Vladimir Baranov, Hansju1rgen Becker, and Diethard K. Bohme*

Transcription

1 J. Phys. Chem. A 1997, 101, Intrinsic Coorination Properties of Iron: Gas-Phase Ligation of Groun-State Fe + with Alkanes, Alkenes, an Alkynes an Intramolecular Interligan Interactions Meiate by Fe + Vlaimir Baranov, Hansju1rgen Becker, an Diethar K. Bohme* Department of Chemistry an Centre for Research in Earth an Space Science, York UniVersity, North York, Ontario, Canaa M3J 1P3 ReceiVe: January 13, 1997; In Final Form: May 6, 1997 X The kinetics an moe of coorination of the electronic groun state Fe + ( 6 D) have been investigate in the gas phase with the organic molecules methane, ethane, propane, butane, ethylene, allene, propene, 1,3-butaiene, isobutene, acetylene, propyne, an iacetylene. Reaction rate coefficients an prouct istributions for sequential ligation were measure with the selecte-ion flow tube (SIFT) technique operating at 294 ( 3K an a helium buffer-gas pressure of 0.35 ( 0.01 Torr. Also, bon connectivities in the ligate species were probe with multicollision-inuce issociation (CID) experiments. Rates of ligation with a single ligan were foun to increase with an increasing number of egrees of freeom, or size, of the ligan an to follow the reactivity orer alkynes > alkenes > alkanes. Ligation with at least two, at least three, an at least five molecules was observe with alkanes, alkenes an alkynes, respectively. Possible moes of boning in the multiply-ligate Fe + cations are briefly escribe. The CID results provie evience for the occurrence of intramolecular interactions between ligans meiate by Fe +, resulting in C-C bon formation in the ligate ions Fe(1,3-C 4 H 6 ) 4+, Fe(C 2 H 2 ) 3+ an Fe(C 2 H 2 ) 5+, Fe(CH 3 C 2 H) 2+ an Fe(CH 3 C 2 H) 4+, an Fe(C 4 H 2 ) 2+ an Fe(C 4 H 2 ) 4+. The postulate interligan interactions are attribute to cyclization or oligomerization reactions leaing to the formation of benzene, imethylcyclobutaiene, an iethynylcyclobutaiene in Fe(C 2 H 2 ) 3+, Fe(CH 3 C 2 H) 2+, an Fe(C 4 H 2 ) 2+, respectively, an the formation of a imer of 1,3-butaiene in Fe(1,3-C 4 H 6 ) 4+. Introuction The organic chemistry of transition-metal ions in the gas phase has been actively investigate in the past, both experimentally an theoretically. 1 Previous stuies involving Fe + an small hyrocarbons have focuse on the thermoynamic, structural, an electronic properties of the boning of Fe + to single ligan molecules with a view to eluciating mechanistic an energetic aspects of C-C an C-H bon activation an insertion important in catalysis an synthetic organometallic chemistry. The small hyrocarbons which have receive attention inclue methane, 2 ethane, 3 propane, 4 butane, 4b ethylene, 5 acetylene, 6 propene, 5a,7 allene, 8 propyne, 8 an butenes. 5a Here we focus on the experimental measurement of the gasphase, room-temperature rates of both single an multiple ligation of Fe + with these an other relate hyrocarbons uner multicollision conitions an on the experimental etermination of bon connectivities in the resulting singly- an multiplyligate species. Such gas-phase measurements provie a useful benchmark for solution behavior an insight into intrinsic aspects of ligation not accessible in solution. Experimental investigations of the room-temperature kinetics of Fe + ligation have not been very common in the past, although these too can provie insights into thermoynamic aspects of Fe + -ligan boning. Furthermore, the earlier investigations have been restricte to single-ligation kinetics. Fast-flow reactor, 9 crosse-beam, 4b chromatographic, 10 an FT-ICR 11 experiments have shown that methane, ethane, an propane will ligate Fe + at room temperature in He bath gas by collisionstabilize attachment with measurable rates but that ligation Fakultät Umweltwissenschaften un Verfahrenstechnik, Branenburgische Technische Universität Cottbus, Karl-Marx-Strasse 17, D Cottbus, Germany. * Corresponing author. X Abstract publishe in AVance ACS Abstracts, June 15, S (97) CCC: $14.00 competes with activation/insertion leaing to issociative aition with propane an butane, particularly at higher collision energies an lower pressures. Slow ligation of Fe + with ethylene, propene, an isobutene has been observe at low pressures. 5a Similar measurements with the remaining alkenes an the alkynes investigate in this stuy appear not to have been reporte previously. A single potential-well moel has been presente for reactions of Fe + with small alkanes which eluciates the critical role of the lifetime of the intermeiate auct ion in etermining the rates of ligation observe in a multicollision environment. 9b The moel preicts that the rate of ligation epens on the size of hyrocarbon ligan an on the thermoynamic stability of the ligate FeL + ion (where L is the ligan). Multiple ligation of Fe + with hyrocarbons in the gas phase may be achieve in at least two ways: by sequential termolecular ligation reactions of type 1 beginning with unligate Fe + or by sequential bimolecular switching reactions of type 2 beginning with Fe + alreay ligate, but with a ifferent ligan. FeL n + + L + He f FeL n He (1) FeL n L m + L f FeL n-1 L m+1 + L (2) Sequential ligation of type 1 was first observe in fast-flow reactor experiments which provie evience for the multiple ligation of Fe + with methane, ethane, an propane in helium bath gas at 0.75 Torr, although rate coefficients were not etermine. Not much is known quantitatively about the sequential kinetics of either reactions 1 or reactions 2, but their occurrence has been exploite in the past in the generation of multiply-ligate FeL + n ions for stuies of their energetics an structures. For example, Fe(CH 4 ) + n ions with n ) 1-4 have been generate from reactions of type 1 in collision-inuce 1997 American Chemical Society

