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Doc Brown's Chemistry  Advanced Level Inorganic Chemistry Periodic Table Revision Notes – Transition Metals

Part 10. Transition Metals 3d–block:  10.9 Cobalt Chemistry

The chemistry of the transition metal cobalt (most common oxidation states +2 and +3) is dominated by the stability of the cobalt(II) ion which forms a wide variety of stable complexes with most ligands such as water, ammonia, chloride ion etc. The cobalt(III) state can be stabilised by a suitable ligand and cobalt(III) complexes are usually made by oxidising a cobalt(II) salt in the presence of the stabilising ligand.

principal 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, formula of compounds

(c) doc b GCSE/IGCSE Periodic Table Revision Notes * (c) doc b GCSE/IGCSE Transition Metals Revision Notes

INORGANIC Part 10 The 3d block TRANSITION METALS sub–index: 10.1–10.2 Introduction 3d–block Transition Metals * 10.3 Scandium * 10.4 Titanium * 10.5 Vanadium * 10.6 Chromium * 10.7 Manganese * 10.8 Iron * 10.9  Cobalt * 10.10 Nickel * 10.11 Copper * 10.12 Zinc * 10.13 Other Transition Metals e.g. Ag and Pt * 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

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

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

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

Z and symbol 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Co 28 Ni 29 Cu 30 Zn
property\name scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc
melting point/oC 1541 1668 1910 1857 1246 1538 1495 1455 1083 420
density/gcm–3 2.99 4.54 6.11 7.19 7.33 7.87 8.90 8.90 8.92 7.13
atomic radius/pm 161 145 132 125 124 124 125 125 128 133
M2+ ionic radius/pm na 90 88 84 80 76 74 72 69 74
M3+ ionic radius/pm 81 76 74 69 66 64 63 62 na na
common oxidation states +3 only +2,3,4 +2,3,4,5 +2,3,6 +2,3,4,6,7 +2,3,6 +2,3 +2,+3 +1,2 +2 only
outer electron config. 3d14s2 3d24s2 3d34s2 3d54s1 3d54s2 3d64s2 3d74s2 3d84s2 3d104s1 3d104s2
Electrode potential M(s)/M2+(aq) na –1.63V –1.18V –0.90V –1.18V –0.44V –0.28V –0.26V +0.34V –0.76V
Electrode potential M(s)/M3+(aq) –2.03V –1.21V –0.85V –0.74V –0.28V –0.04V +0.40 na na na
Electrode potential M2+(aq)/M3+(aq) na –0.37V –0.26V –0.42V +1.52V +0.77V +1.87V na na na

Extended data table for COBALT

property of cobalt/unit value for Co
melting point Co/oC 1495
boiling point Co/oC 2870
density Co/gcm–3 8.90
1st Ionisation Energy/kJmol–1 760
2nd IE/kJmol–1 1646
3rd IE/kJmol–1 3232
4th IE/kJmol–1 4950
5th IE/kJmol–1 7670
atomic radius Co/pm 125
Co2+ ionic radius/pm 74
Relative polarising power Co2+ ion 2.7
Co3+ ionic radius/pm 63
Relative polarising power Co3+ ion 4.8
oxidation states of Co +2, +3
simple electron configuration of Co 2,8,15,2
outer electrons of Co [Ar]3d74s2
Electrode potential Co(s)/Co2+(aq) –0.28V
Electrode potential Co(s)/Co3+(aq) +0.40
Electrode potential Co2+(aq)/Co3+(aq) +1.87V
Electronegativity of Co 1.88

Advanced Inorganic Chemistry Page Index and Links

  • 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. Vitamin B12 (cobalamine) contains a cobalt atom and is necessary for the prevention of pernicious anaemia and the formation of red blood corpuscles.

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

The Chemistry of COBALT

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

  • COBALT(II) chemistry

  • 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 CoSO4(aq) or cobalt(II) chloride CoCl2(aq) are suitable for laboratory experiments investigating the aqueous chemistry of the cobalt(II) ion.

