J. Phys. Chem. 1987, 91, 2037-2045
2037
and it corresponds well with the renumbered B(v=37) and B(v=38) levels predicted by S t ~ a l l e y . ~ Analysis of transition line shapes of double-resonance spectra provides sensitive tests of the accuracy of the weakly bound excited-state wave function. With our present intensities, however, many bands in the ionization spectrum exhibit ac Stark splitting, which can also be used to extract molecular and photophysical parameters in the REMPI process.I6
In summary, we observe distinct differences, many of which are not yet reconciled by theory, in spectra originating from the ground state vs. the inner- and outer-well E,F states that are resonant with the same rovibrational levels at the second dissociation limit. Clearly, double resonance of H2 prepared in unstudied Franck-Condon regions can provide a wealth of spectroscopic and photodynamic data to challenge and expand our understanding of this most fundamental molecule.
(16) Quesada, M. A.; Parker, D. H.; Lau, A.; Chandler, D. W., to be submitted for publication.
Acknowledgment. This work was supported by the U S . Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences.
FEATURE ARTICLE State-Specific Reactions of Atomic Transition-Metal Ions with H,, HD, and D,: of d Orbitals on Chemistry
Effects
J. L. Elkind and P. B. Armentrout*+ Chemistry Department, University of California. Berkeley, California 94720 (Received: October 14, 1986)
Guided ion beam mass spectrometry has been used to examine the kinetic energy dependence of reactions of atomic ions of the entire first-row transition-metal series with molecular hydrogen. By varying the conditions under which the ions are formed, results for specific electronic states can be obtained. The periodic trends in the chemistry and derived thermodynamic information are summarized. The results can be understood by considering the spin and orbital occupation of the atomic ions.
Introduction One of the more challenging aspects of the study of transition-metal chemistry concerns the plethora of electronic states. Because of the availability of d orbitals which lie close in energy to the valence s orbital, there are a multitude of closely spaced electronic levels. This of course is one of the underlying reasons for the versatile reactivity of transition-metal species. Increasingly sophisticated physical chemistry methods are being brought to bear on the characterization of the chemistry of transition-metal atoms, ions, complexes, and clusters. One particularly active area of research concerns the reactions of transition-metal ions in the gas phase. In this report, we summarize our recent results concerning the differences in reactivity exhibited by different electronic states of atomic ions of the first-row transition metals. The reaction system which we have documented most carefully is reaction 1 and its isotopic analogues.'" This is an ideal forum M+ + H2
-+
MH+
+H
TABLE I: Bond Dissociation Enerpies and Promotion Enereies Doo(M+-H), eV M state' exDtlb theory" EmsceV Sc 2A 2.40 (O.lO)d 2.39, 2.29' 2.36k 0.15 0.28 2.34 Ti 3@ 2.39 V 4A 2.05 (0.06)' 1.89 0.7 1 1.99 1.05, 0.98m Cr ?Z+ 1.36 (0.ll)g Mn 6Z+ 2.06 (0.15)h 1.72, 1.77," 1.62O 0.59 2.04 0.49 Fe 2.12 (0.06)' Co 4@ 1.98 (0.06)' 1.89 0.82 1.55 1.38 Ni 3A 1.68 (0.08y Cu *Z+ 0.92 (0.13y 0.91 3.03 'Reference 18 unless otherwise noted. *Uncertaintiesin parentheses. CPromotionenergy from ref 3. dReference 21. eReference 3. fReference 1. XReference 6. Reference 2. 'Reference 4. 'Reference 5. Reference 19c. ' Reference 19d. Reference 19b. " Reference 19a. OReference 19e.
(1)
to investigate the detailed nature of electronic effects in metal ion reactions for several reasons. First, it is simple. Reaction 1 is the only low-energy process available to the system. Second, it is endothermic. The H2 bond energy of 4.5 eV is considerably stronger than the metal-hydride ion bond energies which vary from 1 to 2.5 eV, Table I. Because of this, the presence of electronically excited metal ions may be observed as a shift in the NSF Presidential Young Investigator 1984-1989; Alfred P.Slcan Fellow.
0022-3654/87/2091-2037$01.50/0
onset for reaction 1. This provides an experimental tool for evaluating the effects of electronic excitation. Third, isotopes are available. Examination of reactions 2-4 provides increased in(1) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1985,89,5626-5636. (2) Elkind, J. L.; Armentrout, P. B. J. Chem. Phys. 1986,84, 4862-4871. ( 3 ) Elkind, J. L.; Armentrout, P. B. Inorg. Chem. 1986,25, 1078-1080. (4) Elkind, J. L.; Armentrout, P. B. J. Am. Chem. SOC.1986, 108, 2165-2761. J . Phys. Chem. 1986, 90, 5736-5145. (5) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1986,90,6576-6586. (6) Elkind, J. L.;Armentrout, P. B. J . Chem. Phys., in press.
