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transition metal chemistry of cobalt complexes oxidation states +2 +3 redox chemical reactions physical properties advanced inorganic chemistry of cobalt

3d block-Transition Metals chemistry of cobalt for Advanced level pre-university inorganic chemistry students

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pink octahedral complex ion hexaaquacobalt(II) ion Co2+(aq) [Co(H2O)6]2+Inorganic Part 10. Transition Metals 3d–block Revision Notes

10.9 Cobalt Chemistry including oxidation states and complex ions

[Author ©  Dr Phil Brown PhD: Doc Brown's Chemistry exam revision notes suitable for students of advanced pre-university A level inorganic chemistry for AQA, Edexcel, OCR, Salters, WJEC, CCEA, IB and US grade 11-12 courses: Periodic Table - 3D block and 1st transition metal series: the chemistry of cobalt and its compounds [page updated Ma7 4th 2026 *]

Sub-index for this page on the chemistry of cobalt - a 3d block transition element

1. Introduction to the chemistry of cobalt

2. Data tables for the chemistry of cobalt

3. Uses and biological role of cobalt

4. Electron configuration, oxidation states and electrode potentials

5. Cobalt(II) chemistry, complexes and oxidation to cobalt(III)

6. More on cobalt(III) chemistry and complexes

7. An example of homogeneous catalysis via a cobalt complex:

8. The vertical connection of cobalt with the other d-block elements of Group 9 (IUPAC designation)

9. Learning objectives for the chemistry of cobalt

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 GCSE Level Notes on Transition Metals (for the basics)

1. Introduction to the chemistry of cobalt - a 3d block transition element

Cobalt is the head-top element of Group 9 of the periodic table (modern IUPAC assignment).

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.

Discussed are the 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.

Cobalt is ferromagnetic, meaning it is a strongly magnetisable material that retains its magnetism without the need of an external magnetic field.

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

2. Data for 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 [Ar]... 3d14s2 3d24s2 3d34s2 3d54s1 3d54s2 3d64s2 3d74s2 3d84s2 3d104s1 3d104s2
EØ M(s)/M2+(aq) na –1.63V –1.18V –0.90V –1.18V –0.44V –0.28V –0.26V +0.34V –0.76V
EØ M(s)/M3+(aq) –2.03V –1.21V –0.85V –0.74V –0.28V –0.04V +0.40 na na na
EØ M2+(aq)/M3+(aq) na –0.37V –0.26V –0.42V +1.52V +0.77V +1.87V na na na

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

na = data not applicable to cobalt

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 [beyond argon core] [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

3. Uses and biological role of 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 in a fungus?).

4. The Chemistry of COBALT - electron configuration, oxidation states and electrode potentials

Pd s block d blocks (3d block cobalt) and f blocks of metallic elements p block elements
Gp1 Gp2 Gp3/13 Gp4/14
1

1H
2 3Li 4Be Part of the modern Periodic Table of Elements: ZSymbol, z = atomic or proton number

Sc to Zn are now considered the head-top elements of groups 3 to 12

3d block of metallic elements: Scandium to Zinc focus on cobalt

5B 6C
3 11Na 12Mg 13Al 14Si
4 19K 20Ca 21Sc

[Ar]3d14s2

scandium

 22Ti

[Ar]3d24s2

titanium

 23V

 [Ar] 3d34s2

vanadium

 24Cr

[Ar] 3d54s1

chromium

 25Mn

   [Ar]   3d54s2

manganese

 26Fe

[Ar] 3d64s2

iron

 27Co

[Ar] 3d74s2

cobalt

 28Ni

[Ar] 3d84s2

nickel

 29Cu

[Ar] 3d104s1

copper

 30Zn

[Ar] 3d104s2

zinc

 31Ga 32Ge
5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn
6 55Cs 56Ba 57,58-71 72Hf 73Ta 74W 75Re 76Os 77Ir 78Pt 79Au 80Hg 81Tl 82Pb
7 87Fr 88Ra 89,90-103 104Rf 105Db 106Sg 107Bh 108Hs 109Mt 110Ds 111Rg 112Cn 113Nh 114Fl
 

