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

Introduction

Preparation and Properties

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.