Chapter 16
Reactions of Gas-Phase Transition-Metal Atoms with Small Hydrocarbons James C. Weisshaar Department of Chemistry, University of Wisconsin—Madison, Madison, WI 53706
We compare the chemistry of gas phase transition metal atoms with alkanes and alkenes, surveying the different charge states M, M , and M . Most monopositive cations are highly reactive, dehydrogenating or demethanating linear alkanes in exothermic reactions at room temperature. The dipositive cations abstract electrons or hydride anions from alkanes if the radii of long-range curve crossings permit. If not, M can approach the hydrocarbon to close range, and the same kinds of H and CH elimination reactions occur as observed in M . The neutral atoms are inert to alkanes, but Sc, Ti, V, Ni, Zr, Nb, and Mo react with alkenes. The interplay of the energetics of low-lying metal states of different electron spin and electron configuration; the size of valence d and s orbitals; and the character of long-range forces determines chemical reactivity. +
2+
2+
2
4
+
2
+
The chemistry of gas phase M+ and M is unusual (7,2). Certain ground state transition metal cations can break C - H bonds or even C-C bonds of linear alkanes at room temperature, leading to exothermic dehydrogenation or demethanation products. While solution phase chemists have discovered photochemically driven reactions in which metal centers insert in C - H bonds of alkanes (3) cracking of the C-C skeleton of linear alkanes at room temperature appears unique to the gas phase metal cations. Comparison of the gas phase chemical reactivity of transition metal atoms of different charge, M , M+, and M + , is becoming possible. This article focuses on reactions of such atoms with small hydrocarbons. The bare metal atoms themselves are electronically complex, but well understood (4). Our goal is to understand how metal atom electronic structure controls chemical reactivity. Here electronic structure refers to the pattern of ground and low-lying excited atomic energy levels, including configuration and electron spin, as well as the relative sizes of the valence nd and (w+7)s orbitals. To understand ground state reactivity, we must consider multidimensional intersections among diabatic potential energy surfaces emanating from both ground and low-lying excited states of reactants. The reason is that the ground 2
0097-6156/93/0530-0208S06.00/0 © 1993 American Chemical Society
16. WEISSHAAR
209
Reactions of Gas-Phase Transition-Metal Atoms
states themselves are often badly suited for forming chemical bonds due to highspin coupling of the unpaired electrons or to unfavorable electron configuration (orbital occupancy). Often chemistry can occur only by mixing of excited-state character (hybridization) during the collision. The gas phase itself is unique in allowing the study of isolated, bimolecular collisions between reactants. Both M and M share the enormous experimental advantage of easily tunable kinetic energy and straightforward identification of the mass of the charged product. In addition to traditional kinetics measurements, we can sometimes study collisions between hydrocarbon and a specific electronic state of the metal with simultaneous control of the kinetic energy as well. Recent review articles (1,2,5) describe the chemistry of M+ and of M + quite thoroughly. Here we seek common themes among reactions with different charge. +
2 +
2
Overview of Electronic Structure An atomic state is labeled by its electron configuration, total electron spin S, orbital angular momentum L , and total angular momentum J, as embodied in the symbol L j . This Russell-Saunders coupling scheme works best in the 3d series, where electron spin is a good quantum number. Spin is gradually degraded as a quantum number moving to the right within a series or downward in the periodic table from the 3d to the 4d to the 5d series. The low-lying electronic states of bare metal atoms are well characterized experimentally for M , M+, and M + , at least in the 3d and 4d series (4). Both the sizes of atomic orbitals and their energies influence the strength of metalligand chemical bonding (6). In the 3d series, the 4s orbital is much larger than 3d. In neutral M , most of the ground states (Se, T i , V , M n , Fe, Co, and Ni) have 3d 4s configurations; the lowest excited states are high-spin 3d ~ 4s . In Cr and Cu, exchange interactions make 3d 4s the ground state. Those ground states with 4s configurations have a closed-shell appearance at long range, suggesting they will be chemically inert. This is borne out by experiment. The M+ ground states and low-lying excited states are either 3 d 4 s or 3d ; the 3d 4s states