The Typical Elements By A. J. CARTY Guelph- Waterloo Centre for Graduate Work in Chemistry Departmentof Chemistry University of Waterloo Waterloo Ontario Canada N2L 3G7 R. H. CRAGG Department of Chemistry University of Kent Canterbury CT2 7NH J. D. SMITH School of Molecular Sciences University of Sussex Brighton BN 1 9QJ PART I Groups I and II By R. H. Cragg 1 Group1 One of the major areas of interest in Group I chemistry has been the detailed study of the reactions of alkali metals with hydrogen. The significance of this work arises from the knowledge that in the reactions of alkali metals with hydrocarbons the metal hydride is a possible significant intermediate and it is therefore of importance to have some understanding of these alkali-metal hydrides.For example in the self-hydrogenation of alkynes and alkenes at the surface of liquid sodium molecular hydrogen is formed and the rate at which it is subsequently converted into metal hydride is influenced by the nature of the hydrocarbon. Liquid alkali metals react with hydrogen to form hydrides at the surface and because the reaction is moderately slow at temperatures just above the melting point of the metal it is relatively easy to measure the rate of the reaction by following the decrease in hydrogen pressure manometrically M(1) + iH,(g) +MH(s) The results for the reaction between hydrogen and liquid sodium have been previously reported and in order to obtain comparisons with other alkali metals the rate of adsorption of hydrogen at liquid lithium and potassium surfaces has been determined in the absence of hydrocarbons.The experimental technique used for the study of the reaction of hydrogen with alkali metals is relatively straightforward. A jet of the clean metal is continuously injected into hydrogen and the rate of reaction of hydrogen with the metal surface is measured over a selected temperature and pressure range e.g. in the case of potassium 22.24.3 kNm-' and 210-333 OC.' The reaction between hydrogen and potassium is observed to follow first-order kinetics with an activation energy of 66.5 kJ mol-' a slightly higher value than that reported for sodium (see Table). In contrast potassium is observed to react ' G. Parry and R.J. Pulham J.C.S. Dalton 1975,446. 93 A.J.Carty,R. H. Cragg and J. D. Smith with hydrogen approximately four times faster than with sodium which is a little surprising in view of the chemical similarities of the two metals. It was therefore suggested that in view of the similar activation energies of sodium and potassium the rate-determining step in their reaction with hydrogen is the same and probably involves electron transfer from the metal to adsorbed hydrogen atoms. This proposal is consistent with the observation that the free energies of formation of the two hydrides are similar and that the dissociative adsorption of hydrogen is usually rapid. The mechanism of the reaction is suggested to proceed by successive steps uiz.the conversion of gaseous molecular hydrogen into crystalline potassium hydride with dissociation into atoms and the formation of hydride anions.The adsorbed hydride ions can either dissolve in the metal or alternatively if the potassium is saturated potassium hydride crystals nucleate and grow on the sdrface of the potassium H,(g) -+ H(ads) + H(ads) H(ads) + e-+ H-(ads) It therefore seems reasonable to suggest that the difference in the rates of reaction of hydrogen with potassium and with sodium is mainly due to the relative strength of hydrogen adsorption; the more strongly the atom is adsorbed the easier becomes the electron transfer and the results indicate that hydrogen is more strongly adsorbed on potassium than on sodium. The rates of reaction of hydrogen with a clean liquid lithium surface have also been studied; the reaction is first order with an activation energy of 52.8 kJmol-'.* Lithium was found to react at 250°C about forty times faster than sodium with hydrogen and this result supports previous observations which showed that the addition of small amounts of lithium to sodium increases the rate of hydrogen adsorption.As in the case of potassium the rate-determining step in the reaction of hydrogen with lithium is assigned to the electron transfer from the metal to adsorbed hydrogen atoms. In view of the faster reaction in the caseaf lithium it appears that hydrogen is more strongly adsorbed on lithium than on either sodium or potassium. The reaction of hydrogen with solutions containing lithium strontium or barium is faster than with The rate is directly proportional to the hydrogen pressure and the reaction is first order with respect to the hydrogen pressure provided that the solution composition and the liquid-metal surface remain constant throughout the reaction.The results suggest that the hydrogen molecule is directly involved in the rate-determining step. However it is important to note that the observed changes in the rate constants are not accounted for only by the activation energy changes as the pre-exponential factor changes with both the solute metal and its concentration. Reported values for the activation energies and rate constants for the reaction of liquid metals with hydrogen are given in the Table. Table Metal E*/ kJ mol-' k(250 'C)/mrn s-' (kN m2)-' Na 72.4 2.505 X Na-Li (5.0%) K 64.1 66.5 5.860 X 9.010X Li 52.8 1.065 X Na-Ba (5.0%) 45.0 4.821 X G.Parry and R. J. Pulham J.C.S. Dalton 1975 1915. M. R. Hobdell and A. C. Whittingham J.C.S.Dalton 1975 1591. The Typical Elements Recently the reaction of liquid potassium with ethylene has been investigated over the range 503-67 1K by measuring the pressure changes and analysing the gas as the metal is injected into hydrogen using an electrical pump.4 It is observed that at lower temperatures self-hydrogenation takes place. The amount of ethane produced decreases as the temperature increases and it is suggested that this is due to the loss of hydrogen from the surface by dissolution in the metal. 2 Group11 Alkoxy-derivatives of beryllium are usually associated in keeping with the general property of beryllium to be four-co-ordinate where possible.For example dimethoxyberyllium is polymeric and insoluble in hydrocarbon solvents. Even di-t-butoxyberyllium which is soluble in hydrocarbon solvents is trimeric (1) and the only monomeric alkoxide so far reported is bis(2,6-di-t-butyl-phen0xy)beryllium. Recently a dimeric alkoxide of beryllium bis(nonafluor0-t- butoxy)beryllium(2),one of the most volatile alkoxides known (it sublimes in vacuo at room temperature) has been obtained from the interaction of nonafluoro-t-butyl alcohol and diethylberylli~m.~ Although insoluble in benzene (2) dissolves without reaction in nitrobenzene and is dimeric in hexafluorobenzene. The.lack of extensive association can be attributed to the increase in steric hindrance about the beryllium atom.Rather surprisingly the bridge Be-0 bond in the dimer is cleaved by diethyl ether. This is in contrast to the lack of reaction with pyridine or quinuclidine of (l) the hydrogen analogue of (2). But I Bu' I 0 0 0 /\ /\ \/\/0 0I I Bu'O -Be Be Be-OBu' (CF,),CO-Be /\ I Be-OC(CF,) \O/ But Bu' C(CF3)3 (1) (2) The reaction of sodium nonafluoro-t-butoxide with beryllium(I1) chloride in diethyl ether results in the formation of a distillable liquid Be[(CF,),CO], OEt, which is monomeric in benzene and only the second example of a three-co-ordinate monomeric beryllium alkoxide. On reaction with pyridine or ammonia the first examples of 1:2 beryllium complexes are obtained.