2 5138 J. Phys. Chem. A, Vol. 101, No. 28, 1997 Baranov et al. issociation stuies of the ligan bining energies in these ions, 2a an Fe(CO) + n ions have been transforme into Fe(C 2 H 4 ) + an Fe(C 2 H 2 ) + n (n ) 1-4) ions accoring to reaction 2 in a chemical ionization source in recent NRMS experiments irecte towar the eluciation of the structures of these ions. 5c,10 The latter stuy 10 also provie the first evience for the Fe + -meiate trimerization of C 2 H 2 to benzene. This interesting result suggests a potential role of Fe + in promoting C-C bon formation an in the meiation of ligan/ligan reactions generally. In this stuy we have surveye the interactions of Fe + with selecte alkanes, alkenes, an alkynes in a multicollision environment. Two experimental approaches were use. Sequential aition of ligans to Fe + was explore through measurements of rate coefficients for reactions of type 1 in helium bath gas at 0.35 Torr using the selecte-ion flow tube (SIFT) technique. This is a moerately high-pressure technique which provies the multiple collisions require for the thirboy stabilization of the ligate ions FeL + n ions. The results of these measurements provie insight into the epenence of rates of ligation on the number an size of ligans an on the thermoynamic stability of the ligate ion. Also, they provie intrinsic reactivities an coorination numbers which are useful in unerstaning analogous coorination reactions in solution. Also, we have conucte multicollision-inuce issociation (CID) experiments to explore bon connectivities in the ligate ions irectly after their formation an collisional stabilization. These experiments have reveale the occurrence of several novel intramolecular interligan reactions an speak to the possible role of Fe + as a catalyst for cyclization an oligomerization of unsaturate hyrocarbons. Experimental Section The results reporte here were obtaine using a selecte-ion flow tube (SIFT) apparatus which has been escribe previously. 13,14 All measurements were performe at 294 ( 3K an at helium buffer gas pressure of 0.35 ( 0.01 Torr. The reactant Fe + ions were prouce in a low-pressure ionization source either from Fe(CO) 5 by ev electron bombarment or from ferrocene vapor at ev, mass selecte, injecte into the flow tube, an allowe to thermalize by collisions (ca ) with He atoms before entering the reaction region. Both Fe(CO) 5 an ferrocene were introuce into the ion source in a large excess of helium (at a partial pressure of less than 5%). The ion signal showe a maximum with increasing pressure which is suggestive of ion/he collisions within the source an the occurrence of issociative electron-transfer reactions of He + with the parent gas. Normally the ion signal was tune at the maximum. As we have reporte elsewhere, we coul not fin any evience for the presence of excite states of Fe + in our reacting Fe + population. 15 Reactant neutrals were introuce into the reaction region either as a pure gas or as a ilute (0.2-5%) mixture in helium. All the reagents except iacetylene were obtaine commercially an were of high purity (generally >99%). Diacetylene was synthesize by reacting 1,4-ichloro-2-butyne with aqueous KOH in DMSO solution. 16 The rate coefficients for primary reactions reporte here are estimate to have an uncertainty of (30%. Higher-orer rate coefficients were obtaine by fitting the experimental ata to the solutions of the system of ifferential equations for successive reactions. The accuracy for this fitting proceure epens on several parameters an is reporte separately for every calculate high-orer rate coefficient. The multicollision-inuce issociation (CID) of sample ions was investigate by raising the potential of the sampling nose cone while taking care not to introuce mass iscrimination in the etection system. This technique has been evelope in our laboratory recently an is escribe in etail elsewhere. 17 The technique is most useful for the exploration of bon connectivities in that ion fragmentation can be inuce in a controlle stepwise fashion. However, since the concomitant energy reisposition has not yet been successfully moele, the observe threshols for issociation o not yiel precise thermoynamic information. Bon connectivity stuies become problematic when the ion of interest is one of several ions being sample, viz. cannot be establishe as the preominant ion, or when it has too low an intensity. Results an Discussion Table 1 summarizes the proucts an rate coefficients measure for the primary an higher-orer reactions of Fe + with selecte alkanes, alkenes, an alkynes at 294 ( 3 K in helium buffer gas at a total gas pressure of 0.35 ( 0.01 Torr. A. Reactions with Alkanes: CH 4,C 2 H 6,C 3 H 8, an C 4 H 10. The effective bimolecular rate coefficients measure for the association reactions of Fe + with alkanes uner our SIFT conitions span a large range from < cm 3 molecule -1 s -1 for the reaction with methane to cm 3 molecule -1 s -1 for the reaction with n-butane. Representative ata for the observe chemistry an CID are shown in Figures 1-3. With the exception of the reaction with n-butane, only auct formation was observe uner our operating conitions. The failure to observe bimolecular proucts with methane an ethane is not surprising since these shoul be enothermic. However, bimolecular channels o become exothermic with propane an higher saturate hyrocarbons. Inee, reactions 3a an 3b Fe + + C 3 H 8 f FeC 3 H H 2 f FeC 2 H CH 4 (3a) (3b) which are exothermic by 11 ( 5 an 19 ( 5 kcal mol -1, respectively, 4b have been observe as minor channels with both the flow reactor in He at 0.75 Torr 9b an the chromatographic technique in He at 1.75 Torr. 10 The fast-flow reactor measurements yiele branching ratios of 2.0 ( 0.4 an 4.3 ( 0.3%, respectively, while the chromatographic technique le to branching ratios of 1.4 an 3%, respectively. However, our experiments inicate an upper limit of 1% for both of these channels an that collisional stabilization completely ominates. The reason for this iscrepancy is not clear. One possible explanation is that the previous measurements, in which Fe + ions were prouce ifferently, inclue some contribution from the excite 4 F state of Fe + : both channels 3a an 3b are major channels in reactions with this state. 4b,10 It is interesting to note that, at the very low pressures of Torr at which collisional stabilization is improbable, FT-ICR experiments have shown that Fe + reacts with propane exclusively to yiel the bimolecular channels 3a an 3b in a ratio of 24/ Bimolecular channels efinitely become competitive uner our experimental operating conitions in the reaction of Fe + with n-butane. The ata in Figure 3 shows the occurrence of the reaction channels 4a-4. Analysis of this ata provies a Fe + + n-c 4 H 10 f FeC 4 H 10 + f FeC 2 H C 2 H 6 f FeC 3 H CH 4 f FeC 4 H H 2 (4a) (4b) (4c) (4)

