Kinetics of the Gas-phase Reaction between Iodine andTrimethylsilane and the Bond Dissociation EnergyD( Me, Si-H)?BY ROBIN WALSH" AND JEAN M. WELLSDepartment of Chemistry, The University of Reading,Whiteknights, Reading RG6 2ADReceived 18th April, 1975The title reaction has been investigated in the temperature range 519-618 K. The only products,formed in equaI quantities, were trimethylsilyl iodide and hydrogen iodide, Rates were found to besurface sensitive below about 560 K, but not so in the range 567-618 K where the rate lawd[L1 - k[I&Me,SiH] ---dt 1 +k'[HI]/[I,]was obeyed over a wide range of iodine and Me3SiH pressures. This expression is consistent withan iodine atom abstraction mechanism and for the step.I' + Me,SiH -+ Messis +HIlog (kl/dm3 mol-1 s-l) = 10.9- 82.3 kJ mol-'/RTIn 1 0has been deduced.From this the bond dissociation energy D(Me,Si-H) = (376k 11) kJ mol-I(90 kcal mol-l) is obtained. The implications of this value for the pyrolyses of organosilanes arediscussed.Reliable free radical thermochemistry is crucial to an understanding of gas phasekinetics and mechanisms. Whereas for many organic species good thermochemistryis available and kinetics and mechanisms are reasonably well understood,l* in thearea of organosilicon chemistry this is not so. Organosilane pyrolyses appear tooccur via free radical pathways in many cases and a complete understanding oftheir mechanisms depends heavily on a knowledge of Si-H and Si-Si bond dissocia-tion energies. We decided therefore to attempt a measurement of D(Me,Si-H).The most recent value,4 prior to our work, for D(Me,Si-H) was 340 kJ mol-'.That .this figure might be somewhat low was suggested by Whittles in the light of therelative rates of methyl radical abstraction from SiH4 6* and Me,SiH.6* RelativeHT yields from hot tritium atom abstractions also indicate a higher figure.A recentredetermination by Davidson and Howard lo of D(Me,Si-SiMe,) used in com-bination with the appropriate thermochemistry leads to D(Me3Si-H) = 364 kJ mol-l.Detailed kinetic studies of the gas phase reactions of iodine with many organicmolecules have been exploited by Benson and co-workers to provide reliable bonddissociation energies. We extend this technique into the organosilicon field here bystudy of the reaction of iodine with trimethylsilane.EXPERIMENTALAPPARATUSThe apparatus consisted of a static vacuum system, and quartz reaction vessel located inan electrically heated metal block furnace, designed to allow passage of a light beam throught A preliminary report of this work has appeared R.Walsh and J. M. Wells, Chem. Comm., 1973,513.10R. WALSH AND J . M. WELLS 101the vessel. The vessel contents could be continuously monitored by a single beam spectro-photometer (Hilger-Watts) the optics of which were modified for the purpose. The vessel,of path length N 17 cm, was connected to the vacuum system via a three-way tap the thirdarm of which was attached to a pressure transducer (Bell and Howell type 4-3274003) whichwas heated to -lW"C, along with that section of the line used for handling I2 vapour.Conventional silicone greased taps had to be used in this part of the line but otherwisegreaseless valves (Springham) were employed.Temperatures were measured with one of several chrome1 alumel thermocouples locatednext to the vessel.The calibration of the thermocouples was checked against an N.P.L.standard platinum resistance thermometer. Reaction vessel temperatures were controlledby an AEIRT3R proportional controller and were uniform in both space and time towithin &I*C.Both the unpacked quartz vessel of S/V = 0.91 cm-l, and an alternative one packed withPyrex tubes (which still allowed passage of a narrow light beam) of S/Y= 3.55 ern-', werewashed with a dilute solution of silicone oil DC703 in CCI4 prior to use, to attempt torender their surfaces inactive.12 Dead spaces were -0.6 % and were neglected.MATERIALSTrimethylsilane was prepared by the LiAlH4 reduction of trimethylsilyl chloride in di-n-butyl