0 1987 American Chemical Society
2038
Elkind and Armentrout
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987
M+
+
MH+
+
D
(2)
MD+
+
H
(3)
HD
+
TABLE II: Electronic States of First-Row Transition-Metal Ions ion state confign energp SI popb sc+
'D ID
TiC
4F 4F 2F 2D 2G 5D SF SF
+
M+ D2 MD+ D (4) formation regarding the dynamics of the reaction with hydrogen. Fourth, the diatomic metal hydrides are accessible to high level ab initio calculations and thus are an ideal interface for experiment and theory. Fifth, the activation of molecular hydrogen by transition metals is important to understanding a variety of homogeneous and heterogeneous catalytic processes. Reaction 1 is a model system for understanding details of the electronic interactions of this activation step. By analogy, reaction 1 is also a model system for activation of any covalent hydrocarbon single bond, Le., C-H and C-C bonds. Experimentally, the elucidation of the state-specific chemistry of transition-metal ions is difficult. The close spacing of the states makes simple ion generation schemes unworkable. Also the ground and all low-lying states have only s and d electrons such that transitions between these states are parity forbidden. Therefore the lifetimes of the excited states are expected to be on the order of seconds although no experimental information is yet a ~ a i l a b l e . ~ This precludes optical pumping schemes as efficient modes of state preparation. One method which shows promise is the use of resonantly enhanced multiphoton ionization. This is under development in our labss and by Wei~shaar.~In our work to date,'" we have used a combination of ion sources to produce ions with varying populations of electronic states. While the true distribution of these states is not always known unambiguously, the reactivity of various states can be assessed fairly reliably. -+
Experimental Section The details of the experimental procedures are described elsewhere.I0 Briefly, ions are formed in one of three types of sources: an electron impact ionization source (EI), a surface ionization source (SI), or an E1 source combined with a highpressure drift cell (DC). In the first of these (EI), ions are formed by high-energy electron impact on a volatile organometallic compound. This source is the most intense, supplying ion fluxes after mass analysis of over lo7 s-l, except near the appearance potential (AP) of the atomic metal ion. We use count rates somewhat smaller than this so that ion-counting techniques can be used to monitor the reactant beam intensity (see below). Variation of the electron energy (E,) above the A P allows rough control over the extent of electronic excitation. Generally, we have found that the extent of excitation does not appear to vary substantially once the E, is 10-20 eV above the AP. Also, the states which are formed most abundantly often appear to be the lowest energy state having a particular spin. The second source (SI) is a more controlled means of producing atomic metal ions. Here, the vapor of an organometallic compound or a metal salt is exposed to a rhenium filament heated to ~ 2 2 0 0K. Decomposition occurs and atoms with low ionization potentials desorb from the filament surface. Ion intensities are now in the 105-107-s-'range. It is generally assumed that the ionization process is statistical such that the desorbing ions have reached equilibrium at the filament temperature. Since the energy available is on the order of kT (=0.2 eV at 2200 K), the only states which are populated appreciably are low-lying electronic levels, Table 11. Unfortunately no completely unambiguous measure of the state populations is available; however, this statistical assumption has been tested indirectly by determining the dependence of certain reactions on the filament temperature" and by comparing the experimental cross section magnitudes for specific ground and excited states ~
~~
~
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~~
~~
~~
~~
~
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(7) Garstang, R. H. Mom Not. R. Astron. Sor. 1962, 124, 321. Private
communication.
(8) Georgiadis, R.; Armentrout, P. B., work in progress. (9) Sanders, L.; Sappy, A. D.; Weisshaar, J. C. J. Chem. Phys. 1986,85, 6952-6963. Sanders, L.; Weisshaar, J. C., work in progress. (10) Ervin, K.; Armentrout, P. B. J . Chem. Phys. 1985, 83, 166-189. (11) Aristov, N.; Armentrout, P. B. J . Am. Chem. Sor. 1986, 208, 1806-1 819.
'F
V+
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Mnt
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SD Fe'
co+
6D 4F 4D SF SF
SF Ni+
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4s3d 4s3d 3d2 4s3d2 3d' 4s3d2 4s3d2 3d' 3d4 4s3d3 4s3d' 3d4 3dS 4s3d4 4s3d4 3dS 4s3d5 4s3dS 3d6 4s3d6 3d' 4s3d6 3d8 4s3d7 4s3d' 3d9 4s3d8 4s3d8 3dI0 4s3d9
0.01 0.32 0.61 0.03 0.13 0.59 1.os 1.12 0.03 0.36 1.10 1.45 0.00 1.52 2.46 2.54 0.00 1.17 1.81 0.05 0.30 1.03 0.09 0.52 1.30 0.08 1.16 1.76 0.00 2.81
0.886 0.060 0.054 0.626 0.356 0.016 > B or Thus, the energy available for chemical change in the pairwise frame is always less than in the C M frame. This means that for E(pair) to exceed the thermodynamic threshold Eo, E(CM) must exceed = 2E0 in reaction with H2or D2; =3Eo for reaction 2; and =1.5Eo for reaction 3. This pairwise scheme explains the shifts in threshold observed for the H2, HD, and D2 systems. It also accounts for the enhanced production of MD' since this product can be formed at much lower energies than MH'. Experimentally, we find that some reactions approach the pairwise behavior limit closely2 while other exhibit behavior similar to the model but are not in quantitative agreement with its predictions.4s6 This latter result indicates that the potential energy surface is largely repulsive but that nonimpulsive interactions occur in some collision geometries.
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Results We have published detailed reports'" of reactions 1-4 for all of the first-row transition-metal ions but scandium and titanium (these are forthcoming).20 These results are summarized in the following sections. In all cases, the behaviors of reactions 1 and 4 are very similar and may be viewed interchangeably. Also, the results are qualitatively consistent with the studies of Beauchamp and co-workers who have examined primarily ground-state chemistry.22 (20) Elkind, J. L.; Armentrout, P. B., work in progress. (21) Sunderlin, L.; Aristov, N.; Armentrout, P. B. J . Am. Chem. SOC. 1987, 109, 78-89.
Elkind and Armentrout
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ao
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so ao 7.0 80
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iao
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"Y (PV, Figure 1. Cross sections for reaction of Sc', Ti', and V+ with D, as a function of relative kinetic energy. In all cases, the ions are formed by surface ionization. The arrow indicates the D, bond energy.
