INORGANIC REACTIONS IN LIQUID AMMONIA By G. W. A. FOWLES and D. NICHOLLS (THE UNIVERSITY SOUTHAMPTON. THE UNIVERSITY LIVERPOOL) 1. Introduction LIQUID ammonia which is the best known non-aqueous ionising solvent is quite a good solvent for many inorganic compounds particularly halides and ammonium salts even though it has an appreciably lower dielectric constant than water. It has a modest degree of auto-ionisation 2NH + NH4+ + NH,- so that solutions of ammonium salts act as acids and those of amides as bases; normal acid-base reactions can be carried out. Since N-H bonds of ammonia are less easily broken than 0-H bonds of water compounds are less likely to be ammonolysed than hydrolysed and for this reason liquid ammonia may sometimes be used as a medium for the preparation of compounds which can be obtained only in low yields from aqueous solutions.Germane GeH, for instance which may be prepared by the action of acids on Mg,Ge is obtained only in 20% yields when aqueous acids are used but an 80 % yield is made possible by the use of solutions of ammonium bromide in liquid ammonia. A unique property of liquid ammonia is that of dissolving alkali metals without further reaction. In contrast to the violent reaction between alkali metals and water the liquid ammonia solutions are quite stable although decomposition to the amide and hydrogen take place in the presence of suitable catalysts such as plati- num black. As two recent reviews1$2 have appeared on the nature of the metal-ammonia solutions we need only very briefly summarise the situa- tion. The metals are considered to dissolve in ammonia giving metal cations and electrons M - t M + + e the electrons are associated with the solvent and in concentrated solutions at least the cation also appears to play some part in determining the environment of the electron.The solutions thus contain a very ready source of electrons and they accordingly constitute very powerful homo- geneous reducing agents. In 1950 Watt3 thoroughly reviewed the use of these solutions in the reduction of both inorganic and organic compounds and Birch4 has also summarised many organic reductions. Jolly “Progress in Inorganic Chemistry,” 1959 1 235. Symons Quart. Rev. 1959 13 99. Watt Chem. Rev. 1950 46 289. Birch Quart. Rev. 1950 4 69. 19 20 QUARTERLY REVIEWS Apart from a brief account by Watt5 in 1957 and a survey of non- aqueous solvents by GutmamQ there has been no recent general review of the various lines of liquid-ammonia research that have been developed since the war.In this present Review we shall first discuss the reactions of ammonia with metal halides with especial emphasis on ammonolysis. Fernelius and Bowman’ reviewed early work (prior to 1940) on the ammo- nolysis of both organic and inorganic compounds (including halides) ; in our account we shall restrict ourselves to those halides which either best illustrate the general principles or have been the most fully studied. The remainder of the Review is concerned with the synthetic procedures in both the inorganic and the organometallic field that have been evolved in the past decade or so. 2. Reactions of liquid ammonia with metal halides We can conveniently consider these reactions as falling into two categories one in which simple addition compounds are formed and the other in which one or more metal-halogen bonds are ammonolysed i.e.(i) (ii) MX + mNH -+ MXn-m,a(NH2)m,p + $NH,X MX + xNH +. MX,,xNH The ammonobasic metal halide formed by ammonolysis (reaction ii) may of course take up ammonia and form an addition compound. It is not possible to classify the halides rigidly under these two headings because the products of the reaction depend on experimental conditions and even those halides that are very easily ammonolysed do form addition compounds initially and these intermediates can on occasion be isolated under suitable conditions. In general though it is true to say that am- monolysis occurs only with the more covalent halides.(a) Ionic Halides of Non-transition Elements.-Thus the ionic halides of non-transition metals (e.g. alkali and alkaline-earth elements) form simple “ammoniates” in which the ammonia is but weakly held and can be removed in vacuo at relatively low temperatures leaving the un- changed halide. A great deal of early work in this field has been done by Biltz who in a summary,8 points out that (i) for a given metal halide the heat of formation of the ammoniate increases as the number of bonded ammonia molecules decreases and (ii) for a given Periodic Group ofmetals the heat of formation of similar ammoniates decreases with increasing atomic weight. The ammonia molecules are undoubtedly associated with the cation although the anion plays an indirect part in determining such details as lattice spacings.Watt J. Chem. Educ. 1957 34 538. Gutmann Quart. Rev. 1956,10,451. Fernelius and Bowman Chem. Rev. 1940,26 3. 13 Biltz 2. amrg. Chem. 1923 130 93. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 21 The bonding in these addition compounds is best regarded as ion- dipole in character resulting from interaction between the cation and the lone-pair of electrons of the nitrogen atom and since the degree to which the electron charge cloud is distorted depends on both the size and the charge of the cation small highly-charged cations form the strongest bonds e.g. Mgs>Ca2+> Sr2+>Ba2*. (b) Halides of Transition Metals in their Lower Valency States.-These halides are usually considered to be highly polymeric with fairly polar metal-halogen bonding.The chlorides of bivalent manganese iron cobalt and nickel for instance which come into this category form addition compounds with ammonia and these adducts do not differ markedly from those formed by the alkaline-earth halides since the ammonia is still lost quite easily in vacuo. However these transition elements of the first row have vacant 4d-orbitals energetically available and the bonding is to be considered more donor-acceptor than ion-dipole in character. The ammoniates formed by these halides of the transition metals in their lower-valency states are often true ammines and the bonding of the ammonia groups to the metal ion is best understood in terms of ligand- field or the molecular-orbital theory. However some halides (e.g. VCl, VBr, MoBr,) have been incorrectly described1* as forming hexa- ammines [M(NH,),]X, whereas they do in fact undergo ammonolysis.Thus both the chloride and the bromide of tervalent vanadium are ammonolysed in liquid ammonia,ll and the so-called ammine is really a mixture [V(NH,),]X E VX,(NH,),4NH3 + NH,X Similarly molybdenum(r1I) bromide is ammonolysed rapidly at room temperature with the formation of MoBr(NH2),,NH3. Titanium(rr1) chloride on the other hand is not appreciably ammonolysed evm at room temperature but much of the ammonia associated with the chloride can be removed by heat in vacuu showing that the ammonia is only weakly bonded. The dichlorides of bath titanium and vanadium form adducts when treated with liquid ammonia at room temperature but at -33.5” there is. virtually no reaction.