Summary of oxidation states of the 3d block metals (least important) Ti to Cu are true transition metals
Group 3 Group 4 Group 5 Group 6 Group 7 Group 8 Group 9 Group 10 Gp 11 Group 12
Sc Ti V Cr Mn Fe Co Ni Cu Zn
                +1  
  (+2) (+2) (+2) +2 +2 +2  (3d7) +2 +2 +2
+3 +3 +3 +3 (+3) +3 +3  (3d6) (+3) (+3)  
  +4 +4   +4     (+4)    
    +5              
      +6 (+6) (+6)        
        +7          
3d14s2 3d24s2 3d34s2 3d54s1 3d54s2 3d64s2 3d74s2 3d84s2 3d104s1 3d104s2
Outer electron configurations beyond [Ar] for the ground state of the simple atom (ion configuration)

Note that when 3d block elements form ions, the 4s electrons are 'lost' first.

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

electrode potential chart diagram for the oxidation states of Co(s) 0 +2 +3 Co2+(aq) ions Co3+(ions)

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.

PLEASE note: The electrode potentials (EØ) for cobalt chemistry in the text sometimes vary from the chart above - apologies, but data sources for cobalt chemistry can vary! Any discrepancy shouldn't significantly affect any electrode potential calculation outcomes e.g feasibility (email if concerned?).

5. COBALT(II) chemistry, complexes and oxidation to cobalt(III)

  • pink octahedral complex ion hexaaquacobalt(II) ion Co2+(aq) [Co(H2O)6]2+Electron configuration of Co2+ is [Ar]3d7

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

    • cobalt forms the stable pink hexaaquacobalt(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) 

    • 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) + CO32–(aq) ===> CoCO3 (s) (maybe basic carbonate? – mixture of CoCO3 + Co(OH)2)

  • brown octahedral complex ion hexaamminecobalt(II) ion [Co(NH3)6]2+ oxidation state of Co is +2When 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.

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

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

    • orange octahedral complex ion hexaamminecobalt(III) ion [Co(NH3)6]3+ oxidation state of Co is +3brown ==> yellow-orange colour of the more redox stable hexaamminecobalt(III) ion.

    • 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.

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

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

      • The overall change can be summarised as:

      • [Co(H2O)6]2+  == ligand displacement ==>  [Co(NH3)6]2+  == ox. agent ==> [Co(NH3)6]3+

      • more EØ data & comments?

    • Comparison of the stability of the hexaammine 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 with a considerably greater Kstab value.

  • 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) sulfate 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,

      • 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

6. More on COBALT(III) chemistry and complexes

  • 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) anionic hexanitrocobaltate(III) ion [Co(NO2)6]3- octahedral complex shape, (ii) anionic octahedral complex hexacyanocobaltate(III) ion [Co(CN)6]3-

    • (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–

  • orange octahedral complex ion hexaamminecobalt(III) ion [Co(NH3)6]3+ oxidation state of Co is +3Isomerism 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)

    • octahedral violet complex chloropentaamminecobalt(III) ion [Co(NH3)5Cl]2+(Cl-)2(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 ?, but there are two geometric E/Z isomers (trans/cis) for this 3rd structural isomer incorporating the tetraamminedichlorocobalt(III) ion, or dichlorotetraamminecobalt(III) ion.

      • Z and E isomers of [Co(NH3)4Cl2]+ E/Z isomerism cis/trans geometric isomers 

      • The 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) + 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]

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

      • [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 i.e. the EDTA ligand bonds more strongly to the central cobalt 3+ ion than the 2+ ion.

