10.9. Chemistry of Cobalt Co, Z=27, 1s²2s²2p⁶3s²3p⁶3d⁷4s² 

data comparison of cobalt with the other members of the 3d–block and transition metals

Elect. pot. = standard electrode potential data for cobalt (E^Ø at 298K/25^oC, 101kPa/1 atm.)

na = data not applicable to cobalt

Extended data table for COBALT

• Uses of COBALT

  • Cobalt is a bluish–white solid that is malleable and ductile and ferromagnetic – hence its use in magnets.

  • Cobalt is alloyed with chromium and tungsten to make a metal (e.g. stellite alloy) hard enough, even at red heat, to be used for high speed cutting tools and valves for internal combustion engines.

  • The alloy alnico (Al + Ni + Co) is used to make extremely strong permanent magnets.

  • Cobalt compounds are used in paints e.g. cobalt blue.

  • Cobalt compounds are used as catalysts e.g. the cobalt carbonyl Co₂(CO)₈ is used to catalyse the hydroformulation reaction to produce an aldehyde or alcohol from an alkene.

• Biological role of Cobalt

  • Cobalt is an essential trace element and found at the centre of the vitamin B₁₂ (cobalamine, C₆₃H₈₈CoN₁₄O₁₄P). It contains a cobalt(III) ion and is necessary for the prevention of pernicious anaemia and the formation of red blood corpuscles, but it is involved many other functions too. Vitamin B12 is the largest and most complex of all the vitamins. The vitamin B12 group of molecules are the only cobalt-containing molecules (called 'cobalamins') associated with biochemistry of humans.  It is the cobalt(III) ion complex that gives this water-soluble vitamin its distinctive red colour. In essence it is a classic transition metal complex of cobalt with a nice colour! In school and college you are more likely to come across the more familiar pink or blue cobalt(II) complexes, so this is a good, if rather more complicated structure of a rarer cobalt(III) complex. When the B12 molecule is inactive the cobalt is in the +3 oxidation state, but when biochemically active it adopts oxidations states of + 1 in cobalt(I) complexes and +2 in a cobalt(II) complexes. Its all very complicated biochemistry !

  • In plants cobalt complexes are involved in nitrogen fixation by microorganisms (a cobalt ion in an enzyme?).

The Chemistry of COBALT

The oxidation states and electron configuration of cobalt in the context of the 3d block of elements

• Electrode potential chart for cobalt the context of the 3d block transition metals

The electrode potential chart highlights the values for various oxidation states of cobalt.

The electrode potentials involving cobalt ions correspond to hydrated complex ions where the ligands are water, oxide or hydroxide.

As you can see from the chart, changing either the ligand or the oxidation state, will also change the electrode potential for that half-reaction involving a cobalt ion e.g. comparing the complexes with water or ammonia ligands for the relative stability of Co²⁺ and Co³⁺.

The hexaaquacobalt(III) ion is a strong oxidising agent.

COBALT(II) chemistry

• Electron configuration of Co²⁺ is [Ar]3d⁷

• In aqueous solution, in the absence of complexing agents,

  • cobalt forms the stable pink hexaaqua cobalt(II) ion, [Co(H₂O)₆]²⁺_(aq) 

• Aqueous solutions of cobalt(II) sulfate CoSO_4(aq) or cobalt(II) chloride CoCl_2(aq) are suitable for laboratory experiments investigating the aqueous chemistry of the cobalt(II) ion.

  • For more on ion colours see uv-visible absorption spectra of some cobalt complex ions

• With alkalis sodium hydroxide and ammonia, cobalt(II) ions produce the hydrated cobalt(II) hydroxide blue ppt. which turns pink on standing. There is no further reaction with excess of NaOH or Na₂CO₃, but see further down for excess NH₃.

  • Co²⁺_(aq) + 2OH^–_(aq) ===> Co(OH)_2(s) 

  • The equation product can be written as a neutral octahedral complex [Co(OH)₂(H₂O)₄]⁰ by adding a 4H₂O on the left.

  • This a precipitation reaction, and no change in cobalt's oxidation state.

• With alkaline aqueous sodium carbonate solutions cobalt(II) ions produces a precipitate of pink/blue? cobalt(II) carbonate.