lie high in energy. Accordingly, the M ground states are highly reactive. In M , the ground states are 3d , and again these are highly reactive species. In the 4d series, the valence orbitals (5s and 4d) are larger than in the 3d series. Moreover, 5s and 4d are more similar in size and orbital energy than 4s and 3d. This enhances the of chemical bonding in the 4d series. The ground states are either 4d 5s or high-spin 4d 5s . We might expect the 4d-series neutral atoms to be more chemically active than the 3d-series atoms, in agreement with the early experimental evidence. The 4d-series M+ ground states are 4d 5s or 4d , and the M ground states are 4d . In the 5d series, the lanthanide contraction stabilizes and contracts the 6s orbital relative to 5d. The energy level structure of the neutral atoms is superficially similar to the 3d series, with primarily 5d 6s ground states for neutral M . The M+ ground states are Sd^os on the left-hand side and 5d on the far right-hand side. 2 s + 1
2
x2
2
x
xl
1
1
1
2
xl
x2
2
2 +
1
x
x
x2
xl
1
+
x
2
xl
1
2 +
x
x2
1
2
x
210
LASER CHEMISTRY O F ORGANOMETALLICS
M+ + Hydrocarbon Reactions An impressive arsenal of complementary experimental techniques has been applied to the chemistry of M+. These include guided ion beam measurements of cross sections (7); Fourier transform, ion-cyclotron resonance measurements of reaction rates and product identity (8); bulk kinetics measurements in fast flow reactors with He buffer gas (9); collision-induced dissociation (10); crossed ionneutral beam measurements of total cross sections (77); drift cell measurements (72); and ion beam measurements of metastable kinetic energy release distributions on fragmentation (13). Electron spin conservation during the rate-limiting step of M+ insertion in C-H bonds (9) can qualitatively explain which ground-state, 3d-series M cations react with alkanes in low energy collisions (Figure 1). The chemically active M cations have either a ground state that can conserve spin during insertion ( C o and Ni+) or a low-lying excited state that can do so (Sc , Ti+, and the much less reactive V and F e ) . Here low-lying means smaller than ~ 1 eV, as shown in Fig. 1. The inert metal cations either have promotion energies to states of proper spin that are too large (in excess of 1.3 eV for C r , Mn+, and Cu+) or they cannot form the 3d-4s hybrids necessary to make two σ-bonds (Zn ). Two particularly well-studied reactions are: +
+
+
+
+
+
+
+
V+ + C H - > V C H + + H 3
8
3
6
(1)
2
Fe+ + C H -> FeC H + + H FeC H + + C H . 3
8
3
6
2
(2a) (2b)
2
4
4
State-specific cross sections as a function of kinetic energy have been measured for both reactions. For the V+ + C H reaction, we (14) and others (15) have developed an appealing picture of how electronic factors control chemical reactivity (Figure 2). We assume that the rate-limiting step is M+ insertion in a C-H bond of the alkane and consider the consequences of electron spin conservation and orbital correlation arguments along the path to the key H - M - C H intermediate. In V the low-lying quintet terms 3d 4s( F) and 3d ( D) are quite inert, while the triplet term 3d 4s( F) at 1.1 eV reacts efficiently to eliminate H (76,77). The triplet's absolute reaction efficiency is 41% for the C H reaction (absolute cross section of 37 ± 19 À ) at 0.2 eV collision energy. From these experimental results, we inferred the importance of electron spin conservation during V insertion into a C - H bond of the hydrocarbon in determining the pattern of state-specific reactivity. A similar picture with the added feature of orbital specificity (3d more reactive than 3d 4s) can explain the observed trends in M+ + H reactivity across the 3d series (7). The Fe+ + C H reaction is puzzling when placed in the same conceptual framework that works well for V . The total reaction cross section is rather insensitive to changes in the initial F e electronic state (76,77), in contrast to the behavior of V+ + C H . The initial states sampled vary from 0-1.1 eV in electronic energy, include both 3d 4s and 3d electron configurations, and include both sextet and quartet electron spins. In particular, the F term has both the proper spin and orbital occupancy for C - H bond insertion. Nevertheless, the relative reactivity of D , F , and D terms varies only a factor of four in Fe+ + C H . Absolute cross sections show that the F e + C H reaction is quite 3
8
+
3
+
3
3
5
7
4
5
3
2
3
8
2
+
n
nl
2
3
8
+
+
3
8
6
7
4
6
4
4
+
3
8
3
8
WEISSHAAR
Reactions of Gas-Phase Transition-Metal Atoms
32
τ—ι—ι—ι—Γ h M
28 24 20
4
Levels
π—Γ
π—Γ
80
2}3d1»8
Ε
î}3d
70
n
m
ο "I low Δ
90
Δ
J spin
Δ
• Ihigh
7
160 £
ο 150
* J spin
!