3 Intrinsic Coorination Properties of Iron J. Phys. Chem. A, Vol. 101, No. 28, TABLE 1: Measure Proucts an Rate Coefficients for Reactions of Groun-State Fe + Cations with Selecte Alkanes, Alkenes, an Alkynes at 294 ( 3 K in Helium Buffer Gas at a Total Pressure of 0.35 ( 0.01 Torr; Reaction a an Collision b Rate Coefficients Are Given in Units of cm 3 molecules -1 s -1 reactant molecule reactant/prouct ions c k exp k c k exp/k c CH 4 Fe + /Fe(CH 4) + < < Fe(CH 4) + / C 2H 6 Fe + /Fe(C 2H 6) Fe(C 2H 6) + /Fe(C 2H 6) Fe(C 2H 6) + 2 < C 3H 8 Fe + /Fe(C 3H 8) Fe(C 3H 8) + /Fe(C 3H 8) Fe(C 3H 8) + 2 < C 4H 10 Fe + / Fe(C 4H 10) Fe + /Fe(C 2H 4+ ) 0.38 Fe + /Fe(C 3H 6+ ) 0.13 Fe + /Fe(C 4H 8+ ) 0.09 Fe(C 4H 10) + /Fe(C 4H 10) Fe(C 4H 10) + 2 < C 2H 4 Fe + /Fe(C 2H 4) Fe(C 2H 4) + /Fe(C 2H 4) Fe(C 2H 4) 2+ /Fe(C 2H 4) Fe(C 2H 4) 3+ /Fe(C 2H 4) + 4 (3.0 ( 1.5) Fe(C 2H 4) 4+ / C 3H 4 (allene) Fe + /Fe(C 3H 4) Fe(C 3H 4) + /Fe(C 3H 4) Fe(C 3H 4) 2+ /Fe(C 3H 4) Fe(C 3H 4) + 3 < C 3H 6 (propene) Fe + /Fe(C 3H 6) Fe(C 3H 6) + /Fe(C 3H 6) Fe(C 3H 6) 2+ /Fe(C 3H 6) Fe(C 3H 6) + 3 < C 4H 6 (1,3-butaiene) Fe + /Fe(C 3H 6) Fe(C 3H 6) + /Fe(C 3H 6) Fe(C 3H 6) 2+ /Fe(C 3H 6) Fe(C 3H 6) 3+ /Fe(C 3H 6) + 4 e Fe(C 3H 6) 4+ / i-c 4H 8 (isobutene) Fe + /Fe(C 4H 8) Fe(C 4H 8) + /Fe(C 4H 8) Fe(C 4H 8) 2+ /Fe(C 4H 8) Fe(C 4H 8) + 3 < C 2H 2 Fe + /Fe(C 2H 2) Fe(C 2H 2) + /Fe(C 2H 2) Fe(C 2H 2) 2+ /Fe(C 2H 2) Fe(C 2H 2) 3+ /Fe(C 2H 2) + 4 (2 ( 1) Fe(C 2H 2) 4+ /Fe(C 2H 2) + 5 (5 ( 2) Fe(C 2H 2) 5+ /Fe(C 2H 2) + 6 e Fe(C 2H 2) 6+ / C 3H 4 (propyne) Fe + /Fe(C 3H 4) Fe(C 3H 4) + /Fe(C 3H 4) Fe(C 3H 4) 2+ /Fe(C 3H 4) Fe(C 3H 4) 3+ /Fe(C 3H 4) + 4 (6 ( 3) Fe(C 3H 4) 4+ /Fe(C 3H 4) + 5 (7 ( 4) Fe(C 3H 4) 5+ /Fe(C 3H 4) + 6 e Fe(C 3H 4) 6+ / C 4H 2 (iacetylene) Fe + /Fe(C 4H 2) Fe(C 4H 2) + /Fe(C 4H 2) Fe(C 4H 2) 2+ /Fe(C 4H 2) Fe(C 4H 2) 3+ /Fe(C 4H 2) + 4 (9 ( 5) Fe(C 4H 2) 4+ /Fe(C 4H 2) + 5 (1.0 ( 0.5) Fe(C 4H 2) 5+ /Fe(C 4H 2) + 6 e Fe(C 4H 2) 6+ / a The uncertainty in the reaction rate coefficient is less than 30%, unless inicate otherwise. b The collision rate coefficient is calculate using ADO theory. 16 c The branching ratios are inicate for the reaction with n-butane. Proucts which were observe not to be forme are not inicate. Not observe. e Observe. branching ratio for (4a)/(4b)/(4c)/(4) of 0.40/0.38/0.13/0.09. The bimolecular channels outweigh the association channel by 3 to 2. The branching ratio of the bimolecular channels (4b)/ (4c)/(4c) of 0.67/0.22/0.15 is consistent with the tren reporte recently for the prouction of these channels in crosse-beam experiments at kinetic energies of 1.1 an 0.25 ev. 4b The reporte branching ratios for the reaction of groun-state Fe + - ( 6 D) at these kinetic energies were 0.39/0.43/0.18 an 0.58/ 0.32/0.10, respectively. We i not completely unravel the seconary chemistry evient in Figure 3, although it is clear that the FeC 4 H 8 + prouct ion is less reactive, k ) (5.5 ( 1.7) cm 3 molecule -1 s -1, than the other three prouct ions of reaction 4 which all react further with an ientical rate coefficient, within experimental uncertainty, of (7.8 ( 2.3) cm 3 molecule -1 s -1. The prouct spectrum for the seconary reactions shown in Figure 3 is consistent with n-butane auct formation involving these three prouct ions accompanie by varying egrees of elimination of C 2 H 6,CH 4, an H 2. The reactions of Fe + with methane, ethane, an propane have been investigate previously at thermal energies with a fast flow

4 5140 J. Phys. Chem. A, Vol. 101, No. 28, 1997 Baranov et al. Figure 1. (left) Experimental ata for the reaction of Fe + with ethane. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(C 2H 6) 2+ in helium at 0.35 Torr in the laboratory energy frame. The ethane flow is equal to molecule s -1. Figure 3. (left) Experimental ata for the primary reactions of Fe + with butane. (right) Experimental ata for the seconary reactions of Fe + with butane. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. Figure 2. (left) Experimental ata for the reaction of Fe + with propane. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(C 3H 8) 2+ in helium at 0.35 Torr in the laboratory energy frame. The propane flow is equal to molecule s -1. reactor at the higher helium pressure of 0.75 Torr. Rate coefficients of (1.1 ( 0.3) 10-12, (5.9 ( 1.8) 10-11, an (6.2 ( 1.9) cm 3 molecule -1 s -1 have been reporte for these conitions. 9b A rate coefficient of (5.0 ( 1.5) cm 3 molecule -1 s -1 has been reporte for the aition of propane at the still higher helium pressure of 1.75 Torr at 300 K. 10 These values are all systematically higher than those measure in our stuy an clearly point towar a epenence of the reaction rate on pressure as is expecte if the ligation procees by termolecular association with collisional stabilization. An appropriate kinetic moel for thir-boy collisional stabilization in reactions of Fe + with small alkanes has been iscusse in etail by Weisshaar et al. 9b The moel preicts that the effective bimolecular rate coefficient shoul increase with increasing pressure an saturate at a value equal to the Langevin collision rate coefficient. The tren in k exp with pressure is clearly evient in Figure 4 in which the size of the alkane is expresse in terms of its number of egrees of freeom. The tren in the effective bimolecular rate coefficient with the size of the alkane will be iscusse later. What can be sai about the number of alkane ligans that a sequentially to Fe +? Only the first auct was observe Figure 4. Variation in the effective bimolecular rate coefficient for the reaction of Fe + with alkanes with the size of the alkane expresse in terms of its number of egrees of freeom (N is the number of atoms). Open circles represent the ata of Weisshaar et al. 9b taken with a fastflow reactor at a helium pressure of 0.75 Torr. The soli circles represent the SIFT ata obtaine in this stuy at a helium pressure of 0.35 ( 0.01 Torr. The ashe curve represents the variation of the bimolecular Langevin collision rate constant. when Fe + reacte with methane uner our SIFT conitions, an it was forme with an immeasurably small rate coefficient. In previous measurements with a fast-flow reactor at the higher helium pressure of 0.75 Torr, up to three aitions were observe, although rate coefficients were not reporte for aitions of more than one methane molecule. 