  • Co²⁺_(aq) + CO₃^2–_(aq) ===> CoCO_{3 (s)} (maybe basic carbonate? – mixture of CoCO₃ + Co(OH)₂)

• When excess ammonia is added to a cobalt(II) salt solution, the hexamine complex is formed BUT this is unstable in the presence of dissolved oxygen and is oxidised to the cobalt(III) complex. This change in cobalt's oxidation state from +2 to +3 via an oxidising agent is quite common if a complexing agent is present too.

  • [Co(H₂O)₆]²⁺_(aq) + 6NH_3(aq) ===> [Co(NH₃)₆]²⁺_(aq) + 6H₂O_(l) 

  • pink hexaaquacobalt(II) ion == oxygen ==> brown hexaamminecobalt(II) ion.

    • This is an example of an cobalt complex ligand exchange reaction, 6 ammonia molecules displacing 6 water molecules.

    • Both complexes are octahedral in shape with a co-ordination number of 6 from 6 unidentate ligands.

    • Water and ammonia are of similar size and both are monodentate (unidentate ligands), that is each ligand can donate a single pair of electrons to form one co-ordinate bond (dative covalent bond).

    • In most ligand exchange reactions there is no change in oxidation state unless a reducing agent or oxidising agent is present (see below with oxygen or hydrogen peroxide present).

    • Transition metal commonly form octahedral complexes, like those of cobalt, with small ligands like water, ammonia and hydroxide ion.

    • In this case transition metal complex chemistry, oxidation follows either from dissolved oxygen, or you can add hydrogen peroxide for a more efficient job!

    • The cobalt(III) complex is stabilised compared to the cobalt(II) complex by the change of ligand from water to ammonia.

  • (i) 4[Co(NH₃)₆]²⁺_(aq) + O_2(g) + 4H⁺_(aq) ===> 4[Co(NH₃)₆]³⁺_(aq) + 2H₂O_(l)

  • (ii) 2[Co(NH₃)₆]²⁺_(aq) + H₂O_2(aq) + 2H⁺_(aq) ===> 2[Co(NH₃)₆]³⁺_(aq) + 2H₂O_(l)

  • brown ==> yellow-orange colour of the more redox stable hexaamminecobalt(III) ion.

    • See also The uv-visible absorption spectra of some cobalt complex ions

  • Both the ammonia complexes are octahedral, co-ordination number 6, BUT here we do have a cobalt oxidation state change of +2 to +3,

    • hence the change in charge on the cobalt complex ion from 2+ to 3+ (ammonia is an electrically neutral ligand).

  • Oxidation state changes:

    • in both (i) & (ii) Co from +2 to +3, (i) O from 0 to –2, (ii) O from –1 to –2.

    • E^Ø +1.82V for [Co(H₂O)₆]³⁺_(aq) + e^– [Co(H₂O)₆]²⁺_(aq)

    • E^Ø +0.10V for [Co(NH₃)₆]³⁺_(aq) + e^– [ Co(NH₃)₆]²⁺_(aq)

    • more E^Ø data & comments?

  • Comparison of the stability of the hexaammine complexes irrespective of redox stability

    • [Co(H₂O)₆]²⁺_(aq) + 6NH_3(aq) ===> [Co(NH₃)₆]²⁺_(aq) + 6H₂O_(l)

      • K_stab = [[Co(NH₃)₆]²⁺_(aq)] / [[Co(H₂O)₆]²⁺_(aq)] [NH_3(aq)]⁶

      • K_stab = 7.7 x 10⁴ mol^–6 dm¹⁸  [lg(K_stab) = 4.9]

    • [Co(H₂O)₆]³⁺_(aq) + 6NH_3(aq) ===> [Co(NH₃)₆]³⁺_(aq) + 6H₂O_(l)

      • K_stab = [[Co(NH₃)₆]³⁺_(aq)] / [[Co(H₂O)₆]³⁺_(aq)] [NH_3(aq)]⁶

      • K_stab = 4.5 x 10³³ mol^–6 dm¹⁸  [lg(K_stab) = 33.7]

    • Note that the more highly charged Co³⁺_(aq) ion complexes more strongly than the Co²⁺_(aq) ion i.e. forms a more stable complex with a considerably greater K_stab value.

