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titial nitrides of formulae mn m 2 n and m 4 n although variable composition is perfectly possible where the small nitrogen atoms are positioned in the gaps in a metallic cubic or hexagonal close packed lattice they are opaque very hard and chemically inert melting only at very high temperatures generally over 2500 c they have a metallic lustre and conduct electricity as do metals they hydrolyse only very slowly to give ammonia or nitrogen 56 the nitride anion n 3 is the strongest π donor known among ligands the second strongest is o 2 nitrido complexes are generally made by the thermal decomposition of azides or by deprotonating ammonia and they usually involve a terminal n 3 group the linear azide anion n 3 being isoelectronic with nitrous oxide carbon dioxide and cyanate forms many coordination complexes further catenation is rare although n 4 4 isoelectronic with carbonate and nitrate is known 56 hydrides standard reduction potentials for nitrogen containing species top diagram shows potentials at ph 0 bottom diagram shows potentials at ph 14 57 industrially ammonia nh 3 is the most important compound of nitrogen and is prepared in larger amounts than any other compound because it contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilisers it is a colourless alkaline gas with a characteristic pungent smell the presence of hydrogen bonding has very significant effects on ammonia conferring on it its high melting 78 c and boiling 33 c points as a liquid it is a very good solvent with a high heat of vaporisation enabling it to be used in vacuum flasks that also has a low viscosity and electrical conductivity and high dielectric constant and is less dense than water however the hydrogen bonding in nh 3 is weaker than that in h 2 o due to the lower electronegativity of nitrogen compared to oxygen and the presence of only one lone pair in nh 3 rather than two in h 2 o it is a weak base in aqueous solution p k b 4 74 its conjugate acid is ammonium nh 4 it can also act as an extremely weak acid losing a proton to produce the amide anion nh 2 it thus undergoes self dissociation similar to water to produce ammonium and amide ammonia burns in air or oxygen though not readily to produce nitrogen gas it burns in fluorine with a greenish yellow flame to give nitrogen trifluoride reactions with the other nonmetals are very complex and tend to lead to a mixture of products ammonia reacts on heating with metals to give nitrides 58 many other binary nitrogen hydrides are known but the most important are hydrazine n 2 h 4 and hydrogen azide hn 3 although it is not a nitrogen hydride hydroxylamine nh 2 oh is similar in properties and structure to ammonia and hydrazine as well hydrazine is a fuming colourless liquid that smells similar to ammonia its physical properties are very similar to those of water melting point 2 0 c boiling point 113 5 c density 1 00 g cm 3 despite it being an endothermic compound it is kinetically stable it burns quickly and completely in air very exothermically to give nitrogen and water vapour it is a very useful and versatile reducing agent and is a weaker base than ammonia 59 it is also commonly used as a rocket fuel 60 hydrazine is generally made by reaction of ammonia with alkaline sodium hypochlorite in the presence of gelatin or glue 59 nh 3 ocl nh 2 cl oh nh 2 cl nh 3 n 2 h 5 cl slow n 2 h 5 oh n 2 h 4 h 2 o fast the attacks by hydroxide and ammonia may be reversed thus passing through the intermediate nhcl instead the reason for adding gelatin is that it removes metal ions such as cu 2 that catalyses the destruction of hydrazine by reaction with monochloramine nh 2 cl to produce ammonium chloride and nitrogen 59 hydrogen azide hn 3 was first produced in 1890 by the oxidation of aqueous hydrazine by nitrous acid it is very explosive and even dilute solutions can be dangerous it has a disagreeable and irritating smell and is a potentially lethal but not cumulative poison it may be considered the conjugate acid of the azide anion and is similarly analogous to the hydrohalic acids 59 halides and oxohalides nitrogen trichloride all four simple nitrogen trihalides are known a few mixed halides and hydrohalides are known but are mostly unstable examples include nclf 2 ncl 2 f nbrf 2 nf 2 h nfh 2 ncl 2 h and nclh 2 61 nitrogen trifluoride nf 3 first prepared in 1928 is a colourless and odourless gas that is thermodynamically stable and most readily produced by the electrolysis of molten ammonium fluoride dissolved in anhydrous hydrogen fluoride like carbon tetrafluoride it is not at all reactive and is stable in water or dilute aqueous acids or alkalis only when heated does it act as a fluorinating agent and it reacts with copper arsenic antimony and bismuth on contact at high temperatures to give tetrafluorohydrazine n 2 f 4 the cations nf 4 and n 2 f 3 are also known the latter from reacting tetrafluorohydrazine with strong fluoride