9b Fe(CH 4 ) + n ions with n as large as 4 have been generate in a c ischarge/flow tube source. 2a Our SIFT experiments have shown that two molecules of ethane an propane rapily a sequentially to Fe + an in both cases the secon molecule as with a higher rate: about 25 times higher in the case of ethane an 2 times higher in the case of propane. The rate of ligation rops precipitously for the formation of the thir auct, by a factor of at least in both cases, an the thir auct was not observe. These results agree with those obtaine using the fast-flow reactor which inicate that two, but not three, molecules of ethane

5 Intrinsic Coorination Properties of Iron J. Phys. Chem. A, Vol. 101, No. 28, Figure 5. (left) Experimental ata for the reaction of Fe + with ethylene. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(C 2H 4) 3+ in helium at 0.35 Torr in the laboratory energy frame. The ethylene flow is equal to molecule s -1. Figure 7. (left) Experimental ata for the reaction of Fe + with propene. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(C 3H 6) 3+ in helium at 0.35 Torr in the laboratory energy frame. The propene flow is equal to molecule s -1. Figure 6. (left) Experimental ata for the reaction of Fe + with allene. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(C 3H 4) 3+ in helium at 0.35 Torr in the laboratory energy frame. The allene flow is equal to molecule s -1. an two molecules of propane ae sequentially at the higher He pressure of 0.75 Torr; again however, rate coefficients for the aition of more than one molecule were not reporte for these latter experiments. 9b Our results with butane also inicate rapi sequential aition of only two molecules. The thir aition was immeasurably slow, an no thir auct was observe. The CID profiles in Figures 1 an 2 show clearly that ligate ethane molecules an ligate propane molecules o not react with each other in the presence of Fe +. Collisional activation of the ligate ions removes these molecules sequentially one at a time in a manner reverse to the sequential aition. We take our kinetic an CID results to inicate that Fe + has a coorination number of 2 with the alkane molecules ethane, propane, an butane. B. Reactions with Alkenes: C 2 H 4,C 3 H 6,H 2 CCCH 2, 1,3-C 4 H 6, an i-c 4 H 8. To the extent that stanar enthalpies of formation are available, thermoynamics preicts that bimolecular reactions, incluing electron transfer, are energetically unfavorable with ethylene, propene, allene, 1,3-butaiene, an isobutene. Only sequentail ligation reactions were observe Figure 8. (left) Experimental ata for the reaction of Fe + with 1,3- butaiene. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(1,3-C 4H 6) n+ with n ) 2, 3, an 4 in helium at 0.35 Torr in the laboratory energy frame. The flow of 1,3-butaiene is equal to molecule s -1. uner our operating conitions with these five alkenes. Representative ata are shown in Figures 5-9. Again, the effective bimolecular rate coefficient for the first aition increases substantially with increasing size of the alkene, viz. from cm 3 molecule -1 s -1 for the reaction with ethylene to cm 3 molecule -1 s -1 for the reaction with isobutene. The number of alkene molecules observe to a sequentially to Fe + was at least three, but four molecules were seen to a with ethylene an 1,3-butaiene. A rop in rate by at least a factor of 10 2 was observe after the aition of three molecules except with the larger alkenes, 1,3-butaiene an isobutene, for which such a rop occurre alreay after the aition of two molecules. The CID experiments inicate sequential loss of iniviual molecules from all triply-ligate species. However, Figure 8 clearly shows that the quaruply-ligate species Fe- (1,3-C 4 H 6 ) 4 + issociates in one step exclusively with the loss of the equivalent of two molecules of 1,3-butaiene accoring to reaction 5. Fe(1,3-C 4 H 6 ) He f Fe(1,3-C 4 H 6 ) [(1,3-C 4 H 6 ) 2 ] + He (5)

6 5142 J. Phys. Chem. A, Vol. 101, No. 28, 1997 Baranov et al. Figure 9. (left) Experimental ata for the reaction of Fe + with isobutene. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(i-C 4H 8) 3+ in helium at 0.35 Torr in the laboratory energy frame. The isobutene flow is equal to molecule s -1. Figure 11. (left) Experimental ata for the reaction of Fe + with propyne. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(C 3H 4) n+ with n ) 2, 3, an 4 in helium at 0.35 Torr in the laboratory energy frame. The flow of propyne is equal to molecule s -1. Figure 10. (left) Experimental ata for the reaction of Fe + with acetylene. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(C 2H 2) n+ with n ) 3, 4, an 5 in helium at 0.35 Torr in the laboratory energy frame. The flow of acetylene is equal to molecule s -1. C. Reactions with Alkynes: C 2 H 2,CH 3 C 2 H, an C 4 H 2. Only sequential ligation was observe in the reactions of Fe + with acetylene, propyne, an iacetylene. Representative ata are shown in Figures The measure effective bimolecular rate coefficient for the first aition increases from cm 3 molecule -1 s -1 for the aition of C 2 H 2 to an cm 3 molecule -1 s -1 for the first aition of C 4 H 2 an CH 3 C 2 H, respectively. At least six alkyne molecules were observe to a sequentially to Fe +, but as was the case with the alkenes, a sharp rop in rate (by at least a factor of 30 in this case) was observe after the aition of three molecules. But the rate coefficient i not become immeasurably small. Two further aitions were observe with measurable rates, an the aition of another acetylene to form Fe(C 2 H 2 ) + 6 was also recore. The