• VIEW ppts. with OH^–, NH₃ and CO₃^2–, & complexes, if any, with excess reagent.

• When hydrogen peroxide is added to an alkaline cobalt(II) solution, oxidation occurs to give cobalt(III) complexes.

  • The air oxidation described above in alkaline ammonia solution can also be effected via hydrogen peroxide giving the hexa–amminecobalt(III) ion – described in detail above.

• If e.g. sodium chloride or hydrochloric acid is added to cobalt(II) sulfate solution the blue tetrachlorocobaltate(II) complex ion is formed.

  • [Co(H₂O)₆]²⁺_(aq) + 4Cl^–_(aq) [CoCl₄]^2–_(aq) + 6H₂O_(l) 

  • This particular ligand substitution/exchange reaction involves several changes (L to R):

    • the larger chloride ion ligand leads to a change in co–ordination number from 6 to 4,

    • the complex ion shape changes from octahedral to tetrahedral,

    • it is likely that the more bulky chloride ion (radius Cl > O) 'forces' the formation of the tetrahedral shape of this cobalt complex ion, rather than a square planar shaped complexes,

    • the colour of the complex changes from pink to blue,

    • the complex changes from a cationic to an anionic ion.

    • There is no oxidation state change at all.

  • This is quite a good reaction to demonstrate Le Chatelier's equilibrium principles:

    • dilution shifts the equilibrium to the left, more pink,

    • increasing the chloride ion concentration shifts the equilibrium to the right, more blue,

    • increasing the solution temperature shifts the equilibrium to the right, more blue

    • or if prepared at higher temperature, with just enough chloride to turn the solution blue, on cooling it becomes pink,

    • this shows that left to right is endothermic and right to left is exothermic.

    • For more on ion colours see The uv-visible absorption spectra of some cobalt complex ions

• Summary of some complexes–compounds & oxidation states of cobalt compared to other 3d–block elements

More on COBALT(III) chemistry

• Electron configuration of Co³⁺ is [Ar]3d⁶

• As we have seen above the hexaaquacobalt(III) cation is unstable in aqueous solution but can be stabilised by suitable ligands like ammonia - the reaction is described in the previous section.

• One example has already been described in the introduction, namely the vitamin B12 complex.

• The formation of [Co(NH₃)₆]³⁺ is described above and two other stable complex anions are with the ...

  • (i) , (ii)

  • (i) the nitrate(III) ion (nitrite, ion NO₂^–) ligand forms the anionic octahedral complex, hexanitrocobaltate(III) ion [Co(NO₂)₆]^3–

  • (ii) the cyanide ion CN^– it forms the anionic octahedral complex hexacyanocobaltate(III) ion [Co(CN)₆]^3–

• Isomerism in cobalt(III) complexes

  • e.g. with the ligands ammonia and chloride ion (i)-(iii)

  • (i) crystalline [Co(NH₃)₆]³⁺(Cl^–)₃ is orange–yellow, no isomers possible

    • with the hexaamminecobalt(III) ion (shown on the right)

  • (ii) crystalline [Co(NH₃)₅Cl]²⁺(Cl^–)₂ is violet, no isomers possible

    • with the chloropentaamminecobalt(III) ion (shown on the right)

  •  (iii) crystalline [Co(NH₃)₄Cl₂]⁺Cl^– is violet or green ? – there are two geometric E/Z isomers (trans/cis) for this 3rd structural isomer incorporating the tetraamminedichlorocobalt(III) ion, or dichlorotetraamminecobalt(III) ion.

    • (iii)  Z and E isomers of [Co(NH₃)₄Cl₂]⁺

    • E/Z (geometrical) isomerism diagrams: The Z and E isomers (cis and trans geometrical isomers) of the isomeric octahedral complexes of the dichlorotetraamminecobalt(III) complex ion

    • (1) is the cis or Z isomer, (2) is the trans or E isomer and they can occur because of the two possible non-superimposable forms of this cobalt complex - an example of stereoisomerism.

