlap of electric states, there can be a wavelength-dependent mix of photoproducts. As first shown by Yardley et al., in chemical trapping studies of the dissociation products produced following photolysis of Fe(CO)5 and Cr(C0)6, gas-phase photochemistry of organometallic compounds can be far richer than solution-phase photochemistry.20s21 Yardley et al. observed the production of more highly coordinatively unsaturated species than are produced in solution-phase photolysis. These compounds resulted from the loss of multiple C O ligands following the absorption of a single UV photon. Obviously, the ability to generate these species in the gas phase opens up many more compounds for study than can be conveniently generated in a liquid-phase environment. In addition, since more highly coordinatively unsaturated species than those produced by the loss of one ligand have been postulated to be the active intermediates in a variety of catalytic the ability to generate and study these highly coordinatively unsaturated compounds could have important ramifications in this area.

Experimental Techniques Detailed descriptions of a number of different apparatuses that have been used to observe and study coordinatively unsaturated metal carbonyls have been previously Thus specific apparatus details will not be discussed in this section. Rather an overview will be presented of the basic features of these transient absorption experiments. In any transient absorption experiment, the apparatus must contain a method for producing the transient, a light source for probing the transient species, and a detector for observing the change in absorption of the probe due to the presence of the transient species. Metal carbonyls typically exhibit broad, strong, absorption bands throughout the ultraviolet region of the spectrum. These absorption bands often continue into the visible region of the spectrum manifesting themselves as the absorptions that lead to these compounds being colored. Excitation into these absorption bands typically leads to a dissociative event involving loss of one or more ligands or, as exemplified by Mn2(CO)lo, possibly metal-metal bond cleavage. As previously stated, Yardley showed that excitation of these bands can (and usually does) lead to loss of more than one ligand.20.21 Thus it is normally possible to generate multiple coordinatively unsaturated metal carbonyls via UV photolysis, with an excimer laser being a convenient and currently the preferred choice for the source of photolysis photons. This source can provide more than enough energy (typically pulses of no more than a few mJ/cm2 are used to avoid possible mul(20) Nathanson, G.; Gitlan, B.; Rosan, A . M.; Yardley, J. T. J . Chem. Phys. 1981, 74, 361. Yardley, J. T.; Gitlan, B.; Nathanson, G.; Rosan, A . M. J . Chem. Phvs. 1981. 74. 370. (21) Tumas, W.; Gitlan, B.; Rosan, A . M.; Yardley, J. T. J . Am. Chem. SOC.1982, 104, 5 5 . (22) Ouderkirk, A.; Weitz, E. J . Chem. Phys. 1983, 79, 1089. (23) Ouderkirk, A.; Wemer, P.; Schultz, N. L.; Weitz, E. J . Am. Chem. SOC.1983, 105, 3354. (24) Ouderkirk, A. J.; Seder, T. A.; Weitz, E. Laser Applications to Industrial Chemistry; SPIE: New York, 1984, Vol. 458, p 148. (25) Seder, T. A.; Ouderkirk, A . J.; Weitz, E. J . Chem. Phys. 1986, 85, 1977. (26) Seder, T. A,; Church, S. P.; Ouderkirk, A. J.; Weitz, E. J . Am. Chem. SOC.1985, 107, 1432. (27) Seder, T. A.; Church, S. P.; Weitz, E. J . Am. Chem. SOC.1986, 108, 4721. (28) Seder, T. A.; Church, S. P.;Weitz, E. J . Am. Chem. SOC.1986, 108, 7518. (29) Ouderkirk, A. J. Ph.D. Thesis, Northwestern University, 1983. (30) (a) Fletcher, T. R.; Rosenfeld, R. N. J . Am. Chem. SOC.1983, 105, 6358. (b) Fletcher, T. R.; Rosenfeld, R. N. J . Am. Chem. SOC.1985, 107, 2203. (c) Fletcher, T. R.; Rosenfeld, R. N. J . Am. Chem. SOC.1986, 108, 1686. (31) Rayner, D. M.; Nazran, A. S.; Drouin, M.; Hackett, P.A. J . Phys. Chem. 1986, 90, 2882. (32) Breckenridge, W. H.; Sinai, N. J . Phys. Chem. 1981, 85, 3557. Breckenridge, W. H.; Stewart, G. M. J . Am. Chem. SOC.1986, 108. 364.