(c) Covalent Halides of Non-transition Metals.-Themore covalent halides invariably undergo at least partial ammonolysis when treated with liquid ammonia and the extent to which the halogen atoms are replaced by amido-groups decreases as the metal-halogen bonds become more polar. For a given element in a particular valency state the chloride bromide and * Orgel “An Introduction to Transition Metal Chemistry Ligand-field Theory,” Methuen London 1960. lo Sidgwick “The Chemical Elements and Their Compounds,” Oxford Univ. Press 1950. l1 Fowles Lanigan and Nicholls Chem. and Znd. 1961 1167. 22 QUARTERLY REVIEWS iodide usually react similarly with liquid ammonia but the fluoride is always ammonolysed less readily and often only forms an ammoniate. We can illustrate these points by reference to the halides of the Group I11 and IV elements.Boron(m) fluoride acts as a typical Lewis acid towards ammonia and forms an adduct BF3,NH3 which does not undergo ammonolysis even at 50°.12 Although no analogous adducts have been isolated from the reactions of ammonia with the other three halides it is probable that they are formed initially and subsequently undergo ammonolysis N H4X Further co-ordination and elimination of hydrogen halide then leads to the formation of the amide which readily decomposes to the imide 2B(NHJ -+ B,(NH)S + 3NH The reluctance of boron(rrr) fluoride to undergo ammonolysis is a reflec- tion of the high B-F bond strength. In the presence of amide ions or alkali metals ammonolysis takes place e.g. + K + BH + KF This is to be expected since the amide ion is a much stronger nucleophilic reagent than ammonia while the alkali metals provide a source of elec- trons and help the formation of fluoride ions.It is interesting that when the ammonolytic mixture [BF,(NH& + KF] is passed down a cation-exchange the potassium ions are re- moved and the adduct (BF3,NH3) is re-formed. This is consistent with the initial formation of a complex boron anion which breaks down on removal of excess of ammonia N H (i) BFz(NH2) + KF - K[BF,(NHz)I N H3 (ii) K[BF,(NH,)] + NH,(Resin) -+ NH,[BF,(NH,)] + K(Resin) (ii i) N H4[BF,(N H2)] Removal of + BF,,NH + NH excess Of NH Thus when the excess of ammonia is removed the solid complex is unstable and transference of a proton from the ammonium ion to the amino-group is followed by the loss of a molecule of ammonia.We shall see that the formation of similar complex ions helps to account for the behaviour of a number of other halides that are only partially ammono- lysed. As we go from the boron halides to those of aluminium there is a marked change since none of the aluminium halides is appreciably l2 Jenkins J. Amer. Chem. SOC. 1956 78 5500. l3 McDowell and Keenan J. Amer. Chem. SOC. 1956 78 2065. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 23 ammonolysed in liquid ammonia. The ammoniates are ammonolysed however on treatment with solutions of the alkali metals in liquid ammonia.14 The halides of the Group IV elements carbon to tin all undergo ammonolysis on direct treatment with ammonia (cf. Table 1). TABLE 1. Ammonolysis of Group 1V halides. Halide CCI CBr Cr SiCl Si,CI GeCI GeI SnCI SnBr snI4 PbCI Reaction temp.140" 100-250 - 33.5 25-150 - 50 - 50 - 33-5 - 33.5 - 33.5 - 33.5 -33.5 20 Ammonolysed product HN C(NH& HN:C(NH& CI,,2NH3 HN C(NH,) Si(NH3 Ge( NH) Ge( NH) SnCl(NH,) SnBr(NH,) No ammonolysis [Si( NH)(NH,)I SnI(NH2)3 Ref. 15 16 17 16 18 19 20 21 22 23 23 7 Carbon tetrahalides are not ammonolysed unless the reactions are carried out above room temperature. This is understandable since carbon has no vacant orbitals of low enough energy to accept electrons from an ammonia molecule and no intermediate adduct can be formed. Hence the mechanism proposed for the ammonolysis of the boron halides is not applicable and the breaking of a C-X bond occurs only at higher tem- peratures. It should be mentioned that carbon tetraiodide forms a diam- moniate when treated with liquid ammonia but the ammonia is held only by weak van der Waals interactions and will not modify the C-X bonds sufficiently to facilitate ammonolysis.In contrast to the inertness of the carbon tetrahalides the halides (other than fluorides) of quadrivalent silicon and germanium are readily and completely ammonolysed to the amides M(NH2), at low temperatures ; these readily break down to the imides M(:NH),. Tin@) chloride l4 Watt and Braun J. Amer. Chem. SOC. 1956,78 5494; Taylor Griswold and Klein- berg ibid. 1955,77 294. l5 Stiihler Ber. 1914 47 909. l6 Watt and Hahn J. Amer. Chem. SOC. 1955,77 312. l7 Watt McBride and Sowards J. Amer. Chem. SOC. 1956,78 1562. l8 Vigoroux and Hugot Compt. rend. 1903 136 1670. l9 Schwarz and Sexauer Ber.1926,56,333; Billy Compt. rend. 1960,250,4163. 2o Schwarz Angew. Chem. 1935,48,221. *l Johnson and Sidwell J. Amer. Chem. SOC. 1933 55 1884. 22 Bannister and Fowles J. 1958 751. 23 Bannister and Fowles J. 1958,4374. 24 QUARTERLY REVIEWS bromide and iodide are not completely ammonolysed however and even after prolonged washing with liquid ammonia the last tin-halogen bond remains intact. The ammonobasic tin(1v) halides are almost insoluble in liquid ammonia so that they can be obtained free from ammonium halide by extracting the latter with liquid ammonia. Schwarz and Jeanmai~e,~~ who did earlier work with tin(1v) chloride found that even on prolonged washing with ammonia the insoluble product still had a Sn Cl ratio of 1 1.5 although this could be lowered to 1 1 if the product were heated to 100" between the washings.More recent work has however shown that SnCl(NHA can be obtained without this heating. The insoluble ammonobasic tin(1v) halides are almost certainly polymers resulting from the elimination. of hydrogen halide between neighbouring molecules or from condensation through chlorine or nitrogen bridges or both e.g. The composition SnCI(NH,) is probably only an average one the degree of ammonolysis being above average at the extremities of the polymer. The presence of some tin compounds in the ammonia-soluble portion of the products (solubility increasing in the order Cl<Br<I) suggests that the ammonium halide formed in the ammonolysis produces soluble complex anionic species such as [SnC13(NH,)3]2-. Solubility is not complete however because of the polymeric nature of the bulk of the product and the relatively low concentrations of ammonium halide.The tin(1v) halide-ammonia systems have also been examined in some detail by a tensimetric procedure in which a known excess of ammonia is condensed on to the halide (SnX, NH3 - 1 30) and the resulting mixture is allowed to come to equilibrium over a period of days. Under these experimental conditions a small amount of liquid ammonia is present in the reaction bulb; small quantities of ammonia are then removed pro- gressively and the vapour pressure of ammonia measured for each com- position. The annexed Figure shows tensimetric plots for the ammonium chloride-ammonia and the tin(1v) chloride-ammonia system. From Fig. (a) it is clear that ammonium chloride forms a triammoniate with a character- istic dissociation pressure of 91 mm.at -36" and the similar univariant portion in Fig. (b) shows that ammonium chloride is present in the product forrned by the reaction of tin(rv) chloride with ammonia. The length of this univariant step is a direct measure of the amount of ammonium chloride that is present. * Schwarz and Jeanmaire Ber. 1932 65 1443. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 25 1 I I I t3Oo b 1 I I I I I -200 1 I I I r -100 ,.+d 1,l g13 15 17 19 NH3- SnCL4 Tensimetric plots (-36") for the systems formed by ammonia with (aj ammonium chloride and (b) tin(Iv) chloride. TABLE 2. Tensimetric studies on tin(iv) halide. Halide SnCI Temp. -63" -45 and -36 - 36 - 36 -45 and -36 - 36 - 36 No. of mol. of NH,X forming ammoniates 2.6 1.0 2.8 1.2 3.0 5-0 2.0 At -63" the tin@) chloride-ammonia system shows the presence of almost three mol.of ammonium chloride in agreement with the reaction but at somewhat higher temperatures only one mol. of ammonium chloride can be detected. Now the degree of ammonolysis at the higher temperatures must be at least as great as that at -63" so that the "missing" two mol. of ammonium chloride must for some reason be incapable of forming the characteristic ammoniates. The explanation is that at higher temperatures the ammonium chloride is more soluble in the excess of ammonia and the more concentrated solution reacts with the ammono- basic tin(1v) chloride to give an anionic complex Three mol. of ammonium bromide are detected in the tin(1v) bromide- ammonia system at -45" and -36" however indicating that although SnCI + 6NH3 + SnCI(NH,) + 3NH,CI SnCI(NH,) + 2NH,CI -f (NHJ,[SnCI,(NH,)3] 26 QUARTERLY REVIEWS complex formation may occur (giving the soluble product in washing experiments) the complex breaks down when it is precipitated as a solid upon removal of the excess of liquid ammonia.This agrees with the decrease in stability of complex hexahalogenostannate ions [SnX,I2- in the order I > Br > C1. Only two mol. of ammonium iodide are however found in the tin(1v) iodide-ammonia system and this must be because ammonolysis of tin(~v) iodide is less complete than that of the bromide and chloride. (We have seen that complex formation is ruled out since complexes formed by the bromide are unstable under tensimetric conditions.) The third stage of the ammonolysis of the iodide Snl,(NH,) + 2NH3 -+ Snl(NH,) + NH,I is completed when ammonium iodide is removed by washing but the high concentration of ammonium iodide (saturated solutions) suppresses the ammonolysis in the tensimetric experiments.Lead(1v) chloride is reported to be resistant to ammonolysis although no detailed study appears to have been made. While the lack of ammono- lysis is not too surprising it is in marked contrast to the behaviour of diammonium hexachloroplumbate(Iv) (NHJaPbC1, which is extensively ammonolysed. The explosive nature of the product precludes more rigorous examination but the ratio Pb C1 N appears to be in the region of 1 1 1. The original workers obtained figures of 5 6 4 and proposed a cyclic constitution to account for the instability of the product but it seems more likely to be a polymeric mixture which owes its explosive properties to the presence of unstable Pb-N bonds.(d) Halides of Transition Metals in Their Higher Valency States.-We limit our review of the ammonolysis of these halides to the elements of the titanium vanadium and chromium subgroups as much more work has been done recently in this field. By and large the investigations fall into two categories those in which the extent of ammonolysis has been studied by tensimetric methods and those in which the halides have been allowed to react with liquid ammonia at various temperatures and the products then washed with ammonia to remove the ammonium halide; the latter technique is not always possible since in one or two cases [e.g. niobium@) chloride] the products are completely soluble in ammonia.Table 3 summarises the results obtained by the two methods for many halides and for a number of complex anionic halides of the elements. In their ammonolytic behaviour the titanium(1v) halides resemble the halides of quadrivalent tin rather than those of germanium in that only three titanium-halogen bonds can be ammonolysed even at room tempera- ture. Contrary to the view sometimes expressed the amide Ti(NH& is not prepared by ammonolysis of the titanium(1v) halides although it is formed when potassium amide is added. The ammonobasic titanium(rv) chloride, FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 27 TABLE 3. Ammonolysis of halides and complex halides of the Group IVA VA and VIA transition elements in their higher valency states.Halide Solubility Insoluble product (Reaction (% of metal) at -33-5" then washed with with excess of ammonia) TiCl (NH&TiCI Tar (NH,),TiBr TiI K2TiCI6 ZrC1 (NH&ZrC16 Rb,ZrC16 Cs,ZrCI ZrBr ThCl ThBr ThI4 VF5 VCl NbF NbCI TaCl MoCI wF6 wa * Reaction at 25". ammonia 5 90 70 70 100 100 10 70 20 5 - - - - 0 50 0 100 100 60 15 0 90 TiCl(NH& TiCl(NHd + TiCl 2(N H2) TiCl(NH2) +TiCl,(NH& +KCI TiBr(NH,) +TiBr2(NH2) - ZrCI,(NH,) ZrCl,(NH2) ZrCl,(NH& +RbCl ZrC13(NH,) + CsCl ZrBr,(NH,) + ZrBr2(NH2),* - Th14,7NH3 VCl(NH&,+ VC1,(NH2) NbF5,2NH VFd,NH,+NH,F - TAmmoniates formed. Mol. ratio Ref. NH,X/M detected tensimet- rically 2.7 4.0 2.0 2.7 4.0 2.5 1.0 3.0 1.0 1.0 ot ot - - - 2.0 2.0 2-0 2.0 - - - <2-7 25 26 26 27 26 27 28 29 29 29 30 28 31 32 33 34 33 35 36 37 38 39 - 25 Fowles and Pollard J.1953 2588. 26 Fowles and Nicholls J. 1961 95. 27 Fowles and Nicholls J. 1959 990. 28 Fowles and Pollard J. 1953 4128; Drake and Fowles J . Less-Common Metals 2 9 Drake and Fowles J. Less-Common Metals 1961 3 149. 30 Bowerman and Fernelius J . Amer. Chem. SOC. 1939 61 121. 31 Young J. Amer. Chem. SOC. 1935 57 997. 32 Watt Soward$ and Malhotra J . Amer. Chem. Soc. 1957 79,4908. 33 Cave11 and Clark J. Inorg. Nuclear Chem. 1961 17 257. 34 Fowles and Nicholls J. 1958 1687. 35 Fowles and Pollard J . 1952 4938. 36 Moureu and Hamblett J. Amer. Chem. SOC. 1937 59 33. 37 Edwards and Fowles J. Less-Common Metals 1961 3 181. 38 Clark and Ernel&us J. 1957 4778. 33 Fowles and Osborne J. 1959 2275. 1960,2,401. 