• More examples of the complexes of cobalt(II) and cobalt(III)

  • Both the hexa–aqua ions of cobalt(II) and cobalt(III) readily complex with EDTA

    • [Co(H₂O)₆]²⁺_(aq) + EDTA^4–_(aq) ===> [Co(EDTA)]^2–_(aq) + 6H₂O_(l)

      • K_stab = [[Co(EDTA)₃]^2–_(aq)] / [[Co(H₂O)₆]²⁺_(aq)] [EDTA^4–_(aq)]

      • K_stab = 2.0 x 10¹⁶ mol^–1 dm³ [lg(K_stab) = 16.3]

      • Remember [H₂O] is not included in the equilibrium expression.

    • [Co(H₂O)₆]³⁺_(aq) + EDTA^4–_(aq) ===> [Co(EDTA)]^–_(aq) + 6H₂O_(l)

      • K_stab = [[Co(EDTA)₃]^–_(aq)] / [[Co(H₂O)₆]³⁺_(aq)] [EDTA^4–_(aq)]

      • K_stab = 1.0 x 10³⁶ mol^–1 dm³ [lg(K_stab) = 36.0]

    • Note that the more highly charged Co³⁺_(aq) ion complexes more strongly than the Co²⁺_(aq) ion i.e. the EDTA ligand bonds more strongly to the central cobalt 3+ ion than the 2+ ion.

  • The cobalt(II) ion complexes with 1,2–diaminoethane, a neutral bidentate ligand, the resulting cobalt complex exhibits R/S isomerism (optical isomers, enantiomers).

    • [Co(H₂O)₆]²⁺_(aq) + 3en_(aq) ===> [Co(en)₃]²⁺_(aq) + 6H₂O_(l)

    • K_stab = [[Co(en)₃]²⁺_(aq)] / [[Co(H₂O)₆]²⁺_(aq)] [en_(aq)]³

    • K_stab = 6.3 x 10¹³ mol^–3 dm⁹ [lg(K_stab) = 13.8]

    • Note: en is an abbreviation for the ligands old name ethylenediamine

      • which has the structure:  H₂N-CH₂-CH₂-NH₂

      • so the complex ion can be written as:  [Co(H₂NCH₂CH₂NH₂)₃]²⁺

An example of homogeneous catalysis via a cobalt complex:

• Cobalt(II) ions catalyse the oxidation of the 2,3–dihydroxybutandioate ion (acid/salt, old name 'tartaric/tartrate') to water, methanoate ion and carbon dioxide with hydrogen peroxide solution.

• The likely scheme of events is outlined below, showing the ease of conversion of cobalt between a +2 oxidation state complex and a +3 oxidation state complex.

• The overall equation for this oxidation of 2,3-dihydroxybutanoate acid is:

  • 2,3-dihydroxybutanoate ion  +  hydrogen peroxide  ===> carbon dioxide  +  methanoate ion  + water

  • ^-OOC-CH(OH)-CH(OH)-COO^-  +  3H₂O₂  ===>  2CO₂  +  2HCOO^-  +  4H₂O

• The rest of the equations are NOT meant to be balanced.

• Starting with the pink hexa–aqua Co²⁺ ion, which is a Co^(II) complex 

  • and the carboxylate ion, ^–OOCCH(OH)CH(OH)COO^– (bidentate ^2– anionic ligand)

• [Co(H₂O)₆]²⁺_(aq) ===> [Co(OOCCH(OH)CH(OH)COO)₃]^4–_(aq)  

  • the pink Co^(II) complex changes ligand from water to the organic acid, but no change in oxidation state or co–ordination number, and I don't know its colour?, but it perhaps it doesn't exist long enough to be seen?

• [Co(OOCCH(OH)CH(OH)COO)₃]^4–_(aq) ==via H₂O₂==>  [Co(OOCCH(OH)CH(OH)COO)₃]^3–_(aq)  

  • the Co^(II)–acid complex is oxidised by the hydrogen peroxide to a Co^(III) –acid complex which is green,

  • and this green complex is the intermediate in the reaction profile diagram below.