16 / /
12
Δ/
*Α
•
140 S. 3 ο 30
ο
\ ι
i ο
ο
8
ο
120
4
10 1
±
0
1
Α—Α—Α ι
ι
ίο
Π = 2 3 4 5 6 7 8 9 10 11 Sc V Μη Co Cu* Ti* Cr* Fe* Ni* Zn* +
+
+
+
F I G U R E 1. Low-lying terms of 3d-series metal cations (Réf. 4). Dashed line connects lowest energy terms that can conserve spin while inserting in a C-H bond.
(triplet)
barrier
F I G U R E 2. Schematic potential energy surfaces for V+ + C H along a coordinate leading from reactants to the C-H bond insertion intermediate. 3
8
211
212
LASER CHEMISTRY OF ORGANOMETALLICS
6
4
4
inefficient from all three terms, D , F , and D . Product branching is also insensitive to initial electronic state (16,17) which suggests that the decision whether or not to react precedes the decision between product channels. Similar H and C H elimination chemistry occurs in M collisions with alkenes. Intriguingly, the V+ + C H reaction cross section is much larger than the V + C H cross section. The M 4- alkene mechanisms are less well studied in general. The chemistry of M+ from the 4d and 5d transition series is again surprising. One important recent discovery is the ability of Zr+ from the 4d series and of Ta+, W+, Os+, Ir+, and Pt + from the 5d series to eliminate H from methane in exothermic reactions at low kinetic energy (18): +
2
4
2
6
+
+
2
4
2
M+ + η C H
MC H
4
n
+ ηH .
2 n
(3)
2
As many as η = 4 methane molecules react sequentially. Exothermic reaction for η = 1 implies D ( M - C H ) > 111 kcal/mol, which is remarkable. The lanthanide contraction and spin-orbit (relativistic) effects lead to changes in orbital size and stability which increase bonding overlap and decrease loss of exchange stabilization on bond formation. The unusual ability of the 5dseries M+ to dehydrogenate C H is apparently a direct result of their unique electronic structure. +
0
2
4
2
M + + Hydrocarbon Reactions The second ionization potentials of transition metal atoms lie in the range 12-20 eV, which is larger than the first ionization potential of all alkanes and alkenes except C H . Dipositive metal cations should be extraordinarily strong oxidizing agents capable of removing an electron even from alkanes. When we began studying reactions of T i with alkanes, we expected to observe facile electron transfer followed by explosion of alkane into fragments. In fact, M ions are indeed highly reactive, but the chemistry is often remarkably gentle. In a fast flow reactor at 0.4 torr He, we observed the following (19): 4
2 +
+
2
+
2
Ti + + η C H
2
Ti(CH ) +, η = 1-4
4
4
n
2
Ti + + C H - * T i H + + C H + 2
6
2
5
(adduct stabilization)
(4)
(hydride anion transfer)
(5)
(electron transfer)
(6)
2
Ti + + C H ^ T i + + C H + . 3
8
3
8
2
Ti + is formed by laser vaporization of a rotating T i disk upstream of the flow tube; rate constants are measured by mass spectrometric detection of the decay of Ti + vs calibrated flow of alkane reactant at fixed reaction length. A l l three reactions are fast; efficiencies k/k are ~0.1 for T i + C H and - 1 for T i + C H and C H . Here k is the Langevin rate constant for cation-neutral capture collisions. The second ionization potential of T i is 13.6 eV, so electron transfer is exothermic from all three alkanes. Nevertheless, the collisionally stabilized dipositive adduct ions Ti(CH ) + dominate Rxn. 4, surviving - 1 ms in a 300 Κ 2
2 +
L
2
6
3
8
L
2
4
n
2 +
4
16.