Scandium. Results20*22d for the reaction of Dz and Sc+ produced by surface ionization (SI) are shown in Figure 1. As can be seen in Table 11, this beam comprises largely ground-state Sc+(,D) with small contributions from Sc+('D) and Sc+(,F). The cross section rises from an apparent threshold of ~ 1 . eV 5 up to a peak between 4 and 5 eV before declining. As discussed above, this high-energy behavior is quite common and results from reaction 9. In reaction with HD, Sc+(SI) reacts to form primarily ScH+ by a factor of =2 over ScD'. This isotopic behavior is intermediate between the statistically behaved systems and a direct reaction (see above). This may be due to the differences in behavior between the ground and excited states. Unfortunately, we have been unable to examine the excited states of Sc+more directly because we have yet to find a useful, volatile scandium compound. Titanium. In reaction with D2, Ti+ produced by S I exhibits the cross section shown in Figure 1.20 The general behavior is quite comparable to that for Sc'. In reaction with HD, nearly equal amounts of TiH+ and TiD' are formed throughout the threshold region consistent with statistical behavior. Analysis of these cross sections is complicated by the low-lying excited state, Table 11. When Ti' is passed through the drift cell filled with Ar or CH,, we obtain nearly identical results in magnitude, isotope effect, and threshold. If the ions are completely thermalized to the bath gas temperature (300 K), the beam should contain 98.5% ground-state a4F and only 1.5% excited-state b4F. While this cannot be ascertained unequivocally, it does appear that the two 4Fstates have very similar reactivity. Another possibility is that relaxation of the b4F state is fast even in the absence of quenching collisions. Resultsz0 for Ti+ produced by electron impact (EI) ionization of TiC1, are still undergoing evaluation, but clear evidence of excited doublet states of Tif, Table 11, is present. In reactions with HD, these react to form about twice as much TiH' as TiD+. Vanadium. Results' for the reaction of D2 with Vf produced by S I are shown in Figure 1. The behavior is similar to that of Sc' and Ti+. As shown in Table 11, this reactivity is principally due to the 5Dground state of V+. Examination of the reaction of V+(SI) with H D shows nearly equal amounts of VH+ and VD+, similar to Ti+. When V+ is produced by E 1 of VOCl,, the apparent reaction threshold shifts to about 0.5 eV from the 2 eV observed for the (22) (a) Armentrout, P. B.; Beauchamp, J. L. Chem. Phys. 1980, 50, 37-43. J . Am. Chem. Soc. 1981,103,784-791. (b) Armentrout, P. B.; Halle, L. F.; Beauchamp, J. L. Ibid. 1981, 103, 962-963. (c) Halle, L. F.; Klein, F. S.; Beauchamp, J. L. Ibid. 1984, 106, 2543-2549. (d) Tolbert, M. A,; Beauchamp, J. L. Ibid. 1984, 106,8117-8122.
,
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Feature Article
The Journal of Physical Chemistry, Vol. 91, No. 8,1987 2041 ENERCY (el', Lab)
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SI data.' In addition, the cross section peak shifts to lower energies by a comparable amount. The reactivity at high energies (>4 eV) decreases. A small exothermic reaction is observed. These results clearly indicate that E1 ionization produces electronically excited V+. The shift in the threshold and the peak is consistent with production of the triplet states of V+, Table 11. A more detailed threshold analysis' indicates that the dominant component of the beam is the 3Fstate (-75%). The remainder is largely in other low-lying triplet states with little contribution from 5D or SF. In reaction with HD, V+(3F) yields about 3-4 times as much VH+ as VD',' a very different result from the ground-state reaction. Chromium. When Cr+ is produced by SI, a very pure beam of Cr+(%) is produced, Table 11. Results6,22bfor the reaction of this ion with D2 are shown in Figure 2. Note that this behavior differs from that shown in Figure 1 in several ways. The apparent threshold is higher by -1 eV. The peak occurs at a somewhat higher energy than expected from reaction 9. The magnitude is small. In reaction with HD,6 the behavior is also quite distinct from Sc+,Ti+, or V+. CrD+ is favored at threshold and reaches a maximum at -4.5 eV. The CrH+ product exhibits two peaks, one at about 4.5 eV and one much higher in energy, =7 eV. While not exhibiting the true pairwise behavior discussed above, these results make it clear that Cr+(%) has a component of impulsive reactivity.6 When Cr+ is produced by E1 on Cr(C0)6 or Cr02C12,both the apparent threshold and the peak shift to lower energies by about 2.5 eV.6*22bThis is consistent with production of either the 4D or 4Gstate of Cr+, Table 11. A detailed analysis6 suggests that the Cr+(EI) beam contains -20% Cr+(%) and that Cr+(4D) is the prominent excited state. No evidence for the reaction of Cr+(6D), the first excited state, is found in any of the Cr+ experiments. In reaction with HD, Cr+(EI) forms more CrH+ than CrD+ by a factor of between 2 and 4, similar to V+(3F). Manganese. Results for reaction 4 where M+ = MII+(~S)are shown in Figure 2. This cross section is derived from results where Mn+ is produced by SI after subtraction of the contribution from excited states.2 It is odd that this subtraction is required since the Mnt(SI) beam contains only 0.17% excited state, Table 11. To observe such a small fraction, the excited state must be much more reactive than the 'S ground state which comprises 99.83% of the SI beam. The behavior of M ~ I + ( ~isS clearly ) very different than the more typical results shown in Figure 1. While the cross section actually does rise near the thermodynamic threshold of -2.4 eV (this is difficult to see on the scale of Figure 2), there is no appreciable reactivity until 4-5 eV.2 Also the peak of the cross section is much higher than the thermodynamic onset
0.0
2.0
4.0
6.0
8.0
10.0
EKRGY (eY. CEU Figure 3. Cross sections fcr reaction of Fet(4F) and Fet(6D) and HD to form FeHt (open circles and broken line) and FeDt (closed circles and full line) as a function of relative kinetic energy (lower scale) and ion energy in the laboratory frame (upper scale).