28 QUARTERLY REVIEWS TiCl(NHh, is isolated from the reaction mixture by washing with ammonia although the bromide is still somewhat contaminated with TiBr,(NH,),.With all three halides the tensimetric studies detect rather less than three mol. of ammonium halide indicating that the third stage of the ammonolysis is somewhat suppressed by ammonium halide the effect being greatest with the iodide. The marked increase in solubility of the ammonolysis products along the series TiCl,<TiBr,<TiI may indicate a decreasing degree of polymerisation while the modest solubility of the ammonobasic titanium(1v) chloride in a concentrated solution of am- monium chloride shows that the polymers can be broken down to some extent. Experiments with anion-exchange resins prove that in the iodide solution at any rate all the titanium is present in anionic form so that similar complex anions are probably present in the solutions formed by the other halides.These complexes are stable only in solution and break down to the ammonolytic mixture on removal of the excess of ammonia. Hexachloro- and hexabromo-titanates with liquid ammonia give products that are much more soluble than those formed by the tetrahalides. It is tempting to consider these complexes as though they react as a mixture of the tetra- halide and ammonium halide. e.g. NHS (NH,),TiCI -+ 2NH4CI + TiCI -+ Ammonolytic mixture and then to explain the increased solubility and decreased ammonolysis as resulting from the presence of the extra ammonium halide; but if the potassium salt behaved like this the liberated potassium chloride would have no marked effect on the ammonolysis of the titanium(1v) chloride since it has only a slight solubility in ammonia.A more convincing explanation requires the direct ammonolysis of the anion followed by the precipitation of potassium chloride when the potassium salt reacts i.e. K,TiCI -+ K,[TiCI,(NH,),] + 2NH,CI -f (NH4)2[TiC14(NH,)21 + 2KCI I Removal of .J excess NH TiCI2(NH,) + 2NH4CI Thus virtually all the titanium is found in solution. The chloro-complexes are the least soluble in ammonia so that small amounts remain un- dissolved in the first washing; on removal of ammonia these complexes break down and the usual ammonobasic titanium(1v) chloride is formed on further treatment with ammonia. Zirconium(1v) halides are less extensively ammonolysed and zirco- nium(~~) chloride for instance forms ZrCl,(NH,) when treated with liquid ammonia at -33.5"; the product is again insoluble in ammonia in the absence of ammonium chloride and probably is polymeric in nature.FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 29 It has been observed that when the compound is heated to 100" in vacuo and treated again with ammonia further ammonolysis takes place such that the Zr:C1 ratio may be lowered to 1.0:2.5; this indicates that the heating tends to break down the polymer so that part of it may be further ammonolysed. The diammonium hexachlorozirconate(1v) behaves as might be expected three mol. of ammonium chloride being detected tensimetrically. The products formed from the rubidium and casium salts are not very soluble presumably because the original complexes themselves are almost insoluble.The ammonolysis of zirconium(1v) bromide was studied at 25" by Bowerman and Fernelius who obtained an insoluble product which they formulated as 3Zr(NH),,7NH4Br,5NH,; it seems more likely that this product is either a mixture of ZrBr,(NH,),xNH and ZrBr,(NH,),,yNH, or a polymer with an average composition corres- ponding to Zr Br = 1.0 2.3. At room temperature one might expect the second Zr-Br bond to be partly ammonolysed. None of the thorium(1v) halides shows any evidence of ammonolysis in liquid ammonia although all form well-defined ammoniates. 3. Reactions of ammono-bases In view of its relatively high solubility in ammonia potassium amide is the most extensively used ammono-base. Sodium amide is practically insoluble but can be used as a suspension or when only a low concentra- tion of amide ions is required.These amides are readily prepared in situ by catalytic decomposition of their alkali-metal solutions and their reactions in ammonia show striking resemblances to the reactions of alkali-metal hydroxides in aqueous systems. Thus simple neutralisation reactions occur KNH + NH,CI 3 KCI + 2NH and potassium permanganate (which forms a violet solution in ammonia) gives a green precipitate of the manganate K,Mn04 when treated with potassium amide ; this precipitate redissolves to a violet solution upon treatment with ammonium Ammonolytic reactions proceed much further in basic solutions than in ammonia alone since by removal of ammonium ions ammonolytic equilibria e.g. MX,(NH,) + 2NH3 + MX(NH,) + NH,X are disturbed and many compounds which show practically no ammono- lysis in ammonia alone are ammonolysed in basic solutions.Thus while carbon@) iodide is not ammonolysed in liquid ammonia at low temperatures the addition of potassium amide leads to the formation of iodoform and iodoamine :41 C14,2NH + NH 3 HCI + INH + 2NH 40 Inone Takamoto and Kurokawa Nippon Kaguku Zasshi 1957 78 274. 41 Watt McBride and Sowards J. Amer. Chem. SOC. 1956 78 1562. 30 QUARTERLY REVIEWS The reaction is complicated by the secondary reactions INH + KNHZ + N2H4 + KI 2CI + N,H4 + 2NH3 -+ N + 2NH.J + 2HCI,. The decomposition products of chloramine in liquid ammonia depend upon the basicity of the medium.42 In acidic solutions the formation of hydrazine by the ammonolysis is suppressed whereas in solutions of potassium amide potassium chloride is precipitated and hydrazine produced.Again nitrogen tri-iodide merely forms an ammoniate in liquid ammonia but ammonolysis occurs when potassium amide is added :43 NH,CI + 2NH + NH2-NH + NH4Cl N13 + 3KNH -f 3KNHI + NH (Fast) 3KNHI 3 3KI + N + NH (Slow) Franklin has given an account4* of the reactions of potassium amide with the salts of many non-transition metals which lead to the formation of amides imides or nitrides e.g. AgNO 3 AgNH Pbl + PbNH Hg12 * Hg3N2 Of late interest has centred around the polymeric transition-metal amides which have been prepared from ammonobasic compounds by further solvolysis or by metathetical reactions of the more ionic salts with potassium amide. (a) Reactions of Ammonobasic Compounds with Alkali-metal Ar~ides.~~- The extent to which metal halides are ammonolysed may be greatly in- creased by the addition of potassium amide and in some instances complete ammonolysis can thus be effected.The resulting amides are inclined to lose ammonia and form the corresponding imides [cf. B(NH&]. The halides that are completely ammonolysed are those of the transition metals in their higher valency (i.e. more acidic) states so that the amides formed are invariably amphoteric and react with either potassium or potassium amide to give potassium salts; consequently it is very difficult to isolate these amides in an uncombined state. Table 4 summarises the products obtained by the reaction of the halides of a number of transition metals. It can be seen that potassium salts are formed with the halides of quadri- valent titanium and zirconium and of quinquevalent tantalum molyb- denum and tungsten.42 Jander 2. anorg. Chem. 1955,280,264. 43 Jander and Schmid 2. anorg. Chern. 1957,292 178. 44 Franklin “The Nitrogen Systems of Compounds,” Reinhold Publ. Jnc. New York 45 Levine and Fernelius Chem. Rev. 1954,54,452. 1935. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 31 Since cerium(rI1) iodide and thorium(1v) bromide are considerably less acidic they are incompletely ammonolysed. The product formed by cerium(rr1) iodide was originally considered to be CeI,,Ce(NH,), 10NH3 but it is more likely to be an ammonobasic cerium(Ir1) iodide polymer [cf. SnCl(NH,),] in which the Ce I ratio averages 2 3. For simplicity this is represented in Table 4 as a mixture; in the same way most of the other products listed in this Table are likely to contain polymeric anions.TABLE 4. Reaction of trmsitional-metal halides with potassium arnide- liquid ammonia solutions. Halide CeI TiBr ZrBr ThBr TaBr MoCl wBr5 Product of reaction with potassium amide CeI(NH2),+CeI2(NH2) Ti( NH)( NK) Zr( NK) ThBr,(NH,) Ta(NH&(NHK),+ Ta(NH,),(NHK) W(NH2X NK) Mo(NH,)(:NK) Ref. 46 47 30 48 45 49 49 (b) Reactions of Ammono-bases with Transition-metal Salts.-Where the metal halides are not ammonolysed the metal amides are more conveniently prepared by metathetical reactions of the more soluble nitrates or thio- cyanates with potassium amide. Schmitz-Dumont and his associates have in this way succeeded in preparing a large number of transition-metal amides. The mechanism involved in the formation of these amides from metal salts is comparable to that encountered in the formation of metal hydrox- ides in water.Equilibria of the type [M(NH,),]”+ + NH + [M(NH3)5(NHz)](E-1)+ + NH,+ may exist and in the presence of amide ions the ammonium-ion concen- tration is reduced and ammonolysis is favoured. A process analogous to olation can then occur so that ultimately polymeric metal amide ammoniates are produced; usually these lose their ammonia quite readily in vacuo. Thus hexammine- chromium(II1) nitrate reacts50 with potassium amide to give the bright red amide [Cr(NH,),](NO,) + 3KNH -+ 3KN0 + 6NH + Cr(NH,) 46 Bergstrom J. Amer. Chem. SOC. 1937,59 1374. 4i Franklin and Hine J. Amer. Chem. Soc. 1912 34 1497. 48 Watt and Malhotra J . Inorg. Nuclear Chem.1959 11 255. 49 Bergstrom J. Amer. Chem. SOC. 1925,47 2317. Schmitz-Dumont Pilzecker and Pepenbrink 2. anorg. Chenz. 1941 248 175. 32 QUARTERLY REVIEWS With an excess of potassium amide some of the bridging bonds are broken and a potassium salt of a polymeric anion is obtained { Cr(NH2)3 I n + KNH2 -f (K[Cr(NH2)41>~ The salt produced from the corresponding cobalt(1rr) amide51 appears to be less polymeric and accordingly soluble in liquid ammonia H,N\ /NH,\ 7 2 2 [$0(NH2)J *+ 3 n KNH n K H N-CO-NH 43-NH2 [H2N ’ \NH,’ ‘NH When the solution is evaporated however de-ammoniation results in polymerisation and thermal decomposition of the product gives a nitrido- salt Bergsfroms2 used the anhydrous thiocyanates as starting materials for the preparation of the amides of bivalent manganese and cobalt and more recently complex thiocyanates have been used of metals that do not form simple thiocyanates.Thus the amides of tervalent titaniums3 and vana- d i ~ m ~ ~ can be prepared K,M(SCN) + 3KNH2 -+ M(NH2) + 6KSCN both amides react with an excess of potassium amide to give the imide salt K[M(:NH),]. The same vanadium compounds are formed by reaction between a solution of potassium in ammonia and vanadium(rr1) bromide.55 Because of their solubility in ammonia complex nitrates may also be useful starting materials for the preparation of amides. Thus the unstable amide Th(NH2)4 of quadrivalent thorium may be prepared56 by treating the anhydrous complex nitrate K,Th(NO,), with four equivalents of potassium amide. The amide decomposes spontaneously { K3[Co2(NH)3(NH2)31 I n + K3n[(Co2Ns)nI f 3nNH3 Th(NH,) -f Th(:NH)(NH,) + NH3 and forms a variety of complex imidoamide salts with an excess of potas- sium amide the composition depending on the amount of potassium amide used; e.g.with a 6-fold excess K[Th(:NH\(NH&,] is believed to be formed. The reaction of potassium amide with halides of the Group VIII metals has been studied extensively by Watt and his Cationic nickel(r1) 51 Schmitz-Dumont and Kron 2. anorg. Chem. 1955,280 180. 52 Bergstrom J. Amer. Chem. SOC. 1924,46 1552 2631. 53 Schmitz-Dumont Simons and Broja Z. anorg. Chem. 1949,258 308. 54 Schmitz-Dumont and Broja 2. anorg. Chem. 1948,255 299. 55 Nicholls J. Inorg. Nuclear Chem. in the press. 50 Schmitz-Dumont and Raabe 2. anorg. Chem. 1954 277 297. 57 Watt and Dawes J. Amer.Chem. Soc. 1959 81 8; Watt Walling and Mayfield ibid. 1953 75 6175; Watt and Davies ibid. 1948 70 3753; Watt Choppin and Hall J. Electrochem. SOC. 1954 101 235. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 33 salts react at -33" to give nickel(r1) amide diammoniate and this amide yields elemental nickel when thermally decomposed in vacuo at 585". 'Tetrammineplatinum(n) bromide yields platinum(I1) amide diammoniate This amide also gives the metal when thermally decomposed [Pt(NH,),]Br + ZKNH -+ Pt(NH2),,2NH -l- 2KBr + 2NH 3Pt(NH2),,2NH -+ 3Pt + lONH + N With bis(ethylenediamine)platinum(Ir) iodide potassium amide causes replacement of the iodine atoms according to the equations [Pt en,]l + KNH -+ [Pt( en-H) en11 + KI + NH [Pt en,] I + ZKNH + Pt( en-H) + 2KI + ZNH [where (en-H) represents the abstraction of a proton from a nitrogen atom bonded to the central platinum atom].With an excess of potassium amide the potassium salt K[Pt(en-H)(en-2H)] is formed; the addition of am- monium halides to these compounds regenerates the original complex Pt( en-H) + 2NH,X -f [Pt en,]X + 2NH The titration of iridium(r1I) bromide with potassium amide provides evidence for the stepwise replacement of bromine atoms by amide groups followed by conversion of the resultant yellow iridium(II1) amide into a red-brown solution of potassium hexa-amidoiridate(II1). The reactions (co-ordinated ammonia omitted) are IrBr + KNH + IrBr,NH + KBr IrBr,NH + KNH + IrBr(NH,) + KBr IrBr(NH,) + KNH -f Ir(NH& + KBr lr(NH2)3 + 3KNH2 -+ K,[lr(NH,),] Bromopentamminerhodium(Ir1) bromide reacts similarly with potassium amide but only as far as the formation of rhodium(1u) amide.(c) Reactions of Ammono-acids with Transition-metal hides.-We have seen that ammonobasic metal halides are almost invariably soluble in liquid ammonia solutions of ammonium halides and this solubility has been attributed to the formation of soluble complex anionic species of the metal e.g. TiI(NH,) + 2NHJ -f (NH4),[TiI,(NH,),] With the amides (or potassium derivatives) of those metals whose halides are ammonolysed in ammonia we should expect dissolution to occur upon treatment with ammonium halide solutions (provided that the amides are not resistant to attack through polymerisation) e.g. NH,CI NH,CI NH,CI Mo(NH,)(:NK) Mo(NHJ5 [Mo(NH,)~CI]- -6 [Mo(NH,),CI,]- KNH NHZ- N H,- Where the metal halide or salt is not ammonolysed in ammonia it is often 2 34 QUARTERLY REVIEWS possible to regenerate this compound by treatment of the amide with the corresponding ammonium salt e.g.Ni(NH,),,2NH3 + 2NH,I -+ Nil,,6NH3 Even with the polymeric diamidothorium(1v) imide { HN Th(NH,) In all the existing bridging bonds are broken upon treatment with ammonium iodide in ammonia and colourless crystals of thorium(rv) iodide octa- ammoniate ThI,,8NH3 are obtained. Low-temperature treatment of the polymeric chromium(rI1) and cobalt(rI1) amides with ammonium salts results in dissolution of the amides. Schmitz-Dumont suggests50 that the residues remaining after removal of ammonia from these solutions contain high polymers such as { [Cr(NHzi) 3(NH 2)BrIBr I n and { [Co(NH3) 3(NH 2) 2 IN03 >n - 4.Reactions of alkali-metal solutions (a) Reduction and Effect on Ammono1ysis.