213
Reactions of Gas-Phase Transition-Metal Atoms
WEISSHAAR
He bath. We observe no electron transfer in either the C H or C H reactions. In Ti + + C H , hydride transfer dominates. Electron transfer is observed only in Ti + + C H . A simple one-dimensional curve-crossing model (Figure 3) can explain the remarkable specificity of these reactions. At long range, reactants T i + RH follow the attractive ion-induced dipole potential V ^ r ) = - aq /2r , where a is the R H polarizability and q = +2 is the ion charge. The T i R H adduct-ion potential well may lie below the ion-pair asymptotes. Bimolecular ion-pair products Ti+ + RH+ and TiH+ + R+ follow repulsive Coulomb potentials V (r) = +q /r at long range, crossing the T i + R H curve at a radius that depends sensitively on the ion-pair product's exothermicity. The different alkanes C H , C H , and C H in effect tune the radii of the crossing points between reactants and the two ion-pair product channels, thus controlling the product branching. Known reaction exothermicities and the simple Coulomb potentials allow us to calculate the radii of the crossing points. The calculated electron-transfer curve crossing occurs at 14.4 Â for T i + C H , 7.0 À for C H , , and 6.1 À for C H . Apparently the electron will not jump at 14.4 À or 7.0 A , but it jumps efficiently at 6.1 À, so electron transfer dominates the C H reaction products. The calculated hydride transfer curve crossing occurs at 9.1 À for C H , 4.6 À for C H , and 4.5 A for C H . T i + C H reactants survive the electron transfer crossing and reach r = 4.6 À, where Htransfer occurs and products separate. Ti + + C H survives both the electron and hydride transfer crossings, reaching the adduct ion well at short range. About 10% of the adducts are collisionally stabilized at the He pressure of 0.4 torr. The adducts may be thermodynamically stable relative to bimolecular products, or they may be metastable but separated from bimolecular products by a large barrier. The remaining adducts dissociate back to Ti + + C H , oblivious to the electron transfer and hydride transfer crossings on the way out as they were on the way in. Thus the specificity of the Ti + data strongly suggests that both eand H- transfer are "physical" processes controlled by the radii of long-range curve-crossing points, which are in turn determined by the exothermicities of these two channels. Given the simplicity of the Ti + chemistry, we were quite surprised when Freiser and co-workers (20) found an array of M + alkane reactions that produce the same kinds of H and C H elimination products observed in M+ + alkane reactions (see Rxns. 1-3). The combined data suggest the following simple model. As M approaches the alkane, it always encounters the e- transfer curve crossing first; the efficiency of e- transfer depends critically on the radius of the crossing. Those M + alkane pairs that survive the e- transfer crossing may encounter a H- transfer crossing at 4-6 À; if so, H- transfer is efficient. But if the calculated H- transfer radius is smaller than about 4 À, H- transfer does not occur. Other rearrangement channels leading to H or C H elimination then dominate the products. Apparently the chemical forces become too strong at short range to allow M + to escape with H- attached. Remarkably, gas phase M + and M+ may be rather similar chemicals when the M survives both the electron and the hydride transfer crossings at long range and gains close contact with the alkane. If anything, M at short range appears even better able to break C-H and C-C bonds than M+. This leads us to wonder if neutral transition metal atoms would be active chemicals as well, if only they were able to penetrate long-range potential barriers to bond insertion. 4
2
6
2
2
6
2
3
8
2 +
2
4
2 +
2
2
2 +
4
2
6
3
8
2 +
4
2
3
3
8
8
2 +
4
2
6
3
8
2
2
4
2
4
2
2
2
2
+
4
2 +
2
2
4
2
2
2
+
2
+
+
6
LASER CHEMISTRY OF ORGANOMETALLICS
214
M + Hydrocarbon Reactions Overview of Kinetics Measurements. With notable exceptions, study of the chemistry of neutral gas phase metal atoms has been dominated by oxidation reactions of alkali and alkaline earth metals. Little is known about the chemistry of neutral transition metal atoms. Mitchell and co-workers (21) are systematically studying the kinetics of termolecular association of 3d-series transition metal atoms with small ligands including 0 , N O , C O , N H , C H , and 2
3
2
4
C2H2.