corresponding to reaction 9. In fact, both the apparent threshold and peak of the MII+(~S)cross section occur at approximately twice the thermodynamic values. This is in very good agreement with the pairwise model discussed above.2 The reactivity of M ~ I + ( ~with S ) H D is also unusual. The main product is MnD+ which has an apparent threshold between 3 and 4 eV and peaks at about 6 eV (lower than for H2 and D2).2 This is consistent with the pairwise model which predicts a threshold for MnD+ formation of 3.6 eV and a peak at 6.7 eV. In contrast, S ) is formed. virtually no MnH+ attributable to the M ~ I + ( ~ state The pairwise model predicts that MnH+ cannot be formed until 7.1 eV and should peak at 13.1 eV. This is consistent with the observation that MnD+ is the only product observed at low energy. In contrast to the bizarre behavior displayed by MII+(~S)are the results for excited Mn+.2 When Mn+ is formed by E1 of Mn2(CO)lo,the apparent reaction threshold is observed at = O S eV and the peak at =3 eV. The shift in the threshold and the peak conforms with production of the quintet states of Mn+ in Table 11. A detailed threshold analysis2 indicates that the dominant component of the cross section due to excited states is the 5S state (=75%) with the remainder 5D. Considerable amounts of M ~ I + ( ~ are S ) also present in the E1 beam but, because it is so unreactive, this state contributes little to the cross section. The excited states react with H D to form -3-4 times as much MnH+ as MnD', as seen for V+(3F) and Cr+(4D). The threshold for both products is the same as observed for the H2 and D2 reactions. This demonstrates that the thermodynamic threshold is observed for these excited states. Iron. Results for reaction 4 with M+ = Fe+(6D), the ground state, are shown in Figure These ions are generated by E1 on Fe(CO)5 followed by passing the ions through the drift cell filled with Ar at >0.2 Torr. Excited states are quenched at a rate constant of cm3/s, an efficiency of 1 in every 620 collision^.^ The behavior of the Fe+(6D) cross section also differs from those shown in Figure 1. The cross section rises from the thermodynamic threshold of about 2.4 eV but peaks late and is substantially smaller. In reaction with HD, this state forms much more FeD+ than FeH+, Figure 3. As with Cr+(%), Fet(6D) shows impulsive behavior characteristic of the pairwise collisions discussed above although it does not conform to the pairwise limit as closely as MII+(~S). Fe+(4F), the first excited state, is only 0.25 eV above the ground considerable state, Table 11. When Fe+ is produced by SI,4,22c
2042
Elkind and Armentrout
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2.4
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ENERGY (eV, CEU Figure 4. Cross sections for reaction of Fe+(4F),CO+(~F), Ni+(ZD),and Cu+('S) with D, as a function of relative kinetic energy.
Figure 5. First-row transition-metal-hydride ion bond energies vs. atomic metal ion promotion energy to a 4s3d"' spin decoupled state, see text and Table I. The line is a linear regression fit to the data.
amounts of this state are produced, Table 11. By subtracting the cross section due to Fe+(6D) from results for Fe+(SI) and correcting for the fractional population, the state-specific cross section for reaction of Fe+(4F) can be determined.4 This is shown for reaction 4 in Figure 4 and for reactions 2 and 3 in Figure 3. Note that this state is more than an order of magnitude more reactive than Fe+(6D) and has a very different isotope branching ratio, and its cross section reaches a peak much closer to 4.5 eV. A surprising result is obtained when Fe+ is made by E1 of Fe(CO)5. Unlike the cases above, little reactivity is observed at low kinetic energies4 Reactions corresponding to both Fe+(6D) and Fe+(4F) can be identified and these have smaller magnitudes than observed for Fe+(SI). This demonstrates that a smaller fraction of these states are being produced by EI. Thus,high-lying excited states are being produced but are apparently unreactive. Cobalt, Nickel, and Copper. ResultsS+22a for reaction 4 for Co', Ni+, and Cu+ (all produced by SI) are shown in Figure 4. In all cases, the reactivity observed is due almost exclusively to ground-state ions. For Ni+ and Cu+, this is shown in Table I1 and has been verified by studies of E1 generated ions.5 In the case of Co+, there is potentially a contribution from the first excited state, SF, but detailed studies using the DC source suggest that this state is largely ~ n r e a c t i v e . ~ Note that the shapes of the cross sections for all three systems and for Fe+(4F) are very similar. The differences are due primarily to changes in the reaction thresholds which reflect the decreasing metal-hydride ion bond energy in going from Fe to Co to Ni to Cu, Table I. Reactions of these three ions with H D all show very similar branching ratios to that shown for Fe+(4F) in Figure 3.5 These ions can also be produced by E1 of CO,(CO)8, Ni(C0)4, and Cu(CH,CO,). In each case, the reactivity attributed to the ground electronic state decreases which demonstrates that fewer ground-state ions are being p r o d ~ c e d .However, ~ just as observed for Fe+(EI), no appreciable reaction due to excited states is seen at low kinetic energies. Careful examination shows very small amounts of reactivity having low energy thresholds consistent with Co+(b3F) and Ni+('F), Table 11. No reaction of excited Cu+ can be identified. More detailed analysis5 also finds excited-state reactivity for Ni+ at high kinetic energies. In reaction with HD, this high-energy reactivity produces much more NiD+ than NiH+. This suggests impulsive behavior consistent with a threshold above the thermodynamic limit. For reasons which will become apparent, we have attributed this reactivity to Ni+(4F).