    • R/S isomers of complex of cobalt(II) ion and 1,2-diaminoethane optical isomerism [Co(en)3]2+ [Co(H2NCH2CH2NH2)3]2+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)

      • 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

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

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

7. 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 three equations below 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)  

    • Initially a ligand substitution reaction takes place and the pink Co(II) complex changes from a water ligand (6 x monodentate ligand) to the organic acid (3 x bidentate ligand), but initially 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?

    • The complex catalysed oxidation can be shown to proceed, represented simply as two stages.

  • (1) [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.

  • (2) [Co(OOCCH(OH)CH(OH)COO)3]3–(aq) ===> [Co(H2O)6]2+(aq), H2O(l), HCOO–(aq) and CO2 (aq) 

    • 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. i.e. the 'new' cobalt(II) complex becomes much more susceptible to oxidation.

  • 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.

  • Activation energy Ea3 is for the uncatalysed reaction, much greater than for the catalysed reaction.

  • Activation energy Ea1 is for step (1), the simplified oxidation of the cobalt(II)-carboxylate ion complex.

  • Activation energy Ea2 is for step (2),  the simplified breakdown of the cobalt(III)-carboxylate ion complex, which regenerates the catalyst.

  • The middle trough represents, with a much lower activation energy, the formation of the intermediate cobalt (III) complex which overall reduces the activation energy of the uncatalysed oxidation reaction.

8. The vertical connection of cobalt with the other d-block elements of Group 9 (IUPAC designation)

Modern IUPAC group numbers of 3-12 Outer electron structure of d-block elements which includes the transition metals

Cobalt is the head element of Group 9 plus Rhodium, Iridium and Meitnerium

Their outer electron configurations are nd7(n+1)s2 (n = 3 to 6) (except Rh 4d85s1)
[e- core] Gp 3 Group 4 Group 5 Group 6 Group 7 Group 8 Group 9 Group 10 Group 11 Group 12
P'd 4, 3d block [Ar] core 21Sc

3d14s2

 22Ti

3d24s2

 23V

3d34s2

 24Cr

3d54s1

 25Mn

3d54s2

 26Fe

3d64s2

 27Co

3d74s2

 28Ni

3d84s2

 29Cu

3d104s1

 30Zn

3d104s2
P'd 5, 4d block (Kr] core 39Y

4d15s2

 40Zr

4d25s2

 41Nb

4d45s1

 42Mo

4d55s1

 43Tc

4d55s2

 44Ru

4d75s1

 45Rh

4d85s1

 46Pd

4d10

 47Ag

4d105s1

 48Cd

4d105s2
P'd 6, 5d b'k  [Xe] core 57La

5d16s2

 72Hf

4f145d26s2

 73Ta

4f145d36s2

 74W

4f145d46s2

 75Re

4f145d56s2

 76Os

4f145d66s2

 77Ir

4f145d76s2

 78Pt

4f145d96s1

 79Au

4f145d106s1

 80Hg

4f145d106s2
P'd 7, 6d b'k [Rn] core 89Ac

6d17s2

 104Rf

5f146d27s2

 105Db

5f146d37s2

 106Sg

5f146d47s2

 107Bh

5f146d57s2

 108Hs

5f146d67s2

 109Mt

5f146d77s2

 110Ds

5f146d87s2

 111Rg

5f146d97s2

 112Cn

5f146d107s2

You would expect some similarities in the chemistry of cobalt, rhodium and iridium.

They have an outer electron configuration of d7s2 or d8s1.

Learning objectives for the chemistry of the 3d block of metal cobalt 27Co (a true transition element)

Know that cobalt is a 3d block element and its position in the periodic table.

Cobalt is the top-head element of group 9 (modern IUPAC convention).

Know that cobalt is a 3d block element because it has one or more electrons in the 3d inner shell.

Know that cobalt has relatively high melting point and boiling point, and a relatively high density.

Cobalt is ferromagnetic i.e. a strongly magnetisable material that retains its magnetism without the need of an external magnetic field, so is used in strong magnetic alloys.