CID experiments inicate a variety of ifferent types of behavior for the issociation of alkyne-ligate Fe + species. Loss of iniviual molecules was observe to be less common than for the alkane- an alkene-ligate species. Figure 10 shows that the triply-ligate species Fe(C 2 H 2 ) + 3 issociates in one step exclusively with the loss of the equivalent of three molecules Figure 12. (left) Experimental ata for the reaction of Fe + with iacetylene. The soli lines represent a fit of the experimental ata with the solutions of ifferential equations appropriate for the observe sequential reactions. (right) Multicollisional CID of Fe(C 4H 2) n+ with n ) 3 an 4 in helium at 0.35 Torr in the laboratory energy frame. The flow of iacetylene is equal to molecule s -1. Fe(C 2 H 2 ) He f Fe + + [(C 2 H 2 ) 3 ] + He (6) of acetylene accoring to reaction 6 an that the quintuplyligate Fe(C 2 H 2 ) + 5 issociates in one step to lose the equivalent of two molecules of acteylene accoring to reaction 7. In Fe(C 2 H 2 ) He f Fe(C 2 H 2 ) [(C 2 H 2 ) 2 ] + He (7) comparison, Figure 11 shows that the oubly-ligate propyne species loses the equivalent of two molecules upon issociation accoring to reaction 8 an that the quaruply-ligate species Fe(C 3 H 4 ) He f Fe + + [(C 3 H 4 ) 2 ] + He (8) also loses the equivalent of two molecules of propyne accoring to reaction 9. Although the loss of just one molecule of propyne Fe(C 3 H 4 ) He f Fe(C 3 H 4 ) [(C 3 H 4 ) 2 ] + He (9) coul not be completely exclue on the basis of the shape of the Fe(C 3 H 4 ) 3 + CID profile, the earlier onset for the issociation

7 Intrinsic Coorination Properties of Iron J. Phys. Chem. A, Vol. 101, No. 28, Figure 13. A semilogarithmic correlation of the effective bimolecular rate coefficient in the gas phase in helium buffer gas at 294 ( 3 K an a total pressure of 0.35 ( 0.01 Torr for the single ligation of Fe + with hyrocarbon molecules with the size of the hyrocarbon expresse in terms of its number of egrees of freeom (N is the number of atoms). Soli circles represent alkanes, soli squares represent alkenes, an soli triangles represent alkynes. of Fe(C 3 H 4 ) 3 + implies a greater bining energy for [(C 3 H 4 ) 2 ] within Fe(C 3 H 4 ) 4 + which is more consistent with the occurrence of reaction 9. Figure 12 shows that the issociation of both the oubly-ligate an quaruply-ligate iacetylene species leas to loss of two molecules accoring to reactions 10 an Fe(C 4 H 2 ) He f Fe + + [(C 4 H 2 ) 2 ] + He (10) Fe(C 4 H 2 ) He f Fe(C 4 H 2 ) [(C 4 H 2 ) 2 ] + He (11) 11. The same orer in the issociation threshol, viz. DT- ((C 4 H 2 ) 2 Fe + -C 4 H 2 ) < DT((C 4 H 2 ) 2 Fe + -[(C 4 H 2 ) 2 ]), is observe in this case. D. Variation in the Rate of Ligation with the Size of the Hyrocarbon. Figure 13 presents the epenence of the effective bimolecular rate coefficient, k exp, for all of the Fe + aition reactions observe in this stuy in helium at 0.35 Torr on the number of egrees of freeom of the hyrocarbon molecule, 3N - 6 (where N is the number of atoms). There is an obvious increase in reactivity with increasing size for the alkanes, alkenes, an alkynes, an the rate coefficients begin to saturate at a value corresponing to the collision rate coefficient, about 10-9 cm 3 molecule -1 s -1, with the highest member of each series of hyrocarbons. Also, the relative magnitues of the rate coefficients at a fixe number of egrees of freeom show the reactivity orer alkynes > alkenes > alkanes. These trens in kinetics can be unerstoo in terms of the single-potential well moel for collisional association presente previously by Weisshaar et al. for Fe + -alkane ligation in which variations in rate coefficients are mainly attribute to variations in the lifetime of the intermeiate hot auct against reissociation. 9b This lifetime increases as the vibrational-state ensity of the hot auct increases an is epenent irectly on the vibrational egrees of freeom an the bon energy (or well epth) of the auct. Thus, the tren in kinetics observe for alkane ligation is ue both to the increasing number of egrees of freeom an the increasing bon energies of the auct ions. It is known that the bon strength increases from 0.59 ( 0.03 ev for Fe + -CH 4, 2a to 0.66 ( 0.06 ev for Fe + - C 2 H 3a 6 an 0.78 ( 0.04 ev for Fe + -C 3 H 8. 4a Differences in rate coefficients at a fixe egree of freeom must be attribute largely to ifferences in bining energies as, for example, the ifference observe for the reactions of Fe + with allene an propyne. The bon energies for Fe + -allene an Fe + -propyne are not known, but our results preict a stronger interaction in Figure 14. A semilogarithmic correlation of the effective bimolecular rate coefficient for the sequential ligation of Fe + with hyrocarbon molecules with the number of ligans ae in the gas phase in helium buffer gas at 294 ( 3 K an a total pressure of 0.35 ( 0.01 Torr: left, alkanes; mile, alkenes; right, alkynes. the case of Fe + -propyne. In comparing alkanes, alkenes, an alkynes with the same carbon content such as, for example, ethane, ethylene, an acetylene, one must consier both egrees of freeom an bon energies. Thus, while the orer in bon energies is D 0 (Fe + -C 2 H 4 ) > D e (Fe + -C 2 H 2 ) > D 0 (Fe + -C 2 H 6 ) (1.50 ( > >0.66 ( 0.06 ev 3a ) an the orer in the number of egrees of freeom is Fe + -C 2 H 6 > Fe + -C 2 H 4 > Fe + -C 2 H 2, the orer in the effective bimolecular rate coefficient for ligation is k(c 2 H 4 ) > k(c 2 H 2 ) > k(c 2 H 6 ) ( > > cm 3 molecule -1 s -1 ). E. Variation in the Rate of Ligation with the Number of Ligans. The moerate helium bath-gas pressure of 0.35 Torr employe in these experiments is sufficient to allow for collisional stabilization of the hot intermeiate ligate ions an so to probe the full extent of ligation, or coorination, of Fe + with the various hyrocarbon ligans investigate. Here the coorination number is efine in terms of the observe ligation kinetics. It is taken to be equal to the number of ligans ae sequentially to Fe + before the occurrence of a sharp rop in the rate of ligation. A rop is consiere to be sharp if the measure rate coefficient for ligation