ether under nitrogen.Slight deterioration of the purified gas occurred over longperiods leading to formation of small quantities of higher molecular weight materials (notidentified). Because of this it was always redistilled through a -78°C trap prior to a day'sexperiments. It was identified by its i.r. spectrum l3 and contained no gas chromato-graphically detectable impurities (after redistillation).Trimethylsilyl chloride was a gift from Midland Silicones.Trimethylsilyl iodide was prepared l4 by the reaction of aluminium iodide with hexa-methyl disiloxane (Koch Light).After distillation the fraction boiling at 106-107°C wascollected. The product proved still to have substantial contamination with hexamethyl-disiloxane (which could either be unreacted starting material or a hydrolysis product). Theiodide was identified by its n.m.r. spectrum l5 (single line absorption at 9.28~). Furtherpurification was not attempted because of the ease of hydrolysis of the iodide. The impureiodide was adequate for identification purposes since the contaminant does not absorb inthe U.V. (above 210 nm).Iodine (Koch Light 99.998 % pure grade) was degassed before each experiment.Hydrogen iodide gas was prepared by adding an HI solution (Fisons AnalaR) dropwiseon to Pz05.It was dried by passage through a -78°C trap and collected at - 196°C.It was stored at room temperature in a blackened bulb and degassed before use.PROCEDUREPrior to a kinetic run, iodine (at - 20°C) and trimethylsilane (at - 196°C) were thoroughlydegassed. It was evaporated into the reaction vessel at a known pressure and, in mostexperiments its absorbance at a wavelength of 484 nm was recorded. A run was initiatedby the sharing of a known pressure of trimethylsilane into the vessel. A continuous traceof the absorbance variation with time was recorded during runs which varied (according toconditions) from minutes to several hours. The pressure transducer was used to recordpressures at intervals during a run, but was not left continuously in contact with reactingmixtures because of small losses due to slow absorption of I2 into the transducer. In someruns absorbance traces were obtained at other wavelengths in both the visible and U.V.regions.The 484 nm absorbance traces represent a direct record of iodine consumption as a functionof time since in the reaction mixture only iodine absorbs at this wavelength. Calibrationsof the spectrophotometer showed that Beer's law was obeyed by iodine at 484nm up to2.0 absorbance units.After a run the products were usually frozen out of the vessel and removed; they wereonly tetained in preliminary identification experiments. Some runs were fairly short and soa test was devised to ensure that mixing times (for I2 with Me,SiH) were still negligible102 D(Me,Si-H) BOND DISSOCIATION ENERGYBecause Iz vapour is visible this was easily done by eye in an identical vessel (at about 50°C)outside the oven.Even at 1 atm of added air mixing times were less than 3 s. Lowerpressures and higher temperatures reduce this time.In runs designed to study the inhibition by HI, the procedure was identical apart frominitial addition of HI to the reaction vessel, after the I3 but before Me3SiH.PRODUCT IDENTIFICATIONQualitative identification of the products as HI and Me3SiI was achieved simply byevaporating them sequentially into the vessel after a run, and recording their U.V. spectra.The U.V. spectra were in accord with those of authentic samples (see fig. 1). A more quanti-tative identification was made by attempting to monitor the absorbance at several U.V.wavelengths both during and after a run at 546 K.The absorbances at these wavelengthswere calibrated against those of HI and Me3SiI. The former was a pure sample. The latter,because of contamination of the prepared sample, was the triply distilled recovered samplefrom several actual runs. Fig. 1 shows a comparison between the runs of the absorbanceslpressure of HI and Me3SiI (in Torr, 1 Torr = 133.2 N m-2) and the final absorbance (basedon pressure of I2 consumed) of a particular run. The matching is reasonable although notperfect. The time evolution of U.V. absorbance at 220,230,250,270 and 490 nm shows that[HI] and [Me3SiIl must be within 10 % both of each other and of the decrease in [I2].I220 24 0 260 280 30(A/nmFIG.1.