Electron Impact Studies on Organo-Beryllium and -AluminiumCompoundsBY D. B. CHAMBERS, G. E. COATES AND F. GLOCKLINGDept. of Inorganic Chemistry, The University, DurhamReceived 14th January, 1969Electron impact studies on R2Be, RzAIH and R3Al compounds at low source temperaturesrevealed the presence of associated electron deficient ions. General decomposition modes arediscussed together with bond energy data on beryllium dialkyls.Most alkyl derivatives of beryllium and aluminium are associated through bridgingcarbon atoms to yield dimeric, or for dimethylberyllium polymeric electron deficientstructures.1 Although the energy of bridging metal-carbon bonds is low (-105ourca Temperature 45-W°C .c -_- l5I34I i"9 I0Source TernDerature 1 % " ~mleFro. 1.-Mass spectra of diethylberyllium (70 eV).15158 ELECTRON IMPACT STUDIESkcd per Be-C or A 1 2 bond) it has proved possible to detect associated metalions formed by electron impact, provided close attention is paid to the experi-mental conditions, especially source temperature.For example, the mass spectrumdiethylberyllium (fig. 1 and 2) illustrates how the abundance of monomeric ionsC,HbBe+ increase with source temperature, whereas trimeric ions C,HbBe$ and themuch more abundant dimeric ions C,H,Be; decrease with temperature. Thesechanges are ascribable to increased dissociation of associated molecules at the highersource temperatures prior to ionization. A similar effect is observed with di-n-propyl-'0 H; $ 2 0lo!40 60 80 100source temp "c.1 -----L" - ~ - ~ -40 6'0 8'0 160 120 140 160 7th 2bOsource temp., "CFIO.2.Variation of ion abundances with source temperature for diethylberyllium.and di-iso-propylberyllium although dimer ions are of lower abundance, and above140" the proportion of hydrocarbon ions with m/e<43 rises steeply due to thermaldecomposition of the parent monomer molecules before ionization. At low sourcetemperatures, dimeric ions C,H,Be; are more abundant for PriBe than for PriBe;this may be due to the higher inductive effect of isopropyl groups increasing theelectron density in the bridging bonds. Although di-isobutylberyllium is dimericin benzene only monomer ions have been detected in its mass spectrum, and at highsource temperatures the abundance of hydrocarbon ions with nz/e I 5 6 increasesrapidly.For di-tert-butylberyllium, which is always monomeric, hydrocarbon ionsconstitute 35 % of the total ion current at 60" and 65 % at 240". Solid dimethyl-be1 yllium contains infinite chains of beryllium atoms with bridging methyl groupswhereas its unsaturated vapour consists mainly of monomer units. When thevapour is in equilibrium with solid, dimer and trimer molecules are also present inthe gas phase.l Conditions in a mass spectrometer are similar to that of the unD . B . CHAMBERS, G . E . COATES AND F . GLOCKLING 159saturated vapour, and only very low abundances of dimer and more highly associatedions (with up to 8 Be atoms) are detectable.and Me,Alz) are of appreciable abundance only at low source temperatures. Bycontrast, dimethylaluminium hydride, which exists as a cyclic hydrogen- bridgedtrimer in inert solvents, produces the trimeric ion Me,Al,H; and dimeric ionsC,H,Al; are of high abundance.This is consistent with the higher heat of associa-tion (- 15 to -20 kcal mole-l) of bridging Al---H---A1 bonds. The variation of ionabundances with source temperature (fig. 3) can only be interpreted in terms of thermalrearrangement (disproporti onation) of (Me,AlH), species producing trimethylalu-minium. This is an example where mass spectrometry provides a more sensitivemeans of detecting a reaction than vapour density studies., DiethylaluminiumTrimethylaluminium resembles beryllium dialkyls in that dimer ions (Me,AlCaHbAl+ ions formed bydecomposition of Me3Al+IT .O{CaHbAl+ ion5 formed bydecomposition of methyl-aluminium hydride ions.I* Me3A12H:+Me4A12H+50 60 80 lb lio l;rO 160 lFj3 200source temp., "CFIG.3.-Rearrangement of dimethylaluminium hydride with source temperature.ethoxide is dimeric in solution and in the vapour phase. The greater strength of thebridging Al---0---A1 bonds is reflected in the mass spectrum where, even at asource temperature of 195" most of the ion current is carried by organo-Al,Oz andorgano-Al,O+ species.DECOMPOSITION MODESFor beryllium alkyls even-electron metal-containing ions are not strikingly moreabuiidant than odd-electron ions, whereas for the limited number cf organoaluminiumcompounds examined even-electron ions predominate.Illustrative examples ofmetastable-confirmed decompositions are shown in fig. 4 and 5. Generally theparent ion formed from associated molecules is of low abundance and first loses analkyl radical, producing an even-electron ion which maintains its even-electroncharacter by successive elimination of neutral molecules. With aluminium alkylsthe neutral molecule eliminated can be metal-containing :Et2A12Hi -+Et,Al+ + AIH160 ELECTRON IMPACT STUDIESDi-krt-butylberylliurn produces the following decompositions, although theyBu\Be+'+Me,C+ + Bu'Be'are not metastable-supported :C,H,Be+'-+C,H: +Be'T r i m e r i o n s-C H Et2Be3H3+ ----+ 2 4 EtBe3HqfD i m e r ionsE t B e +' 4 2EL: B e 3- 3 2c €1 Be+' c,H,B~+* __t -HZ 4 10 C 11 Be+ r , 9-C2:i4C 2 5 H I + B e I\, C ,H4Bet 2 -H-----c,H,B~+ 'FIG.4. -Metastable confirmed fragmentation of diethylberyllium at 70 eVD. 3. CHAMBERS, G . E. COATES AND F . GLOCKLING 161If the C3Hz ion has the structure [CH2 : C : CHI+ then the observed process iscompatible with the ionization potentials [I(CH2 : C : CH)' = 8.25 eV ; I(Be) =9-32 eVJ. In the mass spectrum of dimethylberyllium low-abundance polymericions are related by successive loss of C,H,Be (probably Me,Be)- CzH6Be - CzH6BeC12H,3Bei + CloH2,Be~ 4 C,H,,Bel.Since these are all even-electron ions they must be fragments from an unobservedparent ion. It seems unlikely that (Me2Be)i' is the parent ion since four carbonI 'I \FIG. fi.-Fragmentation of aluminium alkyls.*atoms and fifteen hydrogen atoms must then be lost, either as radicals or moleculesto give C12H33Bei.For monomeric dialkylberyllium ions a favourable decompo-sition mode is elimination of alkane producing an odd-electron ion of high abundance :(CnH2n+ 1)2Be+' +CnH2nBe+' + CnHZn+ 2The extent to which structures may be written for fragment ions is extremelylimited, and ions such as Et2Be2H+ derived from diethylberyllium may be formu-lated in a variety of ways. On the basis of what is known of alkylberyllium hydridemolecules the structure [EtBe-- - H--- BeEt]+ might be favoured. Similarly, thetrimeric ion Et,Bef could be formulated as [I] or [II]* dotted arrows indicate processes not supported by metastable peaks.1 62 ELECTRON IMPACT STUDIESEt---Be 1+\ /' z,'*' \EtBe E tEt----BiEt JDlr Et Et l + ,' ',,,' '*\ /' '\ %'\,, ,,." '\\, .,,"Be Be BeEtEt EtAPPEARANCE POTENTIALS OF BERYLLIUM ALKYLSAppearance potentials of R2Be+' ions, at high source temperature (200") aredue almost entirely to the ionization of monomeric R2Be molecules, and are probablya measure of the vertical ionization potential. The ionization efficiency curves aresimilar to those produced by the inert gases (fig. 6) and, using AV, the ionizationpotentials of beryllium dialkyls have been evaluated by the Warren method (table 1).The ionization potential of dimethylberyllium is higher than that of any other fullyalkylated organometallic compound so far examined and the decrease in going todiethylberyllium is similar to that observed in other groups of the periodic table.i'I Im 10electron energy, eV (uncorrected)Fm.6.Tonization efficiency curves for Me2BeD. B . CHAMBERS, G . E . COATES AND F . GLOCKLING 163TABLE 1 .-IONIZATION POTENTIALS OF BERYLLIUM DIALKYLSI.P. (ev)Mez& 10.67 f0-079.46 f0.05 Et,BePrzBe 8-71 f0.06PriBe 8.80 f0.02BuiBe 8-74 f0.05Table 2 lists the appearance potentials of some fragment ions but in most cases" tailing " of the ionization efficiency curves was found as in fig. 7a-d. If an ionRBe+ is formed by the process :R2Befe+RBe++R'+2e20001000500'0050f 103 uc = 0 - -1Ion current 6 8 10 12 14 l 8 - -electron energy, eV (uncorrected)FIG. 7.4onization e9i:ien:y curves for C4H9Be+ and C4HsBe+* derived from BuiBe.then D(RBe-R) = A(RBe)Lze -I(R,Be). The observed " tailing " suggests thatthe dissociative process occurs with excess kinetic energy; hence the derived bonddissociation energies (table 3) must be regarded as upper limits.The low value ofD(Bu'Be-Bu')+ is probably due to inaccuracy in measuring I(BuiBe) because ofthe low abundance of the molecular ion.In beryllium dialkyls it is highly probable that ionization removes a bondingelectron so that the beryllium-carbon bond energy in the molecule, D(RBe-R164 ELECTRON IMPACT STUDIESTABLE 2.-APPEARANCE POTENTIALS OF FRAGMENT IONS FROM BERYLLIUM DIALKYLScompound ion appearance potential (eV)Me2Be CHZBe+' 11.92 f0.052-67 f0-020.35 f0.031.51 fO.059-86 f0.050.81 fO.059 . 6 0 fO.010.65 40.019-14 f0.03C,H,Be+ 1040 A-0.05TABLE 3 .-BOND DISSOCIATION ENERGIES IN BERYLLIUM DIALKYLSdissociation energy(k3.2 kcal mole-')MeBe-Me)+3tBe-Et)+?rr"Be-Prn)+VBe-Pri)+3u'Be-Bu')f3eCHz-H)fkCZHa-H)+3er"C3H6--H)+k'C3H 6-H)'3eiC4H8-H)+46.147.342-748.729.186.771.570.776.077.7will be in excess of -45 kcal mole-l. The bond dissociation energies:D(BeC,H,,-H)+ + (BeC,H2,)+ + Heaxe all about 74 kcal mole-1 with the exception of D(BeCH2-H)+ for which thevalue is 86.7 3-4 kcal mole-1. This difference may be explained if some stabilizingrearrangement occurs when the product ions (BeC,H,,)+' contain more than onecarbon atom. For example, the structure of the BeC,HZo ion m y be representedasl G. E. Coates and K. Wade, Orgmometallic Compounds, vol. 1, (Methuen, 1968).T. H. Wartik and H. I. Schlesinger, J. Amer. Chem. SOC., 1953,75,835.J. W. Warren. Nature, 1950, 165,810