acceptors such as arsenic pentafluoride as is onf 3 which has aroused interest due to the short n o distance implying partial double bonding and the highly polar and long n f bond tetrafluorohydrazine unlike hydrazine itself can dissociate at room temperature and above to give the radical nf 2 fluorine azide fn 3 is very explosive and thermally unstable dinitrogen difluoride n 2 f 2 exists as thermally interconvertible cis and trans isomers and was first found as a product of the thermal decomposition of fn 3 61 nitrogen trichloride ncl 3 is a dense volatile and explosive liquid whose physical properties are similar to those of carbon tetrachloride although one difference is that ncl 3 is easily hydrolysed by water while ccl 4 is not it was first synthesised in 1811 by pierre louis dulong who lost three fingers and an eye to its explosive tendencies as a dilute gas it is less dangerous and is thus used industrially to bleach and sterilise flour nitrogen tribromide nbr 3 first prepared in 1975 is a deep red temperature sensitive volatile solid that is explosive even at 100 c nitrogen triiodide ni 3 is still more unstable and was only prepared in 1990 its adduct with ammonia which was known earlier is very shock sensitive it can be set off by the touch of a feather shifting air currents or even alpha particles 61 62 for this reason small amounts of nitrogen triiodide are sometimes synthesised as a demonstration to high school chemistry students or as an act of chemical magic 63 chlorine azide cln 3 and bromine azide brn 3 are extremely sensitive and explosive 64 65 two series of nitrogen oxohalides are known the nitrosyl halides xno and the nitryl halides xno 2 the first is very reactive gases that can be made by directly halogenating nitrous oxide nitrosyl fluoride nof is colourless and a vigorous fluorinating agent nitrosyl chloride nocl behaves in much the same way and has often been used as an ionising solvent nitrosyl bromide nobr is red the reactions of the nitryl halides are mostly similar nitryl fluoride fno 2 and nitryl chloride clno 2 are likewise reactive gases and vigorous halogenating agents 61 oxides main article nitrogen oxide nitrogen dioxide at 196 c 0 c 23 c 35 c and 50 c no 2 converts to colourless dinitrogen tetroxide n 2 o 4 at low temperatures and reverts to no 2 at higher temperatures nitrogen forms nine molecular oxides some of which were the first gases to be identified n 2 o nitrous oxide no nitric oxide n 2 o 3 dinitrogen trioxide no 2 nitrogen dioxide n 2 o 4 dinitrogen tetroxide n 2 o 5 dinitrogen pentoxide n 4 o nitrosylazide 66 and n no 2 3 trinitramide 67 all are thermally unstable towards decomposition to their elements one other possible oxide that has not yet been synthesised is oxatetrazole n 4 o an aromatic ring 66 nitrous oxide n 2 o better known as laughing gas is made by thermal decomposition of molten ammonium nitrate at 250 c this is a redox reaction and thus nitric oxide and nitrogen are also produced as byproducts it is mostly used as a propellant and aerating agent for sprayed canned whipped cream and was formerly commonly used as an anaesthetic despite appearances it cannot be considered to be the anhydride of hyponitrous acid h 2 n 2 o 2 because that acid is not produced by the dissolution of nitrous oxide in water it is rather unreactive not reacting with the halogens the alkali metals or ozone at room temperature although reactivity increases upon heating and has the unsymmetrical structure n n o n n o n n o above 600 c it dissociates by breaking the weaker n o bond 66 nitric oxide no is the simplest stable molecule with an odd number of electrons in mammals including humans it is an important cellular signalling molecule involved in many physiological and pathological processes 68 it is formed by catalytic oxidation of ammonia it is a colourless paramagnetic gas that being thermodynamically unstable decomposes to nitrogen and oxygen gas at 1100 1200 c its bonding is similar to that in nitrogen but one extra electron is added to a π antibonding orbital and thus the bond order has been reduced to approximately 2 5 hence dimerisation to o n n o is unfavourable except below the boiling point where the cis isomer is more stable because it does not actually increase the total bond order and because the unpaired electron is delocalised across the no molecule granting it stability there is also evidence for the asymmetric red dimer o n o n when nitric oxide is condensed with polar molecules it reacts with oxygen to give brown nitrogen dioxide and with halogens to give nitrosyl halides it also reacts with transition metal compounds to give nitrosyl complexes most of which are deeply coloured 66 blue dinitrogen trioxide n 2 o 3 is only available as a solid because it rapidly dissociates above its melting point to give nitric oxide nitrogen dioxide no 2 and dinitrogen tetroxide n 2 o 4 the latter two compounds are somewhat difficult to study individually because of the