-Comparison of the U.V. spectrum of reaction products at 546 K with the sum of U.V. spectraof HI and Me3SiI. 0, reaction products ; - - - , HI spectrum ; - - - , Me3SiI spectrum ; -,sum of spectra.Several species were specifically ruled out from consideration. Since the reaction productwas completely condensible (G0.5 % of total products remain in the vessel), hydrogen andmethane were absent at this level. Methyl iodide could not have exceeded a few percentof the products based on chromatographic analysis (4 m length 15 % ppG column at 6O"C),and substantial amounts of other iodides (such as Me2SiHCH21) could not have been presentor a U.V. absorption peak at -2260 nm would have been observed.Gas chromatographicanalysis of the -778°C condensible reaction product on the column showed the only sub-stantial peaks to be due to Me3SiH (unreacted) and Me3SiI. Some minor peaks totallinga few percent in area were present. However, as the iodide is partially hydrolysed in thesampling system and co-elutes with (CH&3iOSi(CH3)3 the chromatographic andysis is notparticularly reliableR. WALSH AND J . M. WELLS 103RESULTSPRELIMINARY EXPERIMENTSThe quantitative identification of the products points to the chemical processThis is supported by pressure measurements which indicate no pressure change (towithin k0.3 Torr or better) during reaction. No secondary reaction between tri-methylsilane and HI occurs as evidenced by a complete lack of any absorbancechanges (in both visible and u.v.) when they were mixed at typical pressures at 547 K.Trimethylsilyl iodide was thermally stable under the conditions of our experiments.Table 1 shows an example of some iodine decay data in an early run at 546 K,using 7.02 Torr I2 and 21.3 Torr Me,SiH.(CH,),SiH + I2 (CH3),SiI + HI.TABLE 1 .-IODINE DECAY w r r ~ TIMEtimelmin 0 4 8 12 16 20 24 2812/Torr 7.02 6.26 5.73 5.24 4.74 4.37 4.01 3.71timelmin 32 36 40 44 48 52 56 60I~/ToIT 3.37 3.06 2.75 2.46 2.14 1.87 1.65 1.42These data were tested to find the order with respect to iodine, by the van’t Hoffmethod. It was assumed that the reaction was first order in Me,SiH, although sinceMe,SiH was in excess this assumption was not very critical.A plot was made oflog((AI,)/[Me,SiH],} against log[I,], where A12 is the loss in I2 over an 8 min interval(i.e., is an approximation to the rate) and [Me,SiH], and [IJr were the instantaneousconcentrations at the middle of the interval, [Me,SiH], being determined from stoichio-metry and the I2 decrease.This plot is shown in fig. 2. Such plots employingdifferences are always scattered but a forced straight line fit gives a slope (i.e., anorder) of 0.47k0.16. A further test of this data was made by attempting to fit itto the integrated form of the rate equation on the assumption of half and first orderdependence of the rate on I2 and Me3SiH respectively. The appropriate equation(see appendix) predicts a linear fit of tan-l(fj) with time wheref = [12],/([Me3Si€€Jo -[I& Fig.3 shows the plot for the data of table 1. The fit is reasonable althoughslight departures from linearity at long times were observed in many runs.DETAILED KINETIC MEASUREMENTSFurther tests to establish the rate law consisted in measuring the three halvesorder rate constant, k3, defined byby means of integrated plots and then examining the dependence of k* on startingconditions (particularly the ratio [Me,SiHJ/[I,]). Most of the integrated plots showedslight curvature, indicating inhibition of the reaction at high conversions. This wassubsequently allowed for explicitly but to avoid complications in these tests the decayplots were not examined beyond 50 % conversion of 12. Good linear fits to thetan-l(f*) against time plots were usually obtained up to this point.Table 2 showssome rate constants obtained at about 595 K.Apart from one run the k* values scatter to a maximum of & 15 % around the mean,and cover a range of values of the ratio [Me,SiHI/[I,] from 5-50. At all temperaturesruns with this ratio less than two tended to give high values for k+. Between values- d[I,]/dt = k3[12]+[Me3SiH] (104 D(Me3Si-H) BOND DISSOCiATION ENERGY0 0.2 0.4 0.6 0.8loslo{[Izlr/(Torr)~FIG. 2.