mainly ground-state ions which react fairly efficiently at threshold and produce nearly equal amounts of MH+ and MD' in reaction with HD. According to the isotope effects discussion above, these results imply that these reactions proceed via statistically behaved intermediates. Figure 2 shows ground-state ions which react inefficiently at threshold and yield much more MD+ than MH+ in reaction with HD. This suggests that these reactions occur via impulsive collisions. Figure 4 shows several ions which react efficiently at threshold and yield 2-4 times as much MH+ as MD+ in reactions 2 and 3. This implies that the reactions are direct, as discussed above. Similar behavior is exhibited by several excited-state ions such as V+(3F), Cr+(4D), and Mn+(%). Having grouped the reactions in a purely empirical fashion now shows other correlations. One striking similarity is that all the ions in Figure 4 have 3d" electron configurations. Also, the excited-state ions exhibiting the same behavior have 4s3dp1 electron configurations where the 4s electron is low-spin coupled to the 3d electrons. The ions shown in Figures 1 and 2 are less homogeneous although both Mn+('S) and Fe'(6D), Figure 2, have 4s3d"' configurations where the 4s is high-spin coupled to the 3d electrons. These observations suggest that both the atomic orbital configurations and the spin state of the metal ions are correlated with the reactivity of these species. These ideas are explored below, Periodic Trends in Thermochemistry. The diatomic metal hydride ion bond dissociation energies (BDEs), Table I, show a strong variation with the identity of the metal. This has been discussed several times in the past.3,'8%23J4The first-row metals on the left side of the periodic table, Sc and Ti, have the strongest BDEs while C r and Cu have the weakest. These weak bond energies are easily understood by noting that the ground-state configuration of Cr+ is the very stable half-filled shell, 3d5, while Cu+ has a filled shell ground state, 3d'". To form a covalent bond to either of these ions, one of the electrons must be decoupled from the others. This costs energy. This concept can be formalized in terms of a promotion energy, E,. Here, we define E , as the energy necessary to take a metal ion in its ground state to an electron configuration where only one electron is in the 4s orbital and it is spin decoupled from the 3d electrons. This energy, Table I, is easily calculated as the mean energy of the electronic states which have high-spin and low-spin 4s3d"' c o n f i g ~ r a t i o n s .The ~~~~
Discussion The reactions of the various states of the first-row transitionmetal ions appear to fall into several categories. Figure 1 shows
(23) Mandich, M. L.; Halle, L. F.; Beauchamp, J. L. Ibid. 1984, 106, 4403-4411. (24) Armentrout, P. B.; Halle, L. F.; Beauchamp, J. L. J . A m . Chem. SOC. 1981, 103, 6501-6502.
Feature Article
Figure 6. Qualitative molecular orbital diagram for the interaction of a metal with H2 in C, symmetry (left side) and C,, symmetry (right side). The electron populations of the ug(H2),a(MH), and ls(H) orbitals are shown for any metal ion reacting to form ground-state products.
MH+ BDEs are plotted vs. this promotion energy in Figure 5 . Obviously, the correlation between Doo(M+-H) and Epis excellent. This implies that the dominant binding orbital on the metal is the 4s orbital. Ab initio calculations18support this idea although they establish that there is significant 3d character in the bonds, especially on the left side of the periodic table. The maximum metal hydride ion bond energy occurs at E , = 0. We have argued previously3 that the BDE at this point, 2.42 eV, is a good estimate of the intrinsic metal-hydrogen bond energy. In other words, this is the BDE expected for a hydrogen atom bonded to a metal with a directional and sterically unhindered orbital containing a single electron that is electronically decoupled from the other electrons. An example of such a species is SCH+(~A).We have recently measured that Do(HSc+-H) = 2.53 f 0.16 eV,21 in good agreement with our expectations. Molecular Orbital Considerations. A great deal of insight into these reactions can be obtained by using simple molecular orbital arguments.25 For first-row transition metals, the valence orbitals are the 3d, 4s, and 4p in order of increasing energy. For simplicity, we ignore the p orbitals since they are too high in energy to be very influential in the bonding.% The 4s orbital lies slightly above the 3d in energy (except for ~candium).~'The energies of particular electron configurations depends on the number of electrons and the spin coupling, as is evident in Table 11. Since the 4s is the outermost valence orbital as well as the principal binding orbital in the diatomic metal hydride ions (see above), the interaction of this orbital with H2 is expected to be dominant at long range. Figure 6 shows the most important interactions for approach of the 4s orbital up to a hydrogen molecule in C2, symmetry. The axis of approach is taken to be the z axis and the three atoms define the x-z plane. The 4s and ug orbitals both have a l symmetry and thus combine into bonding and antibonding molecular orbitals (MOs). Since there are two electrons in the ugorbital, occupation of the 4s leads to occupation of the antibonding 4a1* M O and thus a repulsive interaction between M' and H2. In C,, symmetry, the situation changes somewhat. The 4s orbital now interacts with both the ug and u,* MOs of H2 such that three new MOs are formed: one is bonding, another is largely nonbonding, and the third is antibonding. Since there are two electrons in the us orbital, occupation of the atomic 4s orbital leads to occupation of the nonbonding MO. This results in an interaction which is less repulsive than that resulting from occupation of the antibonding 4al* MO in C2, symmetry but is (25) Mahan, B. H. J . Chem. Phys. 1971,55, 1436-1446. Acc. Chem. Res. 1975,8, 55-61. (26) Ab initio calculations (ref 18) indicate that the MH' species contain only =lo% 4p character. (27) Ballhausen, C. J.; Gray, H. B. Molecular Orbital Theory; Benjamin/Cummings: Reading, 1964; Table 8-1 1,12.