Our own contribution (22) has been a broad survey of the reactivity of transition metal atoms with oxidants, alkanes, and alkenes at 300 K . We use the same fast flow reactor as in the T i kinetics measurements, substituting a hollow cathode sputtering source of metal atoms for the laser vaporization source. Laser-induced fluorescence (LIF) probes the decay of specific electronic states of M vs calibrated reactant flow at fixed reaction length. The slope of plots of ln(M) vs reactant number density yields an effective bimolecular rate constant. Thus far, all of the results are in He buffer gas at 0.4-0.8 torr and 300 K . The experiment is described in more detail in the Ph.D. thesis of David Ritter (23). Table I collects the rate constants for reactions of neutral M from the 3d-series with a variety of collision partners. We find no reaction (k < 1 χ 10" cm -s ) of 3d-series metal atoms with the linear hydrocarbons propane and /z-butane. We observe fairly efficient reactions of Sc, T i , V , and N i with alkenes. Rate constants generally increase with the size of the alkene. They are independent of which spin-orbit level J within the ground term is probed by LIF. We observe no dependence ( ± 2 0 % precision) of the effective bimolecular rate constant on He pressure over the range 0.4-1.0 torr. Either the reactions are bimolecular or they have already reached the saturated regime of termolecular kinetics at 0.5 torr. Our quantitative kinetics measurements for ground states of M at 300 Κ provide upper bounds on activation energies that can test ab initio estimates of barrier heights. For example, the Ni(d s, D) + alkene data imply activation energies not larger than 0.4-3.5 kcal/mol, depending on the alkene. Mitchell and co-workers obtain a N i - C H bond energy of 35.5 ± 5.0 kcal/mol (24). Calculations on N i 4- C H find the crossing point between the repulsive A surface and the attractive A surface (from Ό Ni) to lie - 8 kcal/mol above Ni( D) + C H , perhaps a slight overestimate (25). Most recently, we have begun to extend this work to the 4d-series neutral transition metal atoms (26). Zr is a chemically lively metal which reacts with alkenes at roughly one-half the hard-spheres collision rate at 300 K . Zr also abstracts oxygen atom from 0 with rate constant 5 χ 1 0 cm -molec -s at 300 K . That is some 20 times more efficient than the congener Ti. We have recently found that Mo reacts with both alkenes, in contrast with its congener Cr, which is inert. 2 +
14
3
1
9
2
3
4
3
2
4
1
l
ι
l
3
2
4
11
3
1
1
2
Electronic Structure Considerations. It is already clear that the neutral transition metal atoms are much more inert than the cations M . Neutral M does not react with alkanes, and only Sc, T i , V , and N i react with alkenes. The relative inertness of neutral M is easily explained. Most of the ground states are 3d 4s , and the 4s orbital is much larger than 3d. The traditional picture of bonding between metal and alkene is the Dewar-Chatt-Duncanson model, in which two donor-acceptor interactions occur. Doubly-occupied 3d on the metal +
x2
2
xz
16. WEISSHAAR
215
Reactions of Gas-Phase Transition-Metal Atoms
(donor) overlaps the empty n* on ethylene (acceptor) and doubly-occupied π on ethylene (donor) overlaps the empty 4s on the metal (acceptor).