-Log-log plot of rate against concentration to determine reaction order with respect to iodineat 546 K. Error bars represent uncertainty limits from spectrophotometric trace.3timelmindefinition off.FIG. 3.-Plot of the rate data at 546K according to the integrated rate equation.See text forTABLE 2.-A SELECTION OF RATE DATA AT 595 K[Id/ Tom Me3SiH/Torr 104k+/T0d S-12.03 64.6 3.594.95 45.3 4.1 11.30 22.3 3.491.55 7.7 3.214.66 22.5 4.198.28 10.5 5.732.29 117.1 3.5R. WALSH AND J . M. WELLS 105of 3 and 200, k, was constant within the scatter. This moderately high scatter couldbe attributed to instability and noise in the single beam spectrophotometric system.At the lower two temperatures the scatter became greater but this was shown to arisefrom surface effects. Within these limitations the rate data supported the threehalves order expression (A) reasonably well.Several runs were performed in the packed vessel to test for heterogeneity. Table 3shows a selection of the results. It is clear from these data that substantial surfacecatalysis is occurring at 522 and 533 K.To try to reduce this effect both vessels wererewashed with silicone oil solution and the unpacked vessel with hexamethyldisilazanealso. These treatments had little or no effect. At 581 K and higher temperaturesthe surface effect seemed to have disappeared and it probably was not serious in theunpacked vessel above 500 K.TABLE 3.-cOMPARISON OF RATE CONSTANTS ‘ OBTAINED IN DIFFERENT VESSELS, WITH VARIOUSSURFACE TREATMENTStemperature/Kvessel, treatment 522 533 581unpacked, untreated 0.65 1.03 16.09unpacked, silicone oil washed 0.66 1.14 15.89unpacked, HMDS washed 0.78 1.24 14.30packed, untreated 4.86 5.32 22.51packed, silicone oil washed 3.52 4.24 16.94a 105kq/Torr-* s-l, b hexamethyldisilazane.All rate constants were corrected for inhibition which raised their values by about10 %. They were then put into an Arrhenius plot which is Shawn in fig.4. Theline shown is a least squares fit to all the data (over 100 runs in the temperature range519-618 K) apart from runs in the packed vessel and runs with low values for the ratio[Me3SiH]/[12]. This yieldsSince at lower temperatures surface contributions in the unpacked vessel occurred,an alternative calculation was done for the limited temperature range 567-61 8 Kyieldinglog(k+/Torr-* s-l) = (12.50 & 0.30) - (1 8 1.6 & 3.4 kJ mol-l)/RT In 10.The error limits quoted are one standard deviation (68 % confidence level).log(k+/Torr-* s-l) = (9.07 0.20) - (142.8 2.2 kJ mol-l)/RT In 10.To examine the inhibition phenomenon in more detail a series of runs was per-formed in which HI was added initially to the reaction mixtures.the effect and to try to minimise the errors associated with theiodine decay with time were obtained as before but these weresistency with the expressionThis was to enhanceanalysis. Traces ofnow tested for con-shown later (see mechanism section) to fit the proposed mechanism.Rearrangementof eqn (B) gives[12]3[Me,SiH] 1 k’ [HI] = -+--4CIzllclt k, k, CblFig. 5 shows a test of this equation in which - d[I,]/dt is approximated by the change106 D(Me,Si-H) BOND DISSOCIATION ENERGY1.60 1.70 1.80 1.90103 KITFIG. 4.-Arrhenius plot for k+sol I I I I I0 2 4 6 8 10[ K I I I E I Z IFro, 5 .P l o t showing a test of the inhibition eff' of HI on the reaction at 568 RR . WALSH AND J. M. WELLS 107-A12, over a h e d but small time interval (4 min). To use such plots to obtain thevalues of k' (= slope/intercept), the intercepts were fixed by taking an average valueof k3 appropriate to the temperature in question. The data fit linear plots within thescatter where the error bars on each point represent maximum errors arising fromuncertainties in the experimental traces. Fig. 6 shows an Arrhenius plot of the derivedvalues of k' from such experiments. The temperature range here was limited to-1.6 5 1.70 I .75103 KITFIG. 