-Solutions of the alkali and alkaline-earth metals in liquid ammonia have attracted widespread interest not only in the physical nature of the solutions but also in the use of the solutions as reducing agents. Early work utilising alkali- metal solutions as reducing agents for inorganic substances is reviewed el~ewhere.~ Attention in recent years has been largely devoted to the production of unusual valency states of metals and to the synthetic uses of the solutions particularly in the field of organic and organometallic chemistry. With many metal salts reduction by an alkali metal in ammonia yields the elemental metal. Such reductions are usually complicated however since the metal formed is in a finely divided and highly reactive state.Thus many metals e.g. zinc cadmium mercury tin and lead form poly-ide salts with an excess of alkali metal and others catalyse the decomposition of the alkali-metal solution i.e.; K + NH3 + KNH + iH2 so that the final products of the reduction may contain the metal amide; e.g. with nickel salts58 there are the reactions NiBr + 2K -f Ni + 2KBr 2K + 2NH3 + ZKNH + H ZKNH + NiBr + 2NH3 -+ Ni(NH2),,2NH + 2KBr The catalytic breakdown of the alkali-metal solutions to the amides may however occur without permanent reduction of the transition metal and 58 Burgess and Eastes J. Amer. Chem. SOC. 1941 63. 2674; Watt and Davies ibid. 1948,70 3753. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 35 many halides give the same products.with alkali-metal solutions as they do with solutions of alkali-metal amides e.g. N H3 K-NHs ZrBr -+ ZrBr,(NH,) -+ Zr(:NK) NH3 K-N H VBr -+ VBr,(NH,) -2 V(:NH)(NHK) and hydrogen gas is liberated. We have already seen that neither boron(n1) fluoride nor the aluminium(rI1) halides are ammonolysed unless an alkali metal is present and it is interesting that while only one B-F bond is ammonolysed on addition of potassium both sodium and lithium effect greater ammonolysis namely K-NHS BF - -+ BF,(NH2) Na-NH Li-NHa 4 ( N H .J2 B e N H * B F( N H ,) -+ ( N H,),B* N H * B( N H) Sometimes a difference in reactivity of the various alkali-metal solutions may be attributed to the difference in the solubilities of the alkali-metal amides (LiNH and NaNH almost insoluble; KNH soluble).Thus tetramethyl-lead reacts with alkali metals in ammonia to give [Pb(CH,),]- ions and methyl radicals (which yield methane and ethane) :69 Pb(CH,) + e- -+ [Pb(CH,),]- + CH3. In the presence of potassium solvolysis of the [Pb(CH,),]- ions occurs giving lead imide as the final product [Pb(CH,),]- + NH3 -+ [Pb(CHJ,NH,]- + CH [Pb(CH,),NH,]- + NH3 -f [Pb(CH3)(NH,)z]- + CHI [Pb(CH,)(NH,),]- + NH3 + [Pb(NH&]- + CHI [Pb(NH,),]- + PbNH + NH + [NH,]- With lithium and sodium however the insolubility of the amides (at -78”) retards the ammonolysis through the reaction [Pb(CH,),NH,]- + Na+ -f Pb(CH,) + NaNH so that only two methyl groups are removed and the product is dimethyl- lead. With the tetra-alkylammonium salts R,NX the N-C bond is ruptured by potassium to give a trialkylamine and a hydrocarbon; e.g.for (CH,),NCl “(CH,),]+ + e -+ N(CH,) + CH,. [CH& + NH + CH + “41- CH,. + e -+ [CH,]- 69 Holliday and Pass J. 1958 3485. Hazlehurst Holliday and Pass J. 1956 4653. 36 QUARTERLY REVIEWS The amide ion produced yields an olefin if R contains a /3-carbon atom e.g. [N(C2H!5)41+ + lNH21- -f N(C2H5)3 f C2H4 f NH3 [(CH3*CHz*CH2)3NCH3]+ + [NHz]- -+ (CH,CH,.CH2),N*CH3 + C,H6 -t NH Evolution of hydrogen during an alkali-metal reduction was used by earlier workers to infer the presence of an ammonium ion. Thus the “diammoniate” of diborane was formulated61 as an ammonium salt (NH,) [H,B(NH,)BH,] on account of its reaction with a sodium solution NH (NH,)[H,B(NH,)BH,] + Na -+ Na[H,B(NH2)BH,l + 3H2 + NH More recent work by Parry and his co-workerss2 has shown the compound to be a borohydride [NH3(BH2)NH3]BH, whose reaction with sodium may be written [NH,(BH,)NH,]BH + Na -+ NaBH1 + BH2NH2 + 4H2 + NH N H* Trimethylboron gives hydrogen when treated with a solution of sodium or potassium and the salt M[B(CH,),NH,] is formed? the trimethylboron can be regenerated by treatment with a solution of ammonium bromide.Tetramethyldiborane is split symmetrically into BH(CH3),,NH3 and the salt Na,BH(CH,) when treated with an excess of sodium in ammonia at -78°,64 and an analogous calcium salt CaBH(CH,),,NH can be piepared. The [BH(CH,),]- ion in the sodium salt shows the high base strength expected of a boron atom with a lone pair of electrons in that it forms a stable compound Na [BH(CH,),],B(CH,), with trimethylboron; this compound appears to contain B-B links.Conductimetric studies of the reactions of alkali-metal solutions with a number of covalent h y d r i d e ~ ~ ~ have shown that disubstitution by alkali metals can occur in SnH, ASH, SbH, and GeH, but not in PH,. Thus disodiostannane SnH,Na, may be isolated after reaction of stannane with a ten-fold excess of sodium; in the presence of excess of the hydride however conversion into the monosubstituted derivative occurs SnH,Na -t SnH + 2SnH,Na The analogous germanium compound NaGeH, reacts with phenyl bromide to give germanium(1r) hydride NaGeH + C6H,Br -+ C6H6 + GeH + NaBr 61 Schlesinger and Burg J. Amer. Chem. SOC. 1938 60 290. 62 Schultz and Parry J . Amer. Chem. SOC. 1958 80 4. 64 Burg and Campbell J. Amer. Chem. SOC. 1952,74 3744; 1957,79,4023. 65 EmelCus and Kettle J.1958 2445; Emeleus and Mackay J. 1961,2676. Smith and Kraus J . Amer. Chem. SOC. 1951 73 2751; Holliday and Thompson J. 1960 2695. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 37 this hydride may be recrystallised from liquid ammonia and it reacts with one atom of sodium to form a soluble red salt which gives germane GeH, on treatment with ammonium bromide. Alkylgennanes with sodium give the monosubstituted salts,66 e.g. NaEtGeH,. (b) The Production of Unusual Valency States.