• [Co(OOCCH(OH)CH(OH)COO)₃]^3–_(aq) ===> [Co(H₂O)₆]²⁺_(aq), H₂O_(l), HCOO^–_{(aq) and} CO_{2 (aq/g)} 

  • the green Co^(III) complex then breaks down to give the products,

  • and you see the bubbles of carbon dioxide and the 'return' of the pink hexa–aqua Co²⁺ complex ion.

• In the above sequence, the change in ligand affects the relative stability of the oxidation states. The Co(II)–acid complex is stable as regards 'breakdown', but is readily oxidised to the Co(III)–acid complex, which is NOT stable to breakdown.

• Again, the middle trough represents the formation of the intermediate cobalt complexes which overall reduces the activation energy of the uncatalysed reaction.

biological role of cobalt vitamin B12, cobalt(II) chemistry, shape and formula of complexes of cobalt(II) Co2+, ammonia ligand, chloride ion as a ligand, complexes of cobalt with ammonia, oxidation of cobalt(II) ion Co2+ to the cobalt(III) ion Co3+ with hydrogen peroxide, tetrahedral complex of Co2+ with chloride ion, colour and structure of cobalt(III) Co3+ complexes, formula of EDTA complexes of cobalt physical and chemical properties of the 3d block transition metal cobalt, oxidation and reduction reactions of cobalt ions, outer electronic configurations of cobalt, principal oxidation states of cobalt, shapes of cobalt's complexes, octahedral complexes of cobalt, tetrahedral complexes of cobalt, square planar complexes of cobalt, stability data for cobalt's complexes, aqueous chemistry of cobalt ions, redox reactions of cobalt ions, physical properties of cobalt, melting point of cobalt, boiling point of cobalt, electronegativity of cobalt, density of cobalt, atomic radius of cobalt, ion radius of cobalt, ionic radii of cobalt's ions, common oxidation states of cobalt, standard electrode potential data for cobalt, ionisation energies of cobalt, polarising power of cobalt ions, industrial applications of cobalt compounds, chemical properties of cobalt compounds, why are cobalt complexes coloured?, isomerism in the complexes of cobalt, formulae of cobalt compounds, tests for cobalt ions keywords redox reactions ligand substitution displacement balanced equations formula complex ions complexes ligand exchange reactions redox reactions ligands colours oxidation states: cobalt ions Co(0) Co2+ Co(+2) Co(II) Co3+ Co(+3) Co(III) CoCl2 CoSO4 Co2+ + 2OH– ==> Co(OH)2 Co2+ + CO32– ==> CoCO3  [Co(H2O)6]2+ + 6NH3 ==> [Co(NH3)6]2+ + 6H2O  4[Co(NH3)6]2+ + O2(g/aq) + 4H+ ==> 4[Co(NH3)6]3+ + 2H2O (ii) 2[Co(NH3)6]2+ + H2O2(g/aq) + 2H+ ==> 2[Co(NH3)6]3+ + 2H2O +1.82V for [Co(H2O)6]3+ + e– [Co(H2O)6]2+ +0.10V for [Co(NH3)6]3+ + e– [Co(NH3)6]2+ [Co(H2O)6]2+ + 6NH3 ==> [Co(NH3)6]2+ + 6H2O Kstab = [[Co(NH3)6]2+] / [[Co(H2O)6]2+] [NH3]6 Kstab = 7.7 x 104 mol–6 dm18 [lg(Kstab) = 4.9] [Co(H2O)6]3+ + 6NH3 ==> [Co(NH3)6]3+ + 6H2O Kstab = [[Co(NH3)6]3+] / [[Co(H2O)6]3+] [NH3]6 Kstab mol–6 dm18 [lg(Kstab) = 33.7] [Co(H2O)6]2+ + 4Cl– [CoCl4]2– + 6H2O [Co(NH3)6]3+ [Co(NO2)6]3– [Co(CN)6]3– [Co(NH3)6]3+(Cl–)3 [Co(NH3)5Cl]2+(Cl–)2 [Co(NH3)4Cl2]+Cl– [Co(H2O)6]2+ + EDTA4– ===> [Co(EDTA)]2– + 6H2O Kstab = [[Co(EDTA)3]2–] / [[Co (H2O)6]2+] [EDTA4–] Kstab = 2.0 x 1016 mol–1 dm3 [lg(Kstab) = 16.3] [Co(H2O)6]3+ + EDTA4– ===> [Co(EDTA)]– + 6H2O Kstab = [[Co(EDTA)3]–] / [[Co(H2O)6]3+] [EDTA4–] Kstab = 1.0 x 1036 mol–1 dm3 [lg(Kstab) = 36.0] [Co(H2O)6]2+ + 3en ==> [Co (en)3]2+ + 6H2O Kstab = [[Co(en)3]2+] / [[Co(H2O)6]2+] [en]3 [Co(H2O)6]2+ + 3en ==> [Co(en)3]2+ + 6H2O Kstab = [[Co(en)3]2+] / [[Co(H2O)6]2+] [en]3 [Co(OOCCH(OH)CH(OH)COO)3]4– ==via H2O2==> [Co(OOCCH(OH)CH(OH)COO)3]3– [Co(OOCCH(OH)CH(OH)COO)3]3– ==> [Co(H2O)6]2+,H2O,HCOO–,CO2 oxidation states of cobalt, redox reactions of cobalt, ligand substitution displacement reactions of cobalt, balanced equations of cobalt chemistry, formula of cobalt complex ions, shapes colours of cobalt complexes  Na2CO3 NaOH NH3 cobalt chemistry for AQA AS chemistry, cobalt chemistry for Edexcel A level AS chemistry, cobalt chemistry for A level OCR AS chemistry A, cobalt chemistry for OCR Salters AS chemistry B, cobalt chemistry for AQA A level chemistry, cobalt chemistry for A level Edexcel A level chemistry, cobalt chemistry for OCR A level chemistry A, cobalt chemistry for A level OCR Salters A level chemistry B cobalt chemistry for US Honours grade 11 grade 12 cobalt chemistry for pre-university chemistry courses pre-university A level revision notes for cobalt chemistry  A level guide notes on cobalt chemistry for schools colleges academies science course tutors images pictures diagrams for cobalt chemistry A level chemistry revision notes on cobalt chemistry for revising module topics notes to help on understanding of cobalt chemistry university courses in science careers in science jobs in the industry laboratory assistant apprenticeships technical internships USA US grade 11 grade 11 AQA A level chemistry notes on cobalt chemistry Edexcel A level chemistry notes on cobalt chemistry for OCR A level chemistry notes WJEC A level chemistry notes on cobalt chemistry CCEA/CEA A level chemistry notes on cobalt chemistry for university entrance examinations