6.-Arrhenius plot for k'. See text for explanation of the two lines.567-618 K to avoid any risks of surface contributions.The scatter is again largeand the highest temperature point appears somewhat out of line with the others.Since these values depend somewhat on k3 used in deriving them it is interesting tonote that a lower k3 value (which would bring it closer to its Arrhenius line) wouldreduce k' at this temperature and bring it closer to the line suggested by the otherpoints. The two lines drawn correspond to inclusion and exclusion of this point.The corresponding Arrhenius lines are, respectively,andlog k' = (1.28kO.56)-(22.3k6.3 kJ mol-')/RTln 10log k' = (- 0.19 & 0.59) - (6.1 k 6.5 kJ mol-l)/RT In 10.REACTION MECHANISM AND THE BOND DISSOCIATION ENERGYTheand so,likely ;observed kinetics parallel very closely those between I2 and hydrocarbons loby analogy, the iodine atom abstraction chain mechanism below seems mostI2( + M) * 214 + M)123I-+Me,SiH * Me,Si-+HIMe&+ I2 + Me,SiI + I-.A stationary state treatment of this mechanism leads t108 D(Me,Si-H) BOND DISSOCIATION ENERGYThus by comparison with the experimental results both the order dependence andinhibition effect are accounted for.In addition k+ can be identified with k1@ andk' with k2/k3. From the known values l6 of Kr, the following expressions for kl areobtained from either all the data, or only the higher temperature selection, respectively,log(kl/dm3 mol-l s-l) = 9.90-71.2 kJ mol-l/RTln 10orThese figures are unfortunately rather far apart and furthermore the more limited(second) set of parameters, apparently free from surface effects, gives an A factorrather higher than the collision number ( N 1 0 1 1 e 3 dm3 mol-l s-l).The total set ofdata probably gives an A factor which is too low because of the inclusion of surfaceeffects. In the light of this, probably the best procedure is to select A = 1010*9 dm3mol-1 s-I by analogy with the hydrocarbon case,l1. viz., I-+i-C4Hlo --+ But-+ HI.By compensation of parameters this leads tolog(kl/dm3 mol-1 s-l) = 10.90-82.3 kJ mol-l/RTln 10.The Arrhenius line corresponding to these parameters is very similar to one obtainedby omitting the rate constants at the single highest and the two lowest temperatures.However, there is no compelling experimental reason for making such a selection.suggests that of the two possibili-ties for the inhibition constant the one where the highest temperature data are omittedis more likely.Hencelog(kz/k3) = -0.19-6.1 kJ mol-l/RTln 10.It seems reasonable to suppose that E3 = 0 since alkyl radicals react with Iz withno activation energy and Si-I bonds are probably stronger (see discussion) thanC-I bonds., Hence Ez = 6.1 kJ mol-l. Thus, AH1,,(593 K) = El--& = 76.2kJ mol-l. Correction of this enthalpy change to room temperature using he; =- 8.8 J mol-I K-l, estimated by thermochemical methods, yields AH;,* (298 K) =78.8 kJ mol-I. Since AHf.2 = D(Me,Si-H) - D(H-I), then from the knownvalue'lof D(H-I) = 298 kJ mol-1D(Me,Si-H) = 376 kJ mol-l.Error limits have not been included in the above figures because of the assumptionabout the A factor, which lay well outside the apparent precision of the data.Itwould seem most reasonable to assume that the A factor is within a factor of 10 ofthe true value in which case the uncertainty in D(Me,Si-H) is f 11 kJ mol-I.13.35- 110.2 kJ mol-l/RTln 10.Again comparison with hydrocarbon systemsDISCUSSIONDespite the uncertainties in the data, the kinetics found for this reaction offerstrong support for the atomic chain mechanism proposed. The only differencebetween this reaction and its hydrocarbon counterpart is the lack of reversibility