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2043 still less attractive than if the 4s orbital (and hence, the nonbonding MO) were empty. If the 4s orbital is unoccupied, interactions with 3d orbitals become important. In C2, symmetry, the 3du(z2) orbital has a l symmetry and therefore acts like the 4s. Indeed mixing of the 4s and 3du orbitals is inevitable in these systems. The 3d?r(xz) orbital has b2 symmetry and therefore mixes with the uU* orbital of H2 to form bonding (lb,) and antibonding (2b2*) MOs. Since the a,,* orbital is always empty, occupation of the 3d?r(xz) leads to occupation of the bonding 1b2 M O and is therefore attractive. The 3d6(x2-y2) orbital also has a l symmetry but probably overlaps the H2ugorbital weakly enough to be effectively nonbonding. The 3d7rbz) and 3dS(xy) orbitals have the wrong symmetry (b, and a2, respectively) to interact with the MOs of H2in C, symmetry. Both pairs of 3d?r and 3d6 orbitals are nonbonding in C,, geometry. Spin. Another consideration in understanding the reactivity of the first-row metal ions is the spin of the reactant state. Not surprisingly, all reactions observed in Figures 1-4 are spin allowed, Le., they all conform to reaction 11 where the numbers in par-
M'(s f
y2) + H2(0)
-
MH+(s) + H(y2)
(1 1)
entheses are the spin quantum number of the species. For all metal state is observed. ions, efficient reaction of a low-spin (s This corresponds to the ground states of Co+, Ni', and Cu+ and excited states of Ti+, V', Cr+, Mn+, and Fe+. High-spin (s + states are fairly reactive for Sc', Ti+, and V+ but are less reactive as one moves across the periodic table. Note that, when the ground state is high spin, the reactions of both high-spin and low-spin states are observed. Ti+, V+, Cr', Mn', and Fe+ all exhibit this behavior. When the ground state is low spin as for Co', Ni', and Cu+, however, excited-state reactivity is suppressed either because the states are not formed by electron impact ionization or because they are unreactive. Since we know some excited states are formed by El, the latter seems more likely. This systematic correspondence of reactivity with spin is probably related to the stability of the MH2+reaction intermediate. In all cases, the spin of ground-state MH2+should be the same as the low-spin metal ion states. This presumes that ground-state MH2+ can be characterized as having two covalent metal-hydrogen bonds, a reasonable proposition.lgb Therefore, high-spin metal ion states do not have easy access to the ground-state intermediates (unless spin-orbit transitions are efficient). This suggests that the potential energy surfaces of the high-spin states are more repulsive than surfaces of low-spin states. The role of spin in controlling reactivity is clear-cut in the reactivity differences between low-spin and high-spin 4s3dP1 configurations. The M O considerations discussed above do not differentiate between these. The difference must lie in the interactions between the 3d electrons and H2. One way to consider this is to envision the reverse reaction, i.e., approach of MH+ and H. The H ( Is) electron can either be low-spin or high-spin coupled with the nonbonding 3d electrons of MH'. If low-spin coupled, there can be favorable bonding interactions between the H atom and the metal. These are most effective if the reaction geometry differs from C-". If high spin, a node must exist between the incoming H(1s) and the metal 3d electrons. This leads to a repulsive interaction which is weakest for strict C,, symmetry. Thus, the difference in reactivity between low- and high-spin M'(4s3d"') + H2 interactions is explained as a change in the repulsiveness of the potential energy surface as the reaction deviates from a collinear geometry. For high-spin 4s3dn-I states, the repulsiveness increases as this deviation occurs. For low-spin 4s3d"' states, the repulsion is mediated by bonding interactions between the trailing hydrogen atom and the metal 3d electrons. Periodic Trends in Reactivity. Having discussed the fundamental aspects of the interactions of atomic metal ions with hydrogen, we find it reasonably straightforward to understand the variations in reactivity across the periodic table. As discussed above, the states with high-spin 4s3d"' configurations should be unreactive. Not only is the 4s orbital occupied leading to repulsion but the high spin prevents attractive bonding interactions with
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The Journal of Physical Chemistry, Vol. 91, No. 8, 1987
the metal 3d electrons as well. Examples of such states include Mt1+(~S,4s3d~), Fe+(6D,4s3d6), and Ni+(4F,4s3d8). All of these states behave in a manner consistent with an impulsive reaction mechanism which indicates a largely repulsive potential energy surface. Also note that no reactions attributable to the species Cr+(6D,4s3d4),C0+(~F,4s3d~), or C ~ + ( ~ D , 4 s 3 are d ~ )observed. We attribute this to their relative inertness. Now consider the high-spin 4s3d"' states for the metals on the left side of the periodic table, S ~ + ( ~ D , 4 s 3 dTi+(a4F,4s3d2), ), and V+(5F,4s3d3). While experimental results concerning V+(5F) are ambiguous since it is a low-lying excited state, the other two are both ground states and appear to react efficiently. In reaction with HD, Sc+('D) forms twice as much M H + as MD+ while Ti+(a4F) forms equal amounts. These results are in obvious contrast to the reactivity exhibited by the high-spin 4s3d"' states discussed above. While our analysis of the Sc+and Ti' systems is not complete, we believe that the potential energy surfaces evolving from the ground states are repulsive here also. However, unlike the situation for the states discussed above, there are low-lying excited states of Sc', the 3F(3d2),and Ti+, the b4F(3d3), which are also high spin, Table 11. The surfaces evolving from these excited states are attractive (since the 4s is unoccupied) and will therefore cross with the repulsive surfaces from SC+(~D) and Ti+(a4F). Since these diabatic surfaces (Le., pertaining to a particular electron configuration) have the same spin, the crossings are avoided to form adiabatic surfaces. Another way to think about this c r w i n g is that the electron in the 4s orbital moves into an empty 3d orbital of the same symmetry to avoid the repulsive interactions. To the right of chromium (n > 5), there are no empty 3d orbitals in the high-spin 4s3d"' states. These ideas lead to the conclusion that the adiabatic surfaces correlating to S C + ( ~ Dand ) Ti+(a4F) have an electronic character of 3d2 and 3d3, respectively, in the MH2+ intermediate. This explanation is consistent with the observation that the behavior of Ti+ is similar to V+(5D,3d4). Sc+ behaves somewhat differently perhaps because the mixing of these states is less efficient than for Ti+. In the case of Sc+, the 3D(4s3d) state and 3F(3d2)states are separated by 0.6 eV, while for Ti+, the two 4F states are separated by only 0.10 eV, Table 11. The two ions, Ti+(4F)and V+(5D), are the only metal states which clearly yield nearly equal amounts of MH+ and MD+. As discussed above, this is a statistical result which implies that a metal dihydride intermediate is formed. This behavior is consistent with the MO arguments above. These configurations have an unoccupied 4s orbital and thus avoid the repulsive interactions discussed above. In addition, for at least one of the surfaces evolving from these states, the 3da is also empty, again avoiding repulsive interactions, and the 3d?r(xz) orbital is occupied, leading to an attractive interaction. This type of surface is apparently attractive enough to exhibit statistical behavior even though access to the ground state of MH2+ is spin forbidden. While not necessary to explain the experimental results, it is possible that spin-orbit interactions are sufficient to permit crossing to the ground state MH2+ surface. These considerations may also explain the fact that the magnitudes of the reaction cross sections for Ti+(4F) and V+(5D), Figure 1, are somewhat smaller than those for other metals, Figure 4. Namely if the 3da orbital is occupied, the reaction no longer can proceed via insertion and, because the ion is high spin, the reaction is much less efficient. The 3da orbital remains empty on 60% of the surfaces evolving from Sc+(3d2), on 40% from Ti+(3d3), and on 20% from V+(3d4). Note that the magnitude of the Sc+, Ti+, and V+ cross sections are roughly proportional to these percentages. Comparisons to the absolute magnitudes of PST calculations also substantiate this argument.' The importance of the 3da orbital occupation can be explored further by comparing the reactivity of Cr+('%,3d5) and V+(5D,3d4). Rather than being statistically behaved, Cr+ displays a strong component of impulsive reactivity as shown by the isotope effects. The only difference between the high-spin 3d4 and 3d5 configurations is that the 3da orbital must be occupied in the latter. This apparently prevents Cr+(%) from inserting into hydrogen. The