Feature Article tiphoton absorption processes) at a number of wavelengths (35 1, 248, and 193 nm are typically the frequencies used) in a pulse that is short (10-30 ns) compared to the detection time scale. The next question involves detection. The case has already been made for the desirability of detection in the IR. Fortunately, metal carbonyls exhibit some of the strongest infrared absorptions known. E’S of 10000 or more have been reported for metal carbonyls with similar absorption strengths observed for coordinatively unsaturated ~ a r b o n y l s . ~ *These ~ ~ - * absorption ~ strengths are comparable to those normally observed for strongly absorbing electronic transition^.^,^^ Thus even though coordinatively unsaturated metal carbonyls are extremely reactive species, and therefore must be studied at low pressure in order to observe their kinetics in real time, the very strong absorptions they possess mitigate the problem of observing small number densities of these species. Nevertheless, sensitive infrared detectors are needed. Most of the practitioners in the field have chosen to use an InSb detector which is the most sensitive detector available for the region of the mid-IR (-2150-1750 cm-’) that it is necessary to access for most carbonyl absorptions. The next experimental question is that of the probe light source. One of two sources has been used: a CO laser or a glowbar. The C O laser has the advantage that is provides a much larger number of photons/unit bandwidth than the glowbar. The C O laser is also a monochromatic source which obviates the need for a monochrometer and the resulting throughput losses. These features normally result in much greater S / N levels a t the detector than can be obtained with a glowbar. However, the CO laser is only discretely tunable with approximately 4 cm-’ between available vibration-rotation lines. Thus the best that can be done with a C O laser is to construct a time-resolved IR spectrum with approximately 4 cm-’ between adjacent frequency points. In addition, to obtain the greatest versatility in tuning, the CO laser must be liquid nitrogen cooled. Liquid nitrogen cooling is necessary to obtain operation of the C O laser on transitions below u = 4-3. This allows extension of the high-frequency tuning range of the CO laser to approximately 2150 cm-’. However, liquid nitrogen cooling is expensive, complicates laser operation, and presents the possible hazard of an explosion due to ozone build up in the laser tube if the laser is operated improperly. With a glowbar, in principle, a continuous spectrum can be constructed, a significant potential advantage. However the much lower photon flux/unit bandwidth has generally led to poorer signal-to-noise levels for the glowbar source when it is compared with a CO laser under the same conditions. In principle, a diode laser combines the best features of the two aforementioned sources and has sufficient power levels to overcome the signal-to-noise level problems inherent in a glowbar. Diode lasers are being incorporated into some transient absorption setups. Existing apparatus which employ excimer laser photolysis sources, C O probe lasers, and high-quality infrared detectors can achieve detection sensitivities of 1O’O transient species with a time resolution as fast as 30 ns. This is more than adequate to observe even gas kinetic reactions of coordinatively unsaturated species.

The State of the Field A . Photochemistry and Photophysics. Condensed-phase metal carbonyl photochemistry is virtually exclusively dominated by the breaking of a single bond.3 If this bond is a metal-ligand bond, loss of one ligand occurs and a single coordinatively unsaturated product is produced. An example of this is the production of Fe(CO)4 via UV photolysis of Fe(CO)s. In matrices, more highly coordinatively unsaturated species have been produced but this involved photolysis of the initially produced coordinatively unsaturated species to induce successive ligand In the gas phase, Yardley et al. used chemical trapping experiments to show that multiple coordinatively unsaturated species were produced ( 3 3 ) Okabe, H. Photochemistry of Small Molecules; Wiley: New York, 1978. (34) Poliakoff, M. J . Chem. Soc., Dalton Trans. 1974, 210. ( 3 5 ) (a) Poliakoff, M.; Turner, J. J . J. Chem. SOC.,Dalron Trans. 1974, 2276. (b) Poliakoff, M.; Weitz, E. Acc. Chem. Res., to be published.