ofthe overall process undoubtedly due to the more favourably negative enthalpy changewhich in turn arises from the increased strength of the Si-I bond relative to C-I.A possible alternative atomic mechanism is worth considering ;Iz( + M) * 21*( + M)I. /I-+ Me,SiH -+ (Me3% ) + Me,SiI +HR.WALSH AND f . M, WELLS 109The key step in this sequence is the displacement step (via a penta co-ordinated inter-mediate or transition state). However, it seems much more likely that if a displace-ment were to occur, methyl radicals rather than hydrogen atoms would be theproduct. No methyl iodide was detected amongst the products and so this mechanismappears unlikely.Bond dissociation energies determined by this method l1 are usually amongst themost reliable and so it is unfortunate that the error associated with the present deter-mination is so large. Nevertheless our value of 376 kJ mol-1 is in reasonable agree-ment with a figure of 364 kJ mol-l which may be derived from the latest determinationof D(Me,Si-SiMe,)l O and the appropriate heats of formation.20 Davidson andHoward O also quote separate unpublished electron impact measurements leading toa value of 368 kJ mol-l.Clearly the earlier figure of 340 kJ mol-l, obtained fromdata on the pyrolysis of Me,SiH is too low. Our figure substantially reduces thediscrepancy in relative methyl radical abstraction rates from SiH4 6 p ' and Me3SiH.6*On a per Si-H bond basis the rates are closely comparable, and the activationenergies are extremely close, being 29 and 33 kJ mol-1 respectively. The bondstrengths for the two Si-H bonds, with D(SiH3-H) = 397+5 kJ mol-l,18 differby 57 kJ mol-l on the basis of the old value for D(Me,Si-H) but by only 21 kJmol-1 from the present figure.Unfortunately it is not possible to predict methylradical abstraction rates from bond dissociation energies or the converse, but theyare usually c~rrelated.~~ l9 It is still slightly surprising that, on a per bond basis,methyl radicals react faster with SiH4 with its stronger Si-H bond than with Me,SiH.Perhaps D(SiH3-H), determined by electron impact studies,l is slightly too high.The correlation of HT yields from recoil tritium abstraction reactions has ledHosaka and Rowland to suggest that D(Me3Si-H) is around 356 kJ mol-l. How-ever, the correlation line is curved and the present value is not incompatible with thecurve. Yields of HT from Me,SiH, and MeSiH, imply that D(Me,SiH-H) andD(MeSiH,-H) are about 2-3 kJ mol-1 greater than D(Me,Si-H).We intend toinvestigate this point later.Heat of formation data for silicon containing compounds have been somewhatunreliable in the past. A recent compilation 2o gives AHf"(Me,SiH) = - 156 kJmol-lwhich, taken in conjunction with our value for D(Me,Si-H), leads to AHf"(Me,Si*) =+2 kJ mol-l. This figure is only on the borderline of compatibility with the valueof - 11 kJ mol-1 derived by Davidson in the Me,SiSiMe, pyrolysis work usingAHf"(Me,SiSiMe,) = - 359 kJ mo1-1.20 This suggests a possible remaining incon-sistency between AHf"(Me,SiH) and AHf"(Me,SiSiMe,). Used in conjunction withother molecular heats of formation20 our value for AHf"(Me,Si) leads to higherfigures for several bond dissociation energies.Noteworthy amongst these is thevalue of 380 kJ mol-l for D(Me,Si-Me). Although there is undoubtedly still someerror associated with it, this value is quite a bit higher than that for the analogouscarbon-carbon bonds. Apart from being surprising in itself, t h s fact, if true forother Si-C bonds, means that the initiation step in free radical organosilane pyrolyseswill be considerably slower than was tho~ght.