-Whereas simple nickel@) salts are reduced to the metal with alkali metal-ammonia solutions the complex nickel(I1) cyanidess7 yield products in which the metal appears to exhibit a valency of less than two. Thus when an excess of potassium tetracyanonickelate(I1) is treated with sodium or potassium in ammonia a bright red precipitate is formed of composition K,[Ni(CN),] [Ni(CN),I2- + e -+ [Ni(CN),la- + CN- If an excess of alkali metal is used in the reduction a bulky yellow precipi- tate of the tetracyanonickelate(0) separates [Ni(CN),I2- + 2e -+ [Ni(CN),I4- The potassium salt can be isolated as a copper-coloured solid extremely unstable in air and blackening on exposure.It dissolves in water evolving hydrogen and giving a red solution with the characteristic properties of the tricyanonickelate(1) anion. The tetracyanonickelate(0) ion [Ni(CN414- is of course isoelectronic with the carbonyl Ni(CO),. The corresponding zero-valent palladium ion,sa [Pd(CN),I4- may be made by the reduction of [ Pd(CN),I2- salts. In liquid ammonia both of the zero-valent salts reduce azoben2ene to hydrazobenzene and silver and mercury(I1) salts to the metals.The analogous cobalt salt may be prepared69 by reduction of potassium hexacyanocobaltate(n1) K,[Co(CN),] + 3K+ + 3e + K,[Co(CN),] + 2KCN although potentiometric titrations the intermediate formation of a cobalt(I1 complex K3 [Co(CN),]. The reduction of the hexacyano- chromate(II1) yields a product containing chromium(I) while the hexacyanomanganate(Ir1) ion appears to give a product containing both the uni- and the zero-valent state of manganese 2K,[Mn(CN),] + 5K+ + 5e + K,[Mn(CN),],K,[Mn(CN),],2NH This formulation of the product is consistent with the amount of potassium used in the reduction its reducing action towards silver nitrate and chemical analysis. 66 Glarum and Kraus J . Amer. Chem. SOC.1950,72,5398. 67 Eastes and Burgess J . Amer. Chem. SOC. 1942,64,1187. Ga Burbage and Fernelius J. Amer. Chem. SOC. 1943 65 1484. 6 9 Hieber and Bartenstein Nuturwiss. 1952,39 300; 2. unorg. Chem. 1954,276 12. 70 Watt Hall Choppin and Gentile J. Amsr. Chem. SOC. 1954 76 373; Watt and 71 Davidson and Kleinberg J. Phys. Chem. 1953 57 571. Thompson J. Inorg. Nuclear Chem. 1959,9 31 1. 3s QUARTERLY REVIEWS Some unusual ammine complexes of platinum and iridium have been prepared by Watt and his co-~orkers.~~ When tetrammineplatinum(I1) bromide is reduced with potassium in ammonia at the boiling point exactly two gram-atoms of potassium are consumed per mole of the bromide and a yellowish-white solid is precipitated. Thermal decomposi- tion of this solid yields only platinum and ammonia and the solid appears to be tetrammineplatinum(0) [Pt(NH,),]Br + 2K+ + 2e -+ Pt(NH,) + 2KBr Diamagnetic pentammineiridium(0) is similarly ~btained’~ when bromo- pentammineiridium(rr1) bromide is reduced with potassium in boiling ammonia [Ir(NH,) BrIBr + 3K+ + 3e -+ lr(NH3)5 + 3KBr The stability of these zero-valent complex ammines is unusual in that the ligands are incapable of forming 7.r-bonds with the metal.A co-ordination compound with a bidentate nitrogen ligand that may contain platinum(0) has been as an unstable pink precipitate [identifiable as bisethylenediamineplatinum(0) only by its thermal de- composition products] when two equivalents of potassium react with bisethylenediamineplatinum(r1) iodide. At room temperature it de- composes by two paths Pt -/- 2en t Pt en -+ Pt( en-H) + H When only one equivalent of potassium is used in the reduction of the iodide an unstable compound [Pt en,]I is formed which disproportionates at room temperature 2 Pt en,l -+ Pt en + [Pt en,]& 5.Recent synthetical reactions So far we have discussed reactions involving ammonia reactions of alkali-metal amides and reductions with alkali-metal solutions. There remains the large field of reactions in which ammonia acts merely as a solvent medium. The advantages of liquid ammonia as a solvent medium for synthetic procedures are by now well known. The solubility relation- ships shown by ammonia especially those which show different trends from those found in water are often most valuable. The smaller tendency of ammonia than of water to solvolysis and the greater stability of products at the lower temperatures employed often permit the study and isolation of compounds which cannot be prepared or which can be obtained only with difficulty from other solvents.Many examples of metathetical reactions are already reviewed74 and we shall mention only those syntheses 7 2 Watt and Mayfield J. Amer. Chenz. SOC. 1953 75 6178. 73 Watt McCarley and Dawes J. Amer. Chem. SOC. 1957 79 5163; 1959 81 8. 74 Audrieth and Kleinberg “Non-aqueous Solvents,” J. Wiley & Sons Inc. New York 1953. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 39 which have been used in the last decade. These are conveniently divided into two sections (a) inorganic and (b) organo-metallic syntheses. (a) Inorganic Syntheses.-One interesting preparation is that of am- monium nitrite,75 which is obtained when sodium nitrite and ammonium chloride are mixed in ammonia at -33.5".Phosphine reacts rapidly with alkali-metal solutions in ammonia to give the salts MIPH, which are valuable for the synthesis of a variety of phosphine derivatives. Treatment of a solution of the sodium salt (NaPH,) with methyl chloride results in precipitation of sodium chloride and the formation of methylph~sphine~~ which can be obtained in good yield from the filtrate CH3CI + NaPH -+ NaCl + CH3.PH2 When trimethylene dichloride is used in place of methyl chloride tri- methylenediphosphine and polymeric alkylphosphines of formula PH,-CH(CH,)CH,.[PH.CH(CH3)CH,];PH2 are ~btained.~' Heavy- metal hydrogen phosphides have been prepared by metathesis of KPH and an ammonia-soluble metal salt .'* From hexamminecobalt(~rr) nitrate a dark brown pyrophoric phosphine Co(PH,), is obtained [Co(NH,),](NO,) + 3KPH2 + Co(PH2)S + 3KNO3 + 6NH3 This compound decomposes spontaneously at O" to leave a black residue of Co(PH), and from this a crystalline phosphide COP,., is formed at 560".With an excess of KPH and the hexammine nitrate the insoluble amorphous salt KCo2(PH2) is obtained. Nickel(1r) thiocyanate gives Ni(PH,), which is unstable at 0" and reacts with an excess of KPH to give a soluble complex which is probably Kz[Ni(PH,),]. Peroxides of magnesium cadmium and zinc have been isolated for the first time79 by treatment of the metal nitrates with sodium or potassium superoxides (which are sparingly soluble and insoluble respectively in ammonia). The unstable superoxide intermediate dissociates to give the peroxide Zn(NO,) + 2K0 -f Zn(02) + 2KN0 Zn(O2)2 -f ZnO2 + 0 2 Strontium nitrate gives strontium peroxide but reaction of barium nitrate with four equivalents of potassium superoxide gives a compound whose analysis suggests the stoicheiometry :*O Ba(NO,) + 4K0 -+ K,Ba(O,),O -+- 2KN0 + 0 This is the only case found of mixed superoxide-peroxide formation; it is i 5 Larbouillat-Linemann Compt.rend. 1954 238 902. i 6 Wagner and Burg J. Amer. Chem. SOC. 1953,75 3869. '' LefHer Groch and Teach Abs. Papers Amer. Chem. SOC. meeting Sept. 1959. '* Schmitz-Dumont Nagel and Schaal Angew. Chem. 1958,70,105. i Q Schechter and Kleinberg J. Amer. Chem. SOC. 1954,76 3297. * O Seyb and Kleinberg J. Amer. Chem. SOC. 1951,73 2308. 40 QUARTERLY REVIEWS noteworthy that in this compound the cations have almost identical ionic radii K+ 1-33 A; Ba2+ 1.35 A.The reactions of solutions of alkali-metal borohydrides in ammonia have been used to afford,s1 by metathesis borohydrides such as (b) Organo-metallic Syntheses.