12
1
T A B L E I. Effective bimolecular rate constants ( 1 0 cn^-molec^-sec ) for reactions of 3d-series, ground state neutral metal atoms at 0.80 torr He and 300 K. Rate constants precise to ± 1 0 % and accurate to ± 3 0 % .
Sc D
Ti
NR 9.5 8.3 NR 14 20 33 71 29 NR 0.01 NR
NR 6.2 5.0 NR 7.1 7.5 14 52 10 NR NR NR
Reactant
V 4
Cr S
Mn S
Fe *D
Co 4
Ni F
Cu S
NR 9.6 9.6 NR 14 22 30 68 15 NR 0.02 NR
NR NR
NR NR
NR NR
NR NR
0.5 11 21 0.3 140 160 155 67 110 NR 10 NR
NR NR
2
ethene propene-h propene-d C F 1-butene t-2-butene c-2-butene isobutene 1,3-butadiene n-butane cyclopropane propane 6
6
3
6
F
7
6
F
-
-
-
-
NR NR NR NR NR 0.15 NR NR NR
NR NR NR NR NR NR NR NR NR
NR NR NR NR NR NR NR NR NR
NR 0.09 NR NR NR 0.35 NR NR NR
14
3
2
3
NR NR NR NR NR NR NR NR NR
1
NR means no reaction observed; k less than 10" cm -s . Dash (--) means reaction not studied.
None of the neutral M atom ground states is properly prepared for a favorable interaction with C H . As M and alkene approach, the metal atom must hybridize to relieve repulsion between 4s or 4s and the doubly occupied π orbital on alkene. Two mechanisms are possible, sd hybridization and sp hybridization; in reality, the optimal combination of these will occur. In sd hybridization, 3d 2 and 4s mix to form two hybrids, which we label sd+ and sd.. One hybrid (sd+) concentrates probability along the z-axis; this becomes the acceptor orbital. The other (sd.) concentrates probability in the xy-plane, thus relieving repulsion; this becomes the singly or doubly occupied non-bonding orbital. In sp hybridization, 4p and 4s mix to form two hybrids directed towards (sp ) or away from (sp.) the approaching C H rr-orbital. The sp hybrid becomes the acceptor and the sp. becomes the lone pair, polarized to the rear of the metal atom to relieve repulsion (27,28). Why are Sc, T i , V , and N i the only reactive 3d-series metal atoms? Compared with Fe and Co, N i has a very small promotion energy to a state of low-spin, 3d 4s character (10 kcal-mol for N i vs 34 for Fe and 21 for Co). 2
4
2
1
z
z
+
2
xl
1
4
+
LASER CHEMISTRY OF ORGANOMETALLICS
216
Only such a low-spin state can hybridize and accomodate two non-bonding electrons in the resulting sd hybrid. The avoided intersections of the repulsive surfaces from 3d 4s ground state reactants and the attractive surfaces from lowspin 3d 4s excited state reactants produce barriers to M-alkene adduct formation (Figure 4). The barrier height scales roughly as the energy difference between 3d 4s and low-spin 3d 4s atomic states. These ideas are familiar from the V+ and Fe+ + alkane examples. N i overcomes the barrier at 300 Κ while Fe and Co cannot. The sd-hybridization scheme is preferred in N i because the 4p orbitals lie very high in energy. It is less obvious why Sc, T i , and V react with alkenes at such similar rates. Electron spin permits both the sd- and sp-hybridization scheme to operate from the ground states of Sc, T i , and V . Since the promotion energy to low-spin 3d *4s states varies widely from Sc to Ti to V but the rate constants are quite similar, we suggest participation of 4p orbitals in the bonding. The early data indicate that Zr (4d 5s ground state) is much more reactive than its 3d-series congener T i . The energies of corresponding types of terms are very similar in Ti and Zr. Consequently, we suggest that orbital sizes play an important role. In the 4d series, 4d and 5s orbitals have comparable size; in the 3d series, 3d is much smaller than 4s. Repulsive 4s-alkene interactions and bonding 4d-alkene interactions begin at more similar distances, attenuating the steepness of the repulsive surfaces from 4d 5s . In