~. This in turn implies that chainsequences of the Rice-Herzfeld type, previously ruled are likely to occur. Forexample, in Me,SiH pyrolysisCH; + Me,SiH + CH4 + =CH,SiHMe,-CH,SiHMe, + CH,=SiHMe + CH;is a probable cycle. Because some products are common to both propagation andtermination, the determination of chain lengths in these pyrolyses is not straightforward.The participation of divalent silicon intermediates lo* 2 2 is a possible added complica110 D(Me,Si-H) BOND DISSOCIATION ENERGYtion and it is clear that these pyrolyses will need careful reinvestigation if theirmechanisms are to be understood.Another important thermochemical quantity, the n-energy in silico-olefins, wasevaluated 23 from earlier kinetic studies with organosilanes, as lying between 119 and158 kJ moP.In the light of the new Si-C bond dissociation energies this requiresupward revision. A value close to 200 ( & 20) kJ mol-' may be estimated using similarthermochemical argurnent~.~~ This new value means that these n-bonds have about80 % of the strength of the n-bonds in olefins, considerably more than had been24 *APPENDIXIntegration of the approximate rate equation (A) givestan-l(f+)- tar1(& = *(b- a ) + ~ (C)wheref= [12]/(b-a), a = [I2lO and b = [Me3SiHIo.The integrated form of eqn (B) containing both unknown constants k , and k', gave veryimprecise values for them when fitted to the data.The method, described in the resultssection, of enhancing inhibition by HI addition to reaction mixtures gives inherently moreprecise values for k'. The implication of inhibition by HI is that the plots of eqn (C) shouldbe slightly curved. However, although slight curvature is observed, allowance for curvatureis best made by correcting the k, from the linear fit by an aoerage inhibition term (- 5-10 %).The data are insufficiently precise to warrant any more sophisticated treatment.The authors acknowledge the provision of both an equipment grant and a main-We thank Mrs.Diane King for help tenance grant (to J. M. W.) from the S.R.C.with some of the preparative work.S. W. Benson, Thermochemical Kinetics (Wiley, New York, 1968).I. M. T. Davidson, Quart. Reu., 1971, 25, 111.I. M. T. Davidson and C. A. Lambert, J. Chem. SOC. A, 1971,882 ; Chem. Comm., 1969,1276.E. Whittle, Chemical Kinetics, ed. J. C. Polanyi (M.T.P. International Review of Science,Physical Chemistry, Series 1, 1972), vol. 9, p. 75.E. R. Morris and J. C. J. Thynne, J. Phys. Chem., 1969, 73, 3294.552.A. Hosaka and F. S. Rowland, J. Phys. Chem., 1973,77,705.69.D. M. Golden and S .W. Benson, Chem. Rev., 1969,69,125.' H. M. Frey and R. Walsh, Chem. Rev., 1969, 69,103.' 0. P. Strausz, E. Jakubowski, M. S. Sandhu and H. E. Gunning, J. Chem. Phys., 1969, 51,* J. A. Kerr, D. H. Slater and J. C. Young, J. Chem. SOC. A, 1967, 134.lo I. M. T. Davidson and A. B. Howard, Chem. Comm., 1973, 323 ; J.C.S. Faruduy I, 1975, 71,l2 K. W. Egger and S. W. Benson, J. Amer. Chem. Suc., 1965,87,3313.l3 D. F. Ball, P. L. Goggin, D. C. McKean and L. A. Woodward, Spectrochim. Acta, 1960, 16,l4 M. G. Voronkov and Yu. I. Khudobin, Izvest. Akud. Nuuk S.S.S.R. Ser. khim., 1956, 713.l5 E. A. V. Ebsworth and S. G. Frankiss, Trans. Furudzy Soc., 1967, 63,1574.l6 J.A.N.A.F. Thermochemical Tables, ed. D. R. Stull and H. Prophet, NSRDS-NBS 37 (Nat.Bur. Stand., Washington, 2nd edn., 1971).H. Teranishi and S. W. Benson, J. Anzer. Chem. Suc., 1963,87,2887 ; see also J. H. Knox andR. G. Musgrave, Trans. Furaduy SOC., 1967, 63,2201.W. C. Steele, L. D. Nicholls and F. G. Stone, J. Amer. Chem. Suc., 1962, 84, 4441 ; see alsoF. E. Saalfeld and H. J. Svec, J. Phys. Chem., 1966,70, 1753.P. Gray, A. A. Herod and A. Jones, Chem. Rw., 1971,71,247.1972).1358.2o J. B. Pedley and B. S. Iseard, CATCH Tables for Silicon Compuunds (University of Sussex,* Some of these points are also contained in a recent review 2 5 which was not available to us whenthis paper was submittedR. WALSH AND J. M WELLS 11121 R. P. Clifford, B. G. Gowenlock, C. A. F. Johnson and J. Stevenson, J. Organometallic Chem.,22 M . A. Ring, M. J. Puentes and H. E. O’Neal, J. Amer. Chem. SOC., 1970,92,4845.23 R. Walsh, J. Organometallic Chem., 1972, 38, 245.24 R. A. Jackson, Essays on Free Radical Chemistry (Chem. SOC. Spec. Publ. No. 24, London,25 I. M. T. Davidson, Reaction Kinetics (Chem. SOC. Spec. Periodical Rep., London, 1975), vol. 1,1972, 34, 53.1970), p. 295.p. 212.(PAPER 5/738