Elkind and Armentrout high spin then leads to a relatively repulsive surface similar to that for high-spin 4s3d"' configurations. The next ions in the 3d" series (n > 5) differ from those with n 5 5 because they are all low spin with one or more of the 3d orbitals doubly occupied. While the reactivity of Mr1+(~D,3d~) has not been cleanly isolated, results for Fe+(4F,3d7),Co+(3F,3d8), Ni+(ZD,3d9),and Cu+('S,3dIo) are available. They all behave very similarly in reactions 1 and 4, Figure 4. In reaction with HD, Figure 3, they all form MH+ preferentially by a factor of 2-4 indicating a direct mechanism. Unlike the 3d" configurations where n < 5, these states cannot have an empty 3da orbital. As noted for Cr+(%), occupation of this orbital is apparently sufficient that these reactions cannot occur via insertion. However, unlike the high-spin Cr+@), the low spin of these configurations provides attractive bonding interactions between the 3d orbitals and the trailing hydrogen. In C2, geometry, this corresponds to the fact that it is spin-allowed to doubly occupy the 1b2 bonding orbital in Figure 6. Thus, these species react efficiently via a direct mechanism. Because M H + bonding is largely 4s-1s in character, it may appear counterintuitive that 3d" configurations react efficiently to form ground-state MH' while 4s3d"' configurations do not. Indeed diabatically, the 4s-1s bound ground state of MH+ can only be formed from M+(4s3dv') + H2 reactants. However, the surfaces evolving from a low-spin M'(4s3d"') configuration cross with those evolving from the M'(3d") ground state which are also low spin. These crossings are avoided to form adiabatic surfaces which connect ground-state M'(3d") reactants with ground-state MH+ products. This argument helps explain the somewhat surprising result that the closed-shell Cu+('S,3dIo) reacts readily at its thermodynamic threshold. The molecular orbital diagram, Figure 6, shows that the primary interaction is the donation of electron density from the doubly occupied a,(H2) orbital to the empty 4s(M) orbital. The CuH2+intermediate formed in this process should be the ground-state species. An interesting question for future spectroscopic or theoretical characterization is whether this species is best viewed as a metal dihydride with two covalent Cu-H bonds (1b, M O is strongly bonding) or as a two-electron threecenter bond (1b2 M O is nonbonding). Similar questions exist for other MH2+ species. In the exit channel, there is a crossing between the M O evolving to the ls(H) orbital and one of the 3d(M) orbitals in all symmetries. The surface crossing between the 3d" and 4s3d"-' configurations discussed above is the result of this M O crossing. The final type of electron configuration examined in these studies is low-spin 4s3d"'. Because this configuration is always an excited state, there is no cleanly isolated example of this type of state. The best examples are found in E1 data where the reactivity is dominated by such states: e.g., V+(3F,4s3d3), Cr+(4D,4s3d4),and Mn+@,4s3d5). Preliminary results indicate that Ti+(2F,4s3d2)is probably another example. Each of these species preferentially forms MH+ in reaction with HD by a factor of 2 4 , indicative of a direct reaction. Occupation of the 4s orbital is expected to be repulsive in CZusymmetry such that insertion into H2 is not expected. However, this repulsion is relieved in C,, symmetry and the low spin can lead to attractive bonding interactions. These factors are consistent with the efficient reaction of these states (in contrast to the inertness of the high-spin 4s3d-I configuration) but via a direct mechanism. This behavior is similar to that noted above for the 3d" (n > 5 ) configurations. This is consistent with the fact that these species build in low-spin 4s3d"' character as the MH+ products are formed. The low-spin 4s3d"' states of Ti+, V+, Cr', and Mn+ probably do not undergo such a crossing since the low-spin 3d" states of these metal ions are higher in energy, Table 11. Summary The experimental results and theoretical considerations discussed here lead to a reasonably comprehensive view of the reactions of atomic transition-metal ions with molecular hydrogen. Three categories of reactivity appear to exist based on the electron
Feature Article
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2045 strongly endothermic and the intermediate lifetimes are short. As the reactant neutral increases in size, the polarizability, the number of degrees of freedom, the intermediate lifetime, and the availability of low-energy reaction channels all increase. These effects may enhance or alter the interactions between diabatic surfaces. Preliminary work on these questions shows that these rules do indeed break down under some conditions. Comparison of reactions of Fe+(6D) and Fe+(4F) with small alkanes (CH,,, C&, and C3H8),33for instance, shows that, for all endothermic channels, FeC(4F)is much more reactive, just as for the hydrogen system. For exothermic channels, however, Fe+(6D) is more reactive at the lowest energies. Our tentative explanation is that the diabatic surfaces evolving from Fe+(4F) and Fe+(6D)cross and this crossing is avoided due to spin-orbit interactions (j-j coupling rather than 1-s). The adiabatic surfaces thus lead from ground-state reactants to ground-state products. At higher kinetic energies (>0.5 eV), the nuclear motion is sufficiently rapid that the reactants begin to stay on the diabatic surfaces and Fe+(4F) becomes more reactive. The nature of these potential energy surface couplings is an avenue of extensive research opportunities. Ligated Metal Systems and Metal Clusters. Another vista in detailed studies of electronic effects of gas-phase transition-metal species involves more complex metal reactants. Since one of the rationales of this research is to understand the chemistry of real catalytic systems, both homogeneous and heterogeneous, the complexity of the gas-phase metal species must be increasgd h order to approximate these systems more closely. By selectively ligating atomic metal ions, it should be possible to change the electronic and chemical properties of the transition-metal center. By systematically varying both the number and type of ligands, we can continue to build upon the base of knowledge established for the atomic species. One of the more interesting ligands one can consider attaching to an atomic metal ion is more metal atoms. Studies of metal cluster ions are a particularly active area of research at the interface of inorganic and physical ~ h e m i s t r y . ~ ~Investigations ”~ of electronic and chemical properties of these species is of obvious interest but of substantial complexity. Unlike atomic metal ions,36 metal cluster ions (and neutrals) have no compendium of electronic states (yet).