addition, the larger absolute size of 4d compared with 3d ultimately makes stronger chemical bonds due to better spatial overlap. Mo (high-spin, 4d 5s ground state) reacts with alkenes at 300 K , albeit slowly. In contrast, the 3d-series congener Cr (3d 4s*) is inert. The same orbital size considerations come into play comparing Mo to Cr. In addition, electron spin is a poorer quantum number in the 4d series than in the 3d series. This means that crossings between nominal high-spin and low-spin surfaces will be more strongly avoided, making nominal spin-changing events more facile. n2
2
nl
n2
2
nl
n_
2
2
x2
5
2
L
5
Summary and Prognosis At a certain level, we are beginning to understand how the electronic structure of transition metal atoms determines their gas phase chemical reactivity. A recurring theme is the importance of intersections among different kinds of diabatic surfaces arising from the closely-spaced atomic asymptotes. It is really the overall pattern of low-lying states that determines reactivity, because the ground state itself is often ill-prepared to form chemical bonds. Perhaps the presently perceived differences among M , M+, and M + chemistry will someday seem superficial. The current data may show that chemical reactivity is governed by the ability of the metal atom to gain close access to the hydrocarbon, which is determined by the long-range part of the metal-hydrocarbon potential. That is, most of these systems may have lowenergy pathways from M(hydrocarbon) at short range to elimination products, but only some of the systems, primarily those with +1 or +2 charge, see small enough barriers to access those pathways. We have only begun to probe the truly subtle chemical questions involving the geometries and stabilities of the intermediates between F e + C H and FeC H + + C H , for example. Spectroscopic work could prove important here. Finally, we can ask the question whether gas phase work is relevant to solution phase organometallic chemistry. There is an enormous literature of C-H bond activation in solution phase. Typically thermolysis or photolysis initiates 2
+
3
8
2
4
4
16.
WEISSHAAR
Reactions of Gas-Phase Transition-Metal Atoms
F I G U R E 3. One-dimensional model of M2+ + alkane reactions, showing the radii of the electron transfer (r ) and hydride transfer (r*) curve crossings. 1
Ni ( 3 d 9 4 s V D ) + C H 2
4
XT Ni ( 3 d 8 4 s 2 , 3 F ) Ν ΐ Ο , Η ^ )
+
F I G U R E 4. Schematic diabatic potential energy curves describing the approach of neutral Fe, Co, or N i to an alkene. A barrier occurs on the lowest-energy adiabatic surface due to the avoided crossing of the curves.
217
218
LASER CHEMISTRY O F ORGANOMETALLICS
the reaction by creating a coordinatively unsaturated metal center. Examples include complexes of Ru and Rh from the 4d series and of Ir and Pt from the 5d series. This is reminiscent of the list of M+ cations that can dehydrogenate methane at low collision energy (Rxn. 3). The same metals would make a good list of heterogeneous catalysts as well. To some extent the highly active metals remember who they are, whether surrounded by vacuum, ligands and solvent, or other metal atoms! Acknowledgments. I thank the National Science Foundation and the Donors of the Petroleum Research Foundation for continuing support of our research on the structure and reactivity of transition metal species. M y former and current graduate students Dr. Lary Sanders, Dr. Russell Tonkyn, Dr. David Ritter, Dr. Scott Hanton, Dr. Andrew Sappey, M r . Robert J. Noll, and M r . John Carroll deserve most of the credit for the progress we have made.
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