Cobalt is an essential part of diet and is at the heart of the complex vitamin B12 molecule..

Cobalt complies with the definition of a transition metal, because it forms at least one ion with partially filled 3d sub–shell containing at least one electron

Know how to work out the electron configurations of cobalt its simple ions (equal to oxidation states).

Be able to relate the oxidation states of cobalt to their electron configurations.

Know that cobalt can form complex ions, with the variety of ligands and colours characteristic of true transition metals.

Know that cobalt does display compounds or ions in several oxidation states i.e. +2 and +3.

e.g. cobalt forms the stable pink hexaaquacobalt(II) ion, [Co(H2O)6]2+(aq) and [Co(NH3)6]2+(aq) and be able to write balanced equations to explain and describe ligand displacement reactions.

 Know that the +3 oxidation sate can be stabilised by change in ligand on oxidising the +2 state

e.g. [Co(H2O)6]2+  == ligand displacement ==>  [Co(NH3)6]2+  == ox. agent ==> [Co(NH3)6]3+

Be able to describe the colour changes, ligand changes and shape of the complexes when hydrochloric acid is added to pink solutions of cobalt(II) salts to give a blue complex.

pink octahedral [Co(H2O)6]2+(aq) + 4Cl–(aq) blue tetrahedra [CoCl4]2–(aq) + 6H2O(l) and note there is no change in cobalt's oxidation state.

Be able to explain why certain cobalt(II)/(III) octahedral complexes can exhibit E/Z isomerism (i.e. cis/trans geometrical isomers) e.g. the [Co(NH3)4Cl2]+ ion.

Be able to sketch and explain the more complex reaction profiles when a cobalt complex ion is involved in a multiple stage catalysis reaction.

Know that cobalt and its compounds can display the catalytic properties of true transition metals.

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. To account for the d block elements and their 'vertical' similarities, in the modern periodic table, groups 3 to group 0 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 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.

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  Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Doc Brown's Chemistry inorganic chemistry revision notes for pre-university level students on 3d-block elements including the physical and chemical properties reactions equations and trends explained for the 3d-block of transition metals series Explaining the importance of chemistry of cobalt in inorganic chemistry, What you need to know about chemistry of cobalt for inorganic chemistry, Explaining the use of chemistry of cobalt knowledge in inorganic chemistry, Examples of chemistry of cobalt explained when studying inorganic chemistry, What is the significance of chemistry of cobalt in inorganic chemistry, What is the use of chemistry of cobalt in inorganic chemistry  Describing and explaining the theory of chemistry of cobalt when studying inorganic chemistry, exam revision notes for chemistry of cobalt in exams, online help for chemistry of cobalt, revision notes for chemistry of cobalt, what do I need to learn for chemistry of cobalt in exams? revision summary for chemistry of cobalt, help in teaching chemistry of cobalt, learning notes for chemistry of cobalt, help to pass the chemistry of cobalt exam, how to prepare for examination questions on chemistry of cobalt? Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. Website content © Dr Phil Brown 2000+. All copyrights reserved on these organic chemistry exam revision notes on chemistry of cobalt, these A level chemistry revision notes are suitable for use of pre-university students studying AQA advanced A level inorganic chemistry revision notes on chemistry of cobalt, Edexcel advanced A level inorganic chemistry revision notes on chemistry of cobalt, OCR advanced A level inorganic chemistry revision notes on chemistry of cobalt, IB advanced A level inorganic chemistry revision notes on chemistry of cobalt, WJEC (Eduqas) advanced A level inorganic chemistry revision notes on chemistry of cobalt, CIE Cambridge advanced A level inorganic chemistry revision notes on chemistry of cobalt, CCEA advanced A level inorganic chemistry revision notes on chemistry of cobalt, and useful for US grade 11 grade 12 AP honors inorganic chemistry courses involving chemistry of cobalt

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