The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 3947 4

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Figure 1. Portion of the infrared spectrum shown following photolysis of 30 mTorr of Fe(C0)5 in 5 Torr of argon with a KrF excimer laser pulse. Adjacent traces are taken at 3-gs intervals. The largest excursion from the base line for features 2 - 4 is the first trace with that trace taken 3 ws after the excimer laser pulse. Nine traces are shown in addition to

the base line. Also shown is the position of the Fe(CO), absorption in a neon matrix. The tic marks above the abscissa indicate the frequencies of the CO laser lines that were used to construct the traces. Features 2-5 are Fe(C0)2, Fe(C0)3, Fe(C0)4, and Fe(C0)5 absorptions, respectively. Percent change in transmitted intensity refers to CO laser intensity. TABLE I: Infrared Absorptions of Matrix-Isolated and Gas-Phase Cr(CO), Fe(CO),

+

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frequency, cm-l Ar matrix gas phase assignment symmetry ref 1995 2000 AI C2, 25, 35 1988 1973 -2042 1935.6

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“Not observed due to overlap with “hot” CO. bAssigned as either Fe(CO)*,,,see ref 34. ?CH4matrix. “Absorptions tentatively assigned to an excited state of Fe(CO)3. following the absorption of a single UV photon.20 In his experiments, large excesses of PF3 were used to coordinate the coordinatively unsaturated species as illustrated in eq 1. His exFe(CO), (5 - x)PF3 Fe(CO),(PF3)5, (1)

+

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periments also demonstrated that the distribution of coordinatively unsaturated species was wavelength dependent and that the distribution of species shifted to more highly coordinatively unsaturated species as the photolysis energy increased. By then demonstrating that the same trends were also observed with Cr(C0)6 he provided evidence that these were general features of gas-phase photochemistry of metal carbonyls.21 Weitz and co-workers were the first to obtain direct spectroscopic evidence for the production of coordinatively unsaturated metal carbonyls in the gas Their initial efforts also confirmed Yardley’s observations; multiple coordinatively unsaturated species could be produced via single-photon photochemistry and the distribution of coordinatively unsaturated species is wavelength dependent. Figure 1 depicts the time-resolved

3948

The Journal of Physical Chemistry, Vol. 91, No. 15, 1987

Weitz

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Figure 3. Transient absorption spectrum resulting from KrF laser photolysis of a mixture of 30 mTorr of Fe(C0)5 in 5 Torr of argon. The

