11.1: Introduction

Chemistry 242 - Inorganic Chemistry II
Chapter 10 - Lithium, Sodium, Potassium, Rubidium and Caesium

Introduction

• The common metals of this group are sodium (2.6% in the lithosphere, NaCl) and potassium (2.4%, KClMgCl₂.6H₂O, carnallite).
• Salt mines are considered to be good places to leave radioactive waste because they are not subject to groundwater flow - that is why the salt got left there in the first place!
• NaOH, Na₂CO₃, Na₂SO₄, Na₃P₃O₉ and Na₄SiO₄ are among the top 50 industrial chemicals.
• Potassium salts, notable K₂SO₄ or KNO₃, are an important component of fertilizer.
• Lithium alkyls are important reagents in synthesis.
• Na⁺ and K⁺ are very important physiological ions, and Li⁺ salts are used to treat certain mental disorders.
• Except Li, the chemistry is predominantly that of the M⁺ ions. There may be a hint of covalentcy in certain chelate complexes.
• Li⁺ would have a ratio of charge/radius similar to Mg²⁺ hence certain similarities.
• Other "look alikes" are NH4⁺ which is similar to K⁺ and Tl⁺ which is a bit like Rb⁺ or Ag⁺.

Preparation and Properties

• Lithium and sodium are made by electrolysis of the molten salts or low melting eutectics.
• potassium, rubidium and cesium are made by the reaction of sodium vapour with the molten chlorides:

  MCl(l) + Na(g) NaCl(l) + M(g)

  Their vapours are more volatile so that the equilibrium (which is on the side of the halides of the metals desired!)) is disturbed.
• The metals are all silvery in colour except Cs which is yellow. They can all be cut with a knife, lithium being the hardest.
• A sodium - potassium (77.2%) alloy melts at -10 and is a useful, if dangerous, reducing agent.
• The metals are normally protected from air under oil. Lithium, sodium and potassium can be handled quickly in air, and rubidium and cesium are handled under argon.
• All react with water to produce hydrogen: lithium slowly, sodium vigorously, potassium violently enough to ignite the hydrogen, and the rest explosively.
• The products of combustion in a free supply of air are:

  Li Li₂O (oxide) 2OH- in water.

  Na Na₂O₂ (peroxide) 2OH- + 2H₂O₂ in water

  M(K, Rb and Cs) MO₂ (suboxide) O2 + 2OH^- + H₂O₂ with water

  The difference is attributed to lattice energy effects, that is it is the size of the metal ion that makes one of the potential oxides more stable.
• All metals will reat with alcohols to give the alkoxide.
• All dissolve in mercury to give amalgams. The most useful is th liquid amalgam formed when less than about 6% Na is dissolved in mercury. It is a relatively gentle reducing agent compared to pure sodium.

The Metals in Liquid Ammonia

They all dissolve in ammonia to give solutions which are a beautiful royal blue when relatively dilute and take on a metallic bronze appearance when concentrated. The most important equilibria in the more dilute solutions are:

Na(s) Na(NH₃) Na⁺(NH₃) + e^-)NH₃)

2e^-(NH₃) e₂^-(NH₃)

The electrons, which are responsible for the blue colour, are trapped in 3.0-3.4 Å cavities in the solvent. The solutions have a lower density than the pure solvent as a result.

At high concentrations, metal atoms cluster, and the solutions become quite metallic in properties, thus the appearance and high electrical conductivity.

Solutions of sodium in liquid ammonia are slowly decomposed by light an drapidly by the catalytic effect of transition metal ions such as Fe(III) to give sodamide and hydrogen:

Na(NH₃) + NH₃ NaNH₂(s) + ½H₂(g)

For potassium, rubidium an dcesium, whose amides are soluble in liquid ammonia the reaction is reversible under hydrogen pressure:

e^-(NH₃) + NH₃ NH₂^- + ½H₂(NH₃) K = 5x10⁴

The alkali metals are also slightly soluble in other amines, THF and glymes but to a much lesser extent.

Compounds

Oxides - see above.

Hydroxides - The most important are NaOH and KOH which are very deliquescent waxy looking solids, usually sold a pellets or flakes. They are very corrosive alkaline compounds which should be handled with care.

Ionic Salts - Salts of virtually all acids are known. They are colourless unless teh anions are coloured, or there are lattice defects. Lithium is different:

1. Li₃N is formed slowly from Li and N₂ at room temperature and is ruby red.
2. LiOH is a "covalentish" OH bridged polymer which is not very alkaline compared to the rest.
3. LiF is rather insoluble.
4. LiCl and LiBr are quite soluble in a number of polar organic solvents such as alcohols, acetone, ethyl acetate and pyridine (LiCl).
5. Lithium salts are often hydrated, e/g/ LiClO₄.3H₂O.