WHAT NEXT? GCSE Level Notes on Transition Metals (for the basics) The chemistry of Scandium * Titanium * Vanadium * Chromium * Manganese The chemistry of Iron * Cobalt * Nickel * Copper * Zinc * Silver & Platinum Introduction 3d–block Transition Metals * Appendix 1. Hydrated salts, acidity of hexa–aqua ions * Appendix 2. Complexes & ligands * Appendix 3. Complexes and isomerism * Appendix 4. Electron configuration & colour theory * Appendix 5. Redox equations, feasibility, Eø * Appendix 6. Catalysis * Appendix 7. Redox equations * Appendix 8. Stability Constants and entropy changes * Appendix 9. Colorimetric analysis and complex ion formula * Appendix 10 3d block – extended data * Appendix 11 Some 3d–block compounds, complexes, oxidation states & electrode potentials * Appendix 12 Hydroxide complex precipitate 'pictures', formulae and equations Some pages have a matching sub-index Advanced Level Inorganic Chemistry Periodic Table Index: Part 1 Periodic Table history Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr AND important trends down a group * Part 7 s–block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p–block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots All 11 Parts have their own sub-indexes near the top of the pages Group numbering and the modern periodic table The original group numbers of the periodic table ran from group 1 alkali metals to group 0 noble gases (= group 8). To account for the d block elements and their 'vertical' similarities, in the modern periodic table, groups 3 to group 0/8 are numbered 13 to 18. So, the p block elements are referred to as groups 13 to group 18 at a higher academic level, though the group 3 to 0/8 notation is still used, but usually at a lower academic level. The 3d block elements (Sc to Zn) are now considered the head (top) elements of groups 3 to 12.

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