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of 30 mTorr of Fe(C0)4 with 100 Torr of CO. The figure depicts the spectra (from -2060 to 1970 cm-I) over a 5-ps range which has been segmented into ten equal time intervals. The first three are marked. The arrows indicated the partially resolved A,, B,, and B2bands of Fe(C0)4. (Reproduced with permission from ref 25. Copyright 1986, American Institute of Physics.) spectrum generated by photolysis of 30 mTorr of Fe(CO), with a KrF excimer laser pulse. Yardley et al.3 chemical trapping data provided information on the expected amounts of various Fe(CO), species and matrix data provided information on the positions and number of absorptions for the various Fe(CO), species. This data, some of which is contained in Table I, provided the initial basis for the assignment of specific absorption features to a given Fe(CO), species. Based on this information it was possible to tentatively assign the features indicated as 2, 3, 4, and 5 in Figure 1 to the respective Fe(CO), species. However, Weitz and coworkers recognized that it is undersirable to have to rely on the prior existence of matrix and/or chemical trapping data for assignments of absorptions. Thus, they developed a rather straightforward procedure which can be described as a “kinetic bootstrap” procedure which allows for the determination of the nature of specific absorption bands even in the absence of matrix isolation and/or chemical trapping data.22-27This procedure is illustrated in Figure 2 which shows a portion of the spectral region displayed in Figure 1. In Figure 2 Fe(CO)5 has been photolyzed in the presence of a large excess of CO. On this time scale the added CO has already reacted with lower fragments to regenerate Fe(CO),, which can be observed to further react with C O on the time scale that is depicted to generate Fe(C0)5.22 This type of procedure can be utilized for each of the Fe(CO), species to verify that a species assigned as Fe(CO), reacts with C O to form Fe(CO),+,. For the Fe(CO), system this procedure confirmed the preliminary assignments that had been made based on matrix and chemical trapping data.25 As a result of these assignments, gas-phase absorptions for the various Fe(CO), species that are generated on excimer laser photolysis of Fe(CO)5 were obtained and are also presented in Table I. As will be discussed later in the text, the “kinetic bootstrap” procedure proved invaluable for the case of Cr(CO), photolysis where there was more limited matrix isolation data for the highly coordinatively unsaturated Cr species and for the case of Mn2(CO),ophotolysis where there was no chemical trapping data and again limited matrix isolation data. An interesting question raised by this data is why the distribution of photofragments is so different in condensed phase vs.

spectrum is a result of the average of 128 laser pulses. Each trace on the spectrum is at the indicated time delay from the excimer laser photolysis pulse. Decreasing transmitted CO laser intensity is toward the top of the page. Frequencies are indicated with tic marks below the abscissa. The frequency of the laser lines used to construct the spectrum are indicated with small tic marks above the abscissa. Tic marks at the laser line frequencies are also included on the 0%change in transmission line. These are included as an aid in interpreting the stacked spectral plot and are drawn at the same angle as the angle of displacement of the stacked plot. (Reproduced with permission from ref 22. Copyright 1983 American Institute of Physics). the gas phase. Yardley et al. initially proposed that the dissociation event in the gas phase was an RRKM like event which could result in an internally energized molecule losing successive ligands.20 In support of this general picture is the data shown in Figure 3. Figure 3 depicts the time evolution of Fe(CO), absorptions on a much shorter time scale than shown in Figure 1. On this time scale it is apparent that the absorptions attributed to the various Fe(CO), species “grow in” on different time scales. The species that lose the least ligands “grow in” the slowest. As the absorptions “grow in” they shift toward higher frequency and narrow. This picture is compatible with the production of internally excited Fe(CO), species on photolysis which is what could be expected in an RRKM like picture of the dissociation process. Since the loss of each ligand represents a mechanism by which energy can be carried away from the Fe(CO), fragment, due both to the energy necessary to break a Fe-(CO) bond and the energy carried away by the CO, it would be expected that Fe(CO), fragments which lose the most ligands would retain the least amount of internal energy. Similarly, it would be expected that the amount of energy retained in the initial Fe(CO), fragment (Fe(CO),) would increase with increasing photolysis wavelength and that successive fragments would be formed with increasing amounts of internal energy. This would be expected to lead to ejection of CO with higher levels of internal energy and production of either more highly coordinatively unsaturated fragments or retention of more internal energy in the most highly coordinatively unsaturated species produced. This is indeed what is observed for the Fe(CO), system and the other systems for which similar data are As shown in Figure 4 the amount of “hot” CO produced is seen to significantly increase in going from XeF to KrF photolysis. C O appears as absorptions to the high-frequency side of the Fe(CO), absorptions and also overlaps with the Fe(CO), absorptions. This can result in a distortion in the shape of the Fe(CO), absorptions making the higher frequency Fe(CO), absorption appear smaller than it should be relative to the lower frequency Fe(CO), absorption. In addition to more “hot” CO, the wavelength range over which “hot” C O is seen to absorb also increase as the photolysis energy increases indicating there is an increase in the degree of internal excitation of the CO. This trend has recently been confirmed and quantified by much more detailed experiments involving vacuum UV detection of the internal states

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