   Insoluble salts of the others:

   Finding insoluble salts for identification and gravimetric analysis was difficult - there are so few! Examples are: NaZn(UO₂)(CH₃CO₂)₉.6H₂O and K⁺₃[Co(NO₂)₆]^3- (or Rb⁺ or Cs⁺).

   Hydrates and Complexes in Solutions

   Hydrates

   See table 10-1. Li⁺, Na⁺ and K⁺ probably have 4 molecules of water in their first (or primary) hydration sphere, while Rb⁺ and Cs⁺ probably have 6. The larger the central ions, the smaller the area of ordering of the water around it, so the effective size of the ions decreases going down the group. This is important in understanding the mobility of the ions, for example down an ion exchange column.

   The Crown Ethers and Cryptands

   The alkali metals are complexed quite strongly by THF and glymes, but the effect becomes really marked for the so-called "crown ethers". Two examples are shown below:

   Each of these crown ethers has an affinity fro a particular metal ion, for example, for 18-crown-6, the binding constants are in the order:

   Li⁺ < (Na⁺,Cs⁺) < Rb⁺ < K⁺ That is, this crown ether likes K⁺ best.

   Li⁺ is most strongly bound in dicylohexyl-14-crown-4
   Na⁺ "fits" well in benzo-15-crown-5
   Rb⁺ "fits" best in dicyclohexyl-21-crown-7
   Cs⁺ "fits" best in dicyclohexyl-24-crown-8

   The stability orders differ depending on the method of comparison (calculation, gas-phase, solution etc): experimetally in solution, it appears that any crown with –CH₂CH₂– bridges prefers K⁺ because of the 5-membered chelate ring size rather than the size of the hole on the crown - the macrocycle just puckers up to fit,but solvent effects may also be very important.

   The complexes are used to get normally insoluble ionic compounds into organic solutions and can also help produce metal electrides like in ammonia.

   Cryptands and Cryptates

   Cryptands are polycyclic cages, usually including nitrogen as well as oxygen to get the necessary junctions. Metal ions are encapsulated even more securely inside them leadin gto cryptates.

   Some remarkable compounds have been made:

   2Na(NEt₃) + 2,2,2-crypt [Na(2,2,2-crypt)]⁺Na^-(s)

   Bear in mind that:

   2Na Na⁺ + Na^- DH = 438 kJ mol^-1

   This compound is stable up to -10 ^oC and has a structure similar to [Na(2,2,2-crypt)]⁺I^- which has normal stability. There is also a [Na(2,2,2-crypt)]⁺e^- known.

   Encapsulated Metals in Biology

   The transport of alkali metal ions, notably through cell walls, is biologically important and involves certain natural macrocyclic compounds such as valinomycin (Fig 10-VII) and nonactin (Fig 10-3).

   Organometallic Compounds

   Lithium Compounds

   The lithium compounds are very important synthetic reagents. They can be made in hydrocarbon solvents (which is how they are sold) by reactions such as:

   C₂H₅Cl + Li C₂H₅Li + LiCl(S)

   C₄H₉Li + CH₃I C₄H₉I + CH₃Li

   The pure compounds are air-sensitive low melting solids or liquids and are associated into small aggregates with multicentre bonding e.g. Li₄(CH₃)₄ (Figure 10-4) or Li₆(C₂H₅)₆.

   Organosodium and Potassium Compounds

   The organometallic compounds of sodium and potassium are predominantly ionic. The most important are NaC₅H₅ (made from Na in l-NH₃ and C₅H₅ monomer: it is usually a Diels-Alder dimer) and NaCºCR.

   Other Alkali Metal Compounds

   Look at this section independently.

   Summary

   1. Lithium:
      1. Reacts relatively slowly with water or oxygen, but readily forms the nitride.
      2. Has a marked tendency towards covalency, notably in its organometallic compounds.
      3. Is often hydrated in its "ionic" compounds.
      4. The hydroxide is not a strong base.
      5. Some salts are not very soluble in water, but do dissolve in donor organic solvents.
   2. Sodium, Potassium, Rubidium and Caesium:
      1. Are all very reactive with water and oxygen. Nitrides are not stable at room temperature.
      2. Compounds are always predominantly ionic.
      3. Hydroxides are strong bases.
      4. Salts are almost all water soluble.
   3. All elements in the group:
      1. Form blue reducing solutions in ammonia.
      2. Form stable complexes with crown ethers or cryptands which are significantly soluble in organic solvents.