changes by 2 or more orers of magnitue. Such a rop was observe for all of the ligans investigate. This is clearly evient from Figure 14, which shows that the coorination number observe with the alkanes, alkenes, an alkynes investigate in this stuy epens on the nature of the hyrocarbon an occasionally on its size. Thus, the coorination number for all alkanes was observe to be 2, for alkenes to be 3 (or 2 for the larger alkenes 1,3-butaiene an isobutene), an for the alkynes to be 3 (but perhaps only 2 for iacetylene). Also higher orer coorination was observe with the alkynes leaing to seconary coorination number of at least 5. A common feature of the early ligation kinetics is an increase in the rate coefficient of ligation from the first to the secon aition. This increase is particularly large (over 1 orer of magnitue) for the small ligans ethane, ethylene, an acetylene. The ecrease in the rate coefficient for the first two reactions with butane inclues the influence of the occurrence of bimolecular prouct channels in the first reaction with butane. F. Structures an Boning in Ligate Fe +. Fe(alkane) + n. There has been consierable iscussion in the literature of the possible structures an boning of Fe + ligate with a single alkane (HR) molecule incluing cluster ions Fe + (alkane), the C-H bon insertion auct H-Fe + -R, the C-C bon insertion auct CH 3 -Fe + -R, rearrange species of the type Fe + (H 2 )- (alkene) an brige structures of the type Fe + H R. 9b The

8 5144 J. Phys. Chem. A, Vol. 101, No. 28, 1997 Baranov et al. Figure 15. Possible structures an profiles of potential energy vs reaction coorinate for the single an ouble ligation of Fe + with ethane. Structures an energies (in kcal mol -1 ) for single ligation are taken from ref 3c. boning has been treate qualitatively in terms of long-range electrostatic an shorter-range onor-acceptor an chemical attractive forces as well as long-range electron-electron repulsion. The boning in Fe + multiply ligate with alkanes also has been aresse, but only very briefly. 9b The orer in bon issociation energy BDE(Fe + -CH 4 ) < BDE((CH 4 )Fe + -CH 4 ) = BDE((CH 4 ) 2 Fe + -CH 4 ) > BDE((CH 4 ) 3 Fe + -CH 4 ) has been rationalize in terms of spin changes in the core Fe + configuration. 2a The first comprehensive quantum-chemical investigation (a DFT/HF hybri approach) of an FeC n H + 2n+2 cation has appeare only very recently for the interaction of Fe + with ethane. 3b,c The results of these investigation inicate boun states in the potential energy surface involving electrostatic boning, C-C bon insertion an C-H bon insertion. All three states an the connecting transition states lie below the initial energy of Fe + an ethane. Since the PE minimum for H-Fe + -C 2 H 5 is extremely shallow (1 kcal mol -1 ) an only 9 kcal mol -1 below the energy of the separate reactants, we can suggest that the singly-ligate FeC 2 H + 6 species forme uner our operating conitions is likely to be boun as either the electrostatic auct or the C-C insertion auct as inicate in Figure 15 or as a mixture of the two. It follows that a secon molecule of ethane may then bon in an analogous fashion to form the combine electrostatic/inserte species (C 2 H 6 )Fe(CH 3 ) + 2 with a structure also shown in Figure 15. Clearly, coorination with six hyrogen atoms to form an electrostatically-boun sanwich, Fe + (C 2 H 6 ) 2, is not feasible. Also, the oubly-inserte species, Fe(CH 3 ) + 4, is unlikely because there are not enough unpaire electrons to form four covalent bons. The further aition of a thir molecule of ethane to the electronically saturate oubly-ligate (C 2 H 6 )Fe(CH 3 ) + 2 is unlikely for electronic an probably also steric reasons, an this woul explain the sharp rop observe for the rate of ligation after the aition of two molecules of ethane. Our CID measurements suggest that the secon molecule of ethane is weakly boun when comparing the threshol for its issociation with the issociation threshols of other ligate ions. 17 Quantum-chemical investigations have not been reporte for propane or butane, but the Fe + /ethane system may well be representative of Fe + /alkane systems generally. Schematic potential energy curves have been propose for the ligation of Fe + with a single propane molecule but largely with a view to rationalizing H 2 an CH 4 elimination channels via C-H activation an insertion. 4b,c,10 One of these moels proposes a barrier to formation of the C-C insertion auct which lies above the energy of the separate reactants, but this suggestion has not been confirme by quantum-chemical calculations. 4c Fe(alkene) + n. The boning of alkenes to the Fe + ion has two components: π onation to a σ-like sp n hybri orbital an a back-onation from fille orbitals into the antiboning orbital on the CC ouble bon. For aucts of the type FeC 2 R + 4, incluing the aucts of Fe + with ethylene, propene, an isobutene, such boning leas to a configuration in which the R substituents are expecte to be out of the [Fe,CC] plane. The boning of Fe + to allene shoul be similar. The ligation to each ouble bon shoul be inepenent since the ajacent ouble bons have π electrons locate in two perpenicular planes which cannot be onate to the same center. In contrast, Fe + can ligate concomitantly to both ouble bons in 1,3- butaiene in either a cis or a trans configuration. Quantumchemical calculations for these singly-ligate species are generally not available although calculations have been reporte for FeC 2 H b,c The lowest-energy state for this ion has been preicte to be a 4 B 2 state with D e ) 25.7 kcal mol -1 at the MCPF level of theory 5b while an optimize geometry is available at QCISD(T) level of theory. 