-(i) Coupling reactions. In early synthetic work on organometallic compounds Kraus made extensive use of liquid ammonia and alkali metal-ammonia solutions. The reactions of organometallic compounds with alkali-metal solutions were reviewed in 1950,82 so that coupling reactions of the type R,MNa + XR' -+ R,MR' + NaX (R R' = alkyl or aryl X = halogen) need not be considered here in detail; as general illustrations of this type of reaction two examples may be cited 83984 [Cr(NM,),I [BH413 and [Mg(NH3),1 EBH412. 2Ph,GeNa + Br.[CH,],-Br -+ Ph,Ge.[CH,],.GePh + 2NaBr Et,PbCI + 2Li + PhCH,CI -+ Et,PbCH,Ph + 2LiCI When the ainmoniate of trimethyl-lead borohydride is distilled at -5 O a most interesting reaction occurs,s5 in which trimethylplumbane is ob- tained with elimination of ammonia-borine N Ha Me,PbCI + KBH4 -+ [Me,Pb]BH,,xNH -33" -5" -+ Me,PbH + H,B,NH + (x-l)NH This hydride reacts rapidly with ammonia at -78" to form ammonium trimethylplumbate and addition of trimethyl-lead chloride then gives hexame thyldiplumbane Me,PbH + NH -+ NH,[PbMe,] NH,[PbMe,] + Me,PbCI -+ (Me,Pb) + NH,CI (ii) "Sandwich" compounds.Cyclopentadienyl and indenyl compounds of a number of transition metals have been prepareds6 in liquid ammonia by reactions of the type [Co(NH,),](SCN) + 2C5H5Li -+ [Co(NH,),](C,H,) + 2LiSCN The ammoniates are readily decomposed thermally in vacuo and the resulting cyclopentadienyl compounds may then be obtained by sublima- tion.The method has been used in particular for the preparation of the Parry Schultz and Girardot J. Amer. Chem. Soc. 1958 80 1. Watt Chem. Rev. 1950 46 317. Smith and Kraus J . Amer. Chem. SOC. 1952 74 1418. 84 Gilman and Leeper J. Org. Chem. 1951 16,466. 85 Duffy and Holliday J. 1961 1678. 86 Fischer and Fritz Adv. Inorg. Chem. and Radiochem. 1959 1 55. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 4 1 cyclopentadienyls of chromium(III) manganese(rr) iron(rr) cobalt(rr) and nickel(r~) and the indenyl of cobalt(r1). (iii) Acetylene derivatives. Acetylene derivatives of the main-group elements have been known for some time.When acetylene is bubbled through a solution of sodium in ammonia the monosodium derivative NaC,CH can be obtained and this is a useful starting material for liquid-ammonia preparations of metal-acetylene compounds. With the Group IV metals for example,87 those triaryl- or trialkyl-metal halides (R,MX) which are not ammonolysed in ammonia (Le. tin and lead compounds) react with sodium acetylide giving compounds of the type Ph,PbCi C-PbPh and Et,SnCi CaSnEt,. Monosubstituted products are not isolated because they readily disproportionate to the disubstituted product and acetylene R,SnCI + NaCiCH -+ R,SnCiCH + NaCl 4 R,SnCiC.SnR + C,H The R,SnCiCH intermediates do however react with an excess of the sodium salt and form complex compounds of the type R,Sn.[ Ci CH],.Na. An alternative route to the disubstituted derivatives is by the reaction 2Ph,SnNa + C21 -+ Ph,SnCiC.SnPh + 2Nal The phenyl compounds are stable to water but the ethyl compounds are more sensitive ; the ease of hydrolysis decreases with decreasing polarity of the carbon-metal bond i.e.Pb>Sn>Si. Transition-metal acetylides have recently been synthesised by Nast and his co-workers.88 Once again the soluble thiocyanates are used as starting materials [Ni(NH,),](SCN) + 4K.CiC.Ph + Vacuo K,[Ni(Ci C.Ph),],ZNH + 2KSCN + 4NH -+ K,[Ni(Ci C-Ph),] K,[Ni(Ci C.Ph),] + [Ni(NH,),](SCN) + Ni(Ci C*Ph),,4NH -4 (PhX i CeNiCi C.Ph)x Vacuo The ammonia-free complexes are rather unstable decomposing in several days at room temperature and derivatives of alkylacetylenes are prone to detonation. The diamagnetism of K,[Ni(C:C-Ph),] suggests that the nickel atom is in a square-planar environment.Like the analogous complex cyanide the acetylide K,[Ni(Ci CH),] can be reduced to the zero-valent compound K,[Ni(Ci CH),] by the action of a solution of potassium in ammonia. 87 Beerman and Hartmann Z. anorg. Chem. 1954,276 20. Nast Internat. Conference on Co-ordination Chemistry Chem. SOC. Special Publ. No. 13 1959 p. 103; Nast Z. Nuturforsch. 1953,8b 381; Nast and Pfab Chem. Ber. 1956,89,415; Nast and Vester,Z. anorg. Chem. 1955,279,146; Nast and Lewinsky ibid. 1955 282 210; Nast and Urban ibid. 1956 287 17; Nast and Pfab ibid. 1957 292 287. 42 QUARTERLY REVIEWS Analogues of hexacyanoferrate(rI1) and hexacyanoferrate(I1) complexes can be obtained from iron(r1) thiocyanate Fe(SCN),,4NH3 + 6MCiCR -f M,[Fe(CiCR),] + 2MSCN + 4NH 2K,[Fe(CiCH),] + 2NH + O2 -+ 2K,[Fe(CiCH),] + H20 + 2KNH2 The iron(II1) complex is unstable exploding above -30".The correspond- ing cobalt compounds Na,[Co(Ci CMe),] (paramagnetic) and Na [Co(C i CMe),] (diamagnetic) are obtained similarly. Reaction of copper(r) iodide with potassium acetylide in ammonia yields the orange cuprous acetylide which decomposes above -45" to black Cu,C and acetylene CUI + K'CiCH --+ Cu-CiCH -4 C U ~ C ~ + C2H2 -78" -45" Alkynylcuprates(1) of the types M,[Cu(CiCR),] and M [Cu(Ci CR),] can also be obtained e.g. when potassium methyl or phenyl acetylide is used NH Cul + 3KCiCMe -+ K,[Cu(CiCMe),],xNH + KI Vacuo - K,[Cu(Ci CMe),] (iv) Carbonyls. Reactions of alkali-metal-ammonia solutions with metal carbonyls have recently been studied in some detail by Behrens and his co-worker~.~~ With the mononuclear hexacarbonyls of Group VIA metals reduction occurs with sodium in ammonia yielding the com- pounds Na,M(CO) (M = Cr Mo or W).These can be recovered from the solution by evaporation of the solvent; they contain ammonia of crystal- lisation unless warmed to 70" in vacuo. A large excess of alkali metal decomposes them. Sodium reduction of the iron carbonyls Fe(CO), { Fe(CO) j3 Fe,(CO), and Fe(CO),I leads in each case to Na,[Fe(CO),]. At -75" almost no carbon monoxide is liberated when sodium reacts with {CO(CO),}, but at higher temperatures ammonia reacts with the carbonyl 3[CO(CO).J2 + 12NH3 -+ 2[CO(NH,),][CO(CO),]2 + 8CO this ammine is then reduced by sodium to cobalt and Na[Co(CO),] 2Na + [Co(NH,),][Co(CO),] -+ 2Na[Co(CO),] + Co + 6NH Nickel carbonyl also forms anions of the type [Ni,(C0),I2- and [Ni,(C0)J2- when reduced by solutions of alkali metals in liquid am- monia.Carbonylcyano-complexes of nickel have been isolatedgl from liquid 8g Behrens and Weber Z . anorg. Chem. 1955 281 190; 1957 291 122; Behrens Behrens and Lohofer Chem. Ber. 1961,94,1391,1497; Hieber Kroder and Zahn Z. Naturforsch. 1952,7b 321. Z . Naturforsch. 1960,15b 325. 91 Nast and Roos 2. anorg. Chem. 1953 272,242. FOWLES AND NICHOLLS INORGANIC REACTIONS IN LIQUID AMMONIA 43 ammonia as yellow crystals by passage of oxygen-free carbon monoxide through solutions of complex nickel cyanides at -40" K,[Ni,(CN),] + 2CO +- 2K2[Ni(CN),CO] K,[Ni(CN),] + 2CO + K,[Ni(CN),(CO),] + 2KCN The magnetic properties of K,[Ni(CN),CO] indicate that this compound is dimeric. Reaction with potassium acetylidesg2 yields yellow explosive. octaalkynyldinickelates K,[Ni,(CN),(CO),] + 8KCiCH -+ K,[Ni,(CiCH),] + 6KCN + 2CO Thanks are offered to Dr. M. Allbutt for valuable discussions. 92 Nast and Kasperl Chem. Ber. 1959 92 2135.