equilibrium between them although sometimes dinitrogen tetroxide can react by heterolytic fission to nitrosonium and nitrate in a medium with high dielectric constant nitrogen dioxide is an acrid corrosive brown gas both compounds may be easily prepared by decomposing a dry metal nitrate both react with water to form nitric acid dinitrogen tetroxide is very useful for the preparation of anhydrous metal nitrates and nitrato complexes and it became the storable oxidiser of choice for many rockets in both the united states and ussr by the late 1950s this is because it is a hypergolic propellant in combination with a hydrazine based rocket fuel and can be easily stored since it is liquid at room temperature 66 the thermally unstable and very reactive dinitrogen pentoxide n 2 o 5 is the anhydride of nitric acid and can be made from it by dehydration with phosphorus pentoxide it is of interest for the preparation of explosives 69 it is a deliquescent colourless crystalline solid that is sensitive to light in the solid state it is ionic with structure no 2 no 3 as a gas and in solution it is molecular o 2 n o no 2 hydration to nitric acid comes readily as does analogous reaction with hydrogen peroxide giving peroxonitric acid hoono 2 it is a violent oxidising agent gaseous dinitrogen pentoxide decomposes as follows 66 n 2 o 5 no 2 no 3 no 2 o 2 no n 2 o 5 no 3 no 2 oxoacids oxoanions and oxoacid salts many nitrogen oxoacids are known though most of them are unstable as pure compounds and are known only as aqueous solutions or as salts hyponitrous acid h 2 n 2 o 2 is a weak diprotic acid with the structure hon noh p k a1 6 9 p k a2 11 6 acidic solutions are quite stable but above ph 4 base catalysed decomposition occurs via honno to nitrous oxide and the hydroxide anion hyponitrites involving the n 2 o 2 2 anion are stable to reducing agents and more commonly act as reducing agents themselves they are an intermediate step in the oxidation of ammonia to nitrite which occurs in the nitrogen cycle hyponitrite can act as a bridging or chelating bidentate ligand 70 nitrous acid hno 2 is not known as a pure compound but is a common component in gaseous equilibria and is an important aqueous reagent its aqueous solutions may be made from acidifying cool aqueous nitrite no 2 bent solutions although already at room temperature disproportionation to nitrate and nitric oxide is significant it is a weak acid with p k a 3 35 at 18 c they may be titrimetrically analysed by their oxidation to nitrate by permanganate they are readily reduced to nitrous oxide and nitric oxide by sulfur dioxide to hyponitrous acid with tin ii and to ammonia with hydrogen sulfide salts of hydrazinium n 2 h 5 react with nitrous acid to produce azides which further react to give nitrous oxide and nitrogen sodium nitrite is mildly toxic in concentrations above 100 mg kg but small amounts are often used to cure meat and as a preservative to avoid bacterial spoilage it is also used to synthesise hydroxylamine and to diazotise primary aromatic amines as follows 70 arnh 2 hno 2 arnn cl 2 h 2 o nitrite is also a common ligand that can coordinate in five ways the most common are nitro bonded from the nitrogen and nitrito bonded from an oxygen nitro nitrito isomerism is common where the nitrito form is usually less stable 70 fuming nitric acid contaminated with yellow nitrogen dioxide nitric acid hno 3 is by far the most important and the most stable of the nitrogen oxoacids it is one of the three most used acids the other two being sulfuric acid and hydrochloric acid and was first discovered by alchemists in the 13th century it is made by the catalytic oxidation of ammonia to nitric oxide which is oxidised to nitrogen dioxide and then dissolved in water to give concentrated nitric acid in the united states of america over seven million tonnes of nitric acid are produced every year most of which is used for nitrate production for fertilisers and explosives among other uses anhydrous nitric acid may be made by distilling concentrated nitric acid with phosphorus pentoxide at low pressure in glass apparatus in the dark it can only be made in the solid state because upon melting it spontaneously decomposes to nitrogen dioxide and liquid nitric acid undergoes self ionisation to a larger extent than any other covalent liquid as follows 70 2 hno 3 h 2 no 3 no 3 h 2 o no 2 no 3 two hydrates hno 3 h 2 o and hno 3 3h 2 o are known that can be crystallised it is a strong acid and concentrated solutions are strong oxidising agents though gold platinum rhodium and iridium are immune to attack a 3 1 mixture of concentrated hydrochloric acid and nitric acid called aqua regia is still stronger and successfully dissolves gold and platinum because free chlorine and nitrosyl chloride are formed and chloride anions can form strong complexes in concentrated sulfuric acid nitric acid is protonated to form nitronium which can act as an electrophile for aromatic nitration 70 hno 3...