configuration and spin state of the metal. (1) If the 4s and 3du orbitals are unoccupied, the systems react efficiently. The branching ratio in reaction with H D is nearly 1:l. Overall the behavior is near statistical. MO concepts indicate that these states should be able to insert into H2 to form a metal dihydride intermediate. (2) If either the 4s or 3du orbital is occupied, the systems can react efficiently via a direct mechanism if they have low spin (Le., the same spin as the ground state of MH2+). The branching ratio in reaction with H D is =3:1 in favor of the MH+ product. M O concepts indicate that these states may prefer a collinear reaction geometry but that other geometries are not unfavorable. (3) If either the 4s or 3du orbital is occupied and the ion has a high spin, the systems react inefficiently at the thermodynamic threshold and via an impulsive, pairwise mechanism at elevated energies. The branching ratio in reaction with H D favors production of the MD+ product at low energies. M O concepts indicate that these states should have repulsive surfaces which strongly favor a collinear reaction geometry. Note that these reactivity “rules” are formulated for the diabatic character of the reaction surfaces, i.e., they pertain to specific electron configurations of the metal ions. Their success in explaining our experimental observations demonstrates what kind of metal orbital character is necessary for efficient activation of molecular hydrogen. The ideas forwarded here are consistent with - 0 * H2 with metal atoms,% theoretical discussm metal c o m p l e ~ e sand , ~ ~metal ~ ~ ~s ~ r f a c e s . ’ ~ * ~ ~ Despite the utility of these rules, they clearly can be broken. For instance, the reactivity of the ground states of Sc+ and Ti+ do not fall neatly into these categories. This is postulated to be due to mixing of the potential energy surfaces evolving from these states with those from more reactive excited states. Thus, the diabatic rules can fail when adiabatic interactions become important. Such interactions also occur in other systems, for example in the exit channel of the reactions of 3d” (n > 5 ) configurations. Further investigation of the importance of such surface interactions may be accomplished by comparison of first-, second-, and third-row transition metals3~20~23*32 since this progression changes the relative energies of the interacting states. Other effects that need to be considered in these studies are the orbital size, the population of f orbitals, and increased spin-orbit interactions. Beyond Hydrogen. A question of obvious interest is whether the “rules” of transition-metal ion reactivity outlined above continue to hold for systems larger than hydrogen. These diabatic rules may work best for the H2 system where the reactions are
Acknowledgment. This research has been funded by the National Science Foundation under Grant No. CHE-8608847. 1333-74-0; D1,7782-39-0. Registry No. H2,
(28) Ruiz, M. E.; Garcia-Prieto, J.; Novaro, 0. J. Chem. Phys. 1984, 80, 1529-1534. Siegbahn, P.E. M.; Blomberg, M. R. A.; Bauschlicher, C. W. Ibid. 1984,81, 1373-1382. Seven, A.; Chaquin, P. Nouv.J. Chim.1983, 7, 353-360. (29) See, for example: Brothers, P. J. Prog. Inorg. Chem. 1981, 28, 1 . Noell, J. 0.; Hay, P. J. J . Am. Chem. SOC.1982, 104,4578-4584. (30) Saillard, J.; Hoffmann, R. Ibid. 1984, 106, 2006-2026. (31) Shustotovich, E.; Baetzold, R. C.; Muetterties, E. L. J. Phys. Chem. 1983,87, 1100-1 1 1 3. Siegbahn, P. E. M.; Blomberg, M. R. A.; Bauschlicher, C. W. J . Chem. Phys. 1984,8J, 2103-2111. (32) Tolbert, M. A.; Beauchamp, J. L. J . Am. Chem. SOC.1986, 108, 5675-5683.
(33) Schultz, R. S.; Elkind, J. L.; Armentrout, P. B., submitted for publication in J. Am. Chem. SOC. (34) See, for example, Loh, S. K.; Hales, D. A.; Armentrout, P. B. Chem. Phys. Lett. 1986, 129, 527, and references therein. (35) Recent reviews of cluster work include Castleman, A. W.; Keesee, R. G. Chem. Rev. 1986, 86, 589-618. Phillips, J. C. Ibid. 1986, 86, 619-634. Morse, M. D. Ibid.,in press. (36) Moore, C. E. Atomic Energy Levels; National Bureau of Standards: Washington, DC, 1949. Sugar, J.; Corliss, C. J . Phys. Chem. Ref. Data 1977, 6, 317-383, 1253-1329. 1978, 7, 1191-1262. 1979, 8, 1-62. 1980, 9, 473-511. 1981, 10, 197-289, 1097-1174. 1982, 11, 135-241.