5b No quantum-chemical calculations or qualitative potential energy curves are available for higher levels of ligations with any of these hyrocarbons. However, the egree of ligation may be rationalize in terms of consecutive π-electron-pair onation which is etermine by the number of available empty orbitals an so the multiplicity of Fe +. For example, the Fe + - ( 6 D) groun-state sp 3 configuration has only three places available for two-electron onor ligation (the fourth is occupie by one unpaire electron) while the 4 F state allows four twoelectron onors to be ligate. The results in Figure 14, which inicate high rates of aition up to three alkene ligans, are consistent with three two-electron-onor ligation to the Fe + - ( 6 D) groun state or three two-electron-onor ligation to the Fe + ( 4 F) state in which the aition of the fourth ligan is sterically hinere. Some support for the latter moe of ligation comes from the observation of the slow aition of a fourth molecule of ethylene an the alreay relatively slow aition of the thir molecule of isobutene. Another possible moe of boning involves oxiative aition which leas to conventional covalent bons. Six covalent bons (two per ligan) are possible since Fe + has seven available electrons. The remaining one electron cannot form an aitional two bons with a fourth ligan. Ligation of Fe + with 1,3-butaiene is a special case because, as escribe in the next section, it involves intramolecular interligan interactions. Fe(alkyne) + n. The boning of Fe + to alkynes is expecte to be similar to that with alkenes: π onation to a σ-like sp n hybri orbital of Fe + an back-onation from fille orbitals into an antiboning orbital on the CtC unit. The aitional onation of the perpenicular electron pair woul turn alkynes formally into four-electron-onor ligans, but we have nothing to support this moe of ligation. The observe fast aition of the first three ligans is similar to the behavior with alkenes. A theoretical treatment of the boning of Fe + to acetylene inicates electrostatic boning an a ligan geometry very similar to that of free acetylene. 6 No quantum-chemical

9 Intrinsic Coorination Properties of Iron J. Phys. Chem. A, Vol. 101, No. 28, TABLE 2: Collision-Inuce Dissociations Observe for Selecte Ligate Fe + Ions in Which Intramolecular Interactions between Ligans Has Been Propose To Occur reaction threshol energy a Fe(1,3-C 4H 6) 4+ + He f Fe(1,3-C 4H 6) 2+ + [(1,3-C 4H 6) 2] + He ( Fe(C 2H 2) 3+ + He f Fe + + [(C 2H 2) 3] + He 1.64 ( 0.13 Fe(C 2H 2) 5+ + He f Fe(C 2H 2) 3+ + [(C 2H 2) 2] + He ( Fe(CH 3C 2H) 2+ + He f Fe + + [(CH 3C 2H) 2] + He 1.27 ( 0.11 Fe(CH 3C 2H) 4+ + He f Fe(CH 3C 2H) 2+ + [(CH 3C 2H) 2] + He ( Fe(C 4H 2) 2+ + He f Fe + + [(C 4H 2) 2] + He 1.58 ( 0.10 Fe(C 4H 2) 4+ + He f Fe(C 4H 2) 2+ + [(C 4H 2) 2] + He ( a Dissociation threshol in ev (in the center-of-mass energy frame) measure in helium at 0.35 Torr. The quote uncertainty is equal to the stanar eviation in the threshol energy at the junction of the best fit to the initial ion signal an the best fit to the slope of the fastest ecaying portion of the isappearance of the parent ion. calculations or qualitative potential energy curves have been reporte for other alkynes or higher levels of ligations with alkynes. The results in Figure 14 inicate high rates of aition up to three alkyne ligans, which is consistent with three twoelectron-onor ligation to the Fe + ( 6 D) groun state. However, the aition of iacetylene appears to be an exception since a significant rop in rate is observe alreay after the aition of two molecules. Furthermore, the aition of up to six ligans was observe with all three of the alkynes investigate in this stuy, which suggests higher-orer ligation moes absent with the alkenes. Inee, the CID measurements suggest the occurrence of intramolecular interligan interactions meiate by Fe + for all three alkynes. G. Intramolecular Interligan Interactions. Now we consier possible intramolecular interactions between ligans meiate by Fe +. Evience for such interactions comes from the results of the CID experiments. Although in most instances issociation of the ligate species was observe to procee one ligan at a time, this was not the case with Fe(1,3-C 4 H 6 ) + 4 (see Figure 8), Fe(C 2 H 2 ) + 3 an Fe(C 2 H 2 ) + 5 (see Figure 10), Fe- (CH 3 C 2 H) + 2 an Fe(CH 3 C 2 H) + 4 (see Figure 11), an Fe(C 4 H 2 ) + 2 an Fe(C 4 H 2 ) + 4 (see Figure 12) for which observe issociations le to the loss of the equivalent of two or three molecules. The observe issociations threshols for these ions are summarize in Table 2. We propose that in each of these multiply-ligate ions a chemical interaction, meiate by Fe +, occurs intramolecularly between two or more ligans. Thus, we attribute the observe exclusive loss of [(C 2 H 2 ) 3 ] from Fe(C 2 H 2 ) + 3 to the intramolecular isomerization of this ion to Fe(C 6 H 6 ) + with loss of C 6 H 6. Evience for this particular isomerization was sought in separate CID experiments with Fe- (C 6 H 6 ) + prouce irectly from the aition of benzene to Fe + accoring to reaction 12. Figure 16 compares the CID spectra Fe + + C 6 H 6 + He f Fe(C 6 H 6 ) + + He (12) of the Fe(C 2 H 2 ) + 3 an Fe(C 6 H 6 ) + ions prouce in these two ifferent ways uner otherwise similar operating conitions, an clearly there is a match in the issociation threshols measure for these two ions. The actual values obtaine for the issociation threshol (in the center-of-mass energy frame) 17 are 1.64 ( 0.13 an 1.70 ( 0.10 ev, respectively. The similarity in these two values provies strong evience for the occurrence of the isomerization of Fe(C 2 H 2 ) + 3 to Fe(C 6 H 6 ) +. The failure of the Fe(C 2 H 2 ) + 3 ion prouce in reaction 12 to issociate back to the reactants Fe(C 2 H 2 ) C 2 H 2 uner CID conitions inicates that any barrier to isomerization lies below the initial energy of the reactants an that the isomerization is energetically feasible. The Fe(C 6 H 6 ) + isomer must be consierably more stable than the Fe(C 2 H 2 ) + 3 isomer given the high exothermicity for the trimerization of acetylene to benzene (-143 kcal mol -1 ), 19 although not enough thermochemical Figure 16. Multicollisional CID results for Fe(C 2H 2) 3+ an Fe(C 6H 6) + in helium at 0.35 Torr. Fe(C 2H 2) 3+ was forme upstream of the CID region from the sequential ligation of Fe + with acetylene (at a flow of molecule s -1 of acetylene) while Fe(C 6H 6) + was prouce from the ligation of Fe + with a single molecule of benzene (at a flow of molecule s -1 of benzene vapor). information is available to be completely quantitative. Asie from the