  • 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)  (can be written as [Co(OH)2(H2O)4])

    •    (a precipitation reaction)

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

    • Co2+(aq) + CO32–(aq) ==> CoCO3 (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) + 6NH3(aq) ==> [Co(NH3)6]2+(aq) + 6H2O(l) 

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

      • The uncharged ligand molecules ammonia NH3 and water H2O are similar in size and ligand exchange occurs without change in co–ordination number (stays at 6).

      • Oxidation then follows from dissolved oxygen, or you can add hydrogen peroxide for a more efficient job!

    • (i) 4[Co(NH3)6]2+(aq) + O2(g/aq) + 4H+(aq) ==> 4[Co(NH3)6]3+(aq) + 2H2O(l)

    • (ii) 2[Co(NH3)6]2+(aq) + H2O2(g/aq) + 2H+(aq) ==> 2[Co(NH3)6]3+(aq) + 2H2O(l)

    • brown ==> colour? hexaamminecobalt(III) ion.

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

      • +1.82V for [Co(H2O)6]3+(aq) + e– rev [Co(H2O)6]2+(aq)

      • +0.10V for [Co(NH3)6]3+(aq) + e– rev [ Co(NH3)6]2+(aq)

      • more EØ data & comments?

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

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

        • Kstab = {[Co(NH3)6]2+(aq)} / {[Co(H2O)6]2+(aq)} [NH3(aq)]6

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

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

        • Kstab = {[Co(NH3)6]3+(aq)} / {[Co(H2O)6]3+(aq)} [NH3(aq)]6

        • Kstab = 4.5 x 1033 mol–6 dm18  [lg(Kstab) = 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

  • VIEW ppts. with OH–, NH3 and CO32–, & 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) sulphate solution the blue tetrachlorocobaltate(II) complex ion is formed.

    • [Co(H2O)6]2+(aq) + 4Cl–(aq) rev [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

      • 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.

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

  • –

  • COBALT(III) chemistry

    • As we have seen above the hexaaquacobalt(III) cation is unstable in aqueous solution but can be stabilised by a suitable ligand.

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

      • (i) , (ii)

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

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

  • Advanced Inorganic Chemistry Page Index and LinksIsomerism in cobalt(III) complexes e.g. with the ligands ammonia and chloride (i)–(iii)

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

      • the hexaamminecobalt(III) ion

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

    • (iii) crystalline [Co(NH3)4Cl2]+Cl– is violet or green – there are two geometrical E/Z isomers (trans/cis)

      • (iii)(c) doc b

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

      • (1) is the cis or Z isomer, (2) is the trans or E isomer

    • (iv) –

  • 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) + EDTA4–(aq) ===> [Co(EDTA)]2–(aq) + 6H2O(l)

        • Kstab = {[Co(EDTA)3]2–(aq)} / {[Co(H2O)6]2+(aq)} [EDTA4–(aq)]

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

      • [Co(H2O)6]3+(aq) + EDTA4–(aq) ===> [Co(EDTA)]–(aq) + 6H2O(l)

        • Kstab = {[Co(EDTA)3]–(aq)} / {[Co(H2O)6]3+(aq)} [EDTA4–(aq)]

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

      • Note that the more highly charged Co3+(aq) ion complexes more strongly than the Co2+(aq) ion.

    • The cobalt(II) ion complexes with 1,2–diaminoethane, a bidentate ligand

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

      • Kstab = {[Co(en)3]2+(aq)} / {[Co(H2O)6]2+(aq)} [en(aq)]3

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

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

    • –

  • An example of heterogeneous catalysis:

    • 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, the equations are NOT meant to be balanced.

    • Starting with the pink hexa–aqa 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,

    • [Co(OOCCH(OH)CH(OH)COO)3]3–(aq) ==> [Co(H2O)6]2+(aq),H2O(l),HCOO–(aq),CO2 (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–aqa Co2+ complex ion.

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

  • –

Scandium * Titanium * Vanadium * Chromium * Manganese * Iron * Cobalt * Nickel * Copper * Zinc * Silver & Platinum

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

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Scandium * Titanium * Vanadium * Chromium * Manganese * 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

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