match in the CID spectra, the substantial magnitue of the threshol observe for the issociation (1.64 ( 0.13 an 1.70 ( 0.10 ev) is consistent with the substantial bining energy of 49.6 ( 2.3 kcal mol -1 etermine experimentally for Fe + - C 6 H Our failure to observe the irect formation of Fe + + C 6 H 6 accoring to reaction 13, which is also expecte to be Fe(C 2 H 2 ) C 2 H 2 f Fe + + C 6 H 6 (13) consierably exothermic, suggests that the bulk of the excess energy associate with the isomerization appears in the C 6 H 6 ligan rather than the Fe + -C 6 H 6 bon. Consequently, we conclue that the thir aition of acetylene to Fe + occurs in the three steps (14a) to (14c). Apparently, the lifetime for isomerization is shorter than the time require for stabilizing collisions with helium. Fe(C 2 H 2 ) C 2 H 2 f (Fe(C 2 H 2 ) 3 + )* (Fe(C 2 H 2 ) 3 + )* f (Fe(C 6 H 6 ) + )* (Fe(C 6 H 6 ) + )* + He f Fe(C 6 H 6 ) + + He (14a) (14b) (14c) Other experimental evience for the isomerization of Fe- (C 2 H 2 ) + 3 to Fe(C 6 H 6 ) + has been reporte previously for Fe- (C 2 H 2 ) + 3 prouce in a chemical ionization source from sequential bimolecular isplacement reactions of type High-

10 5146 J. Phys. Chem. A, Vol. 101, No. 28, 1997 Baranov et al. Figure 17. Propose mechanisms for the multiple ligation of Fe + with actylene, propyne (R ) CH 3) an iacetylene (R ) C 2H) involving intramolecular interligan interactions. energy collision-inuce issociation experiments inicate that at least a fraction of these ions ha isomerize to Fe(C 6 H 6 ) +. More recently, FT-ICR/CID experiments have shown in a relate stuy that at least a fraction of the Fe 4 (C 2 H 2 ) 3 + ions prouce by the sequential ehyrogenative aition of ethylene accoring to reaction 15 isomerizes to Fe 4 (C 6 H 6 ) + prior to collisional issociation. 21 Fe 4 (C 2 H 2 ) n + + C 2 H 4 f Fe 4 (C 2 H 2 ) n H 2 (15) Other higher-orer intramolecular interligan chemistry with acetylene is reveale by the results of the CID experiments reporte here. We attribute the observe loss of the equivalent of two molecules of acetylene from Fe(C 2 H 2 ) + 5 compare to the loss of one molecule of acetylene from Fe(C 2 H 2 ) + 4 (see Figure 10) to the formation of a cyclobutaienyl ring (a rich electron onor) in Fe(C 2 H 2 ) + 5 as shown in Figure 17. The CID spectrum inicates that Fe(C 2 H 2 ) + 5 is more strongly boun than Fe(C 2 H 2 ) + 4 : the issociation threshols for the loss of (C 2 H 2 ) 2 from Fe(C 2 H 2 ) + 5 an loss of C 2 H 2 from Fe(C 2 H 2 ) + 4 are ( 060 an ( ev, respectively. The intensity of the Fe(C 2 H 2 ) + 6 ion forme by ligating Fe(C 2 H 2 ) + 5 with one aitional molecule of acetylene was too small for a CID stuy. The T-shape structure for Fe(C 2 H 2 ) + 6 propose in Figure 17 involves sieways 4π electron boning to a secon benzene ring to be consistent with the 18-electron rule. The observe issociations of the higher-orer Fe + ions ligate with propyne an iacetylene are markely ifferent from that observe with acetylene: the thir auct loses one ligan molecule while the secon auct loses two accoring to the generalize reactions 16 an 17. We propose in this case that Fe(L) He f Fe(L) L (16) Fe(L) He f Fe + + [(L) 2 ] (17) Figure 18. Propose mechanism for the multiple ligation of Fe + with 1,3-butaiene involving intramolecular interligan interactions. Fe + meiates the cyclization of the first two ligans into imethyl- an iethynylcyclobutaiene, respectively, as shown in Figure 17. The antiaromatic cyclobutaiene ring is intrinsically unstable, but in these ligate ions we propose that it is stabilize by boning to Fe + an by the presence of two methyl or ethynyl substituents. We note in comparison that Fe + oes not seem to meiate the isomerization of two molecules of acetylene to cyclobutaiene since the CID of Fe(C 2 H 2 ) + 2 inicate exclusive loss of one molecule of acetylene. Apparently, the aitional electron onor properties of two methyl groups an two electron-rich ethynyl groups are require to stabilize a four-membere ring in the multiple ligation of Fe + by propyne an iacetylene, respectively. The aition of two further molecules of propyne to Fe- (propyne) + 2 an of iacetylene to Fe(iacetylene) + 2 resulte in ligate species which again issociate with the loss of the equivalent of two molecules of propyne or iacetylene, respectively, accoring to reaction 18. Again we propose the occurrence of intramolecular cyclization as shown in Figure 17. Fe(L) He f Fe(L) [(L) 2 ] (18) Finally, we note that the collision-inuce issociation of the fourth auct of Fe + with 1,3-butaiene, Fe(1,3-butaiene) 4 +, was observe to result in the loss of the equivalent of two molecules of 1,3-butaiene. We interpret this observation in terms of a imerization of 1,3-butaiene after the aition of the fourth molecule in a η 6 coupling reaction as illustrate in Figure 18. Such a reaction may lea to an open Fe + (η 4 -cis- 1,3-C 4 H 6 ) 2 (η 3 -C 8 H 12 ) structure (which has several cis/trans isomers) an a close Fe + (η 2 -trans-1,3-c 4 H 6 ) 2 (η 6 -C 8 H 12 ) structure in which the imer is coorinate as a three- an a six-entate ligan, respectively. Conclusions The experimental results reporte here provie a broa survey of the intrinsic kinetics for ligation of Fe + with saturate an unsaturate acyclic hyrocarbons at room temperature in helium at 0.35 Torr. Trens in the rates of aition of one ligan with the size of the ligan are consistent with expectations in terms of the egrees of freeom an stability of the ligate species accoring to current moels of ion/molecule association reactions. The observe variations of the measure rate coefficients for the sequential ligation of Fe + provie insight into the intrinsic coorination number of Fe + for acyclic hyrocarbons. Intramolecular interligan interactions meiate by Fe + were foun to be relatively common among the larger unsaturate hyrocarbon ligans. This unexpecte result has been attribute to intramolecular oligomerization an cyclization reactions. Acknowlegment. D.K.B. is grateful to the Natural Sciences an Engineering Research Council of Canaa for the financial support of this research an to S. Wloek an H. Wincel for