10.10. Chemistry of Nickel Ni, Z=28, 1s²2s²2p⁶3s²3p⁶3d⁸4s² 

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

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

na = data not applicable to nickel

Extended data table for NICKEL

• Uses of NICKEL

  • Nickel is a moderately hard silvery–white metal, lustrous like most transition metals and malleable and ductile.

  • Nickel is quite resistant to corrosion and not affected by water but will dissolve slowly in most strong acids.

  • Nickel has many uses from 'silver' coinage metals like cupro–nickel, which is an alloy of nickel and copper that doesn't readily corrode.

  • Along with chromium, nickel is used in stainless steels.

  • Alnico alloy (Al + Ni + Co) is used to make permanent magnets.

  • Nichrome wire (Ni + Cr) is used to make wire for windings in electric motors.

  • Nickel is a constituent of monel metal alloy used to make ships propeller shafts and chemical reactor vessels because of its strength and anti–corrosion properties.

  • Nickel is an important hydrogenation catalyst in converting unsaturated vegetable oils to saturated fats like margarine.

    • unsaturated oil + hydrogen ==> low melting solid more saturated fat

    • Along the carbon chain of the vegetable oil you get: –CH=CH– + H₂ ===> –CH₂–CH₂–

    • This reaction is described in detail at the end of my nickel notes.

  • Solutions of nickel(II) salts or complexes are used in electroplating nickel onto other metal surfaces.

    • e.g. the complex ion salt Ni(NH₄)₂(SO₄)₂.6H₂O

  • Nickel(II) oxide, NiO, is used in pigments.

• Biological role of nickel

  • It is apparently found in human tissue, but its role is unknown.

The Chemistry of NICKEL

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

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

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

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

If you change either the ligand or the oxidation state, will also change the electrode potential for that half-reaction involving a nickel ion.

NICKEL(II) CHEMISTRY

• Electron configuration of Ni²⁺ is [Ar]3d⁸

• In aqueous solution nickel forms the green stable hexaaquanickel(II) ion, [Ni(H₂O)₆]²⁺_(aq) from eg nickel(II) chloride solution NiCl_2(aq) or nickel(II) sulfate NiSO_4(aq), both of which are suitable for laboratory experiments for investigating the aqueous chemistry of the nickel(II) ion.

•   emphasising the octahedral shape of the [Ni(H₂O)₆]²⁺ ion or   emphasising the six dative covalent bonds between the lone electron pair donating ligand and the central metal ion.

• With alkalis sodium hydroxide or ammonia, nickel(II) ions produce the hydrated nickel(II) hydroxide green? precipitate. There is no further reaction with excess of NaOH, but see further down for excess NH₃.

  • Ni²⁺_(aq)  +  2OH^–_(aq) ===>  Ni(OH)_2(s) 

    • This precipitation reaction can be written as

    • [Ni(H₂O)₆]²⁺_(aq) +  2OH^–_(aq) ===>  [Ni(OH)₂(H₂O)₄]_(s)  +  2H₂O_(l)

    • The two nickel(II) complexes are octahedral in shape with a co-ordination number of 6 from 6 unidentate ligands.

    • The overall charge on the nickel(II) hydroxide precipitate complex is zero, the 2OH^- cancelling out the Ni²⁺.

    • This is an example of a nickel complex ligand exchange reaction, two hydroxide ions displacing two water molecules.

    • Water and the hydroxide ion 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.

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

• With alkaline aqueous sodium carbonate solutions, nickel(II) ions produces a precipitate of green ppt. of nickel(II) carbonate.

  • Ni²⁺_(aq) + CO₃^2–_(aq) ===> NiCO_3(s) 

    • Its actually a basic carbonate – a mixture of the hydroxide and carbonate, you can make the pure carbonate by using sodium hydrogencarbonate solution.

    • Ni²⁺_(aq) + 2HCO₃^–_(aq) ===> NiCO_3(s) + 4H₂O_(l) + CO_2(g)

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

• With excess aqueous ammonia the blue hexaammine complex ion is formed from the hexaaquanickel(II) ion – a typical ligand substitution reaction giving the hexaamminenickel(II) ion:

• [Ni(H₂O)₆]²⁺_(aq) + 6NH_3(aq) [Ni(NH₃)₆]²⁺_(aq) + 6H₂O_(l)

  • See also the absorption spectra and colours of nickel compounds

  • The two nickel(II) complexes are octahedral in shape with a co-ordination number of 6.

  • The overall charge on the nickel(II) hydroxide complex remains 2+ because both ligands are electrically neutral.

  • This is another example of a nickel complex ligand exchange reaction where six ammonia molecules replace six water molecules.

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

  • K_stab = 4.8 x 10⁷ mol^–6 dm¹⁸  [lg(K_stab) = 7.7]

  • You can also write the equation of the ammine complex from the dissolving of nickel(II) hydroxide precipitate.

    • Ni(OH)_2(s) + 6NH_3(aq) [Ni(NH₃)₆]²⁺_(aq) + 2OH^–_(aq)

  • Ligand substitution may be incomplete, so, with lower concentrations of ammonia the pale blue complex can also have other structures e.g. [Ni(H₂O)₂(NH₃)₄]²⁺_(aq)   and [Ni(H₂O)₄(NH₃)₂]²⁺_(aq) 

    • Both of these octahedral complex ions exhibit E/Z (cis/trans) isomerism, diagrams below)

• The hexaaquanickel(II) ion also forms complexes with other amine ligands

  • e.g. the bidentate ligand 1,2–diaminoethane (H₂N–CH₂–CH₂–NH₂), often abbreviated to en from its old trivial name of ethylenediamine). Each of the lone pairs of electrons on the nitrogen atoms can form a co-ordinate bond

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

    • This is an example of a chelation substitution reaction where a bidentate or multidentate ligand displaces a numerically greater monodentate (unidentate) ligands.

    • The resulting nickel complex is described as an example of a chelate.

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

    • K_stab = 2.0 x 10¹⁸ mol^–3 dm⁹ [lg(K_stab) = 18.3]

    • The reaction is almost completely 100% to the right.

    • Notice that the K_stab is greater than the K_stab for the formation of the ammonia complex, so you can correctly predict that the following ligand exchange will take place ...

    • [Ni(NH₃)₆]²⁺_(aq) + 3en_(aq) [Ni(en)₃]²⁺_(aq)  +  6NH_3(aq)

    • I've used en for simplicity, but the formula of the complex ion formed is [Ni(H₃NCH₂CH₂NH₃)₃]²⁺

    • Although then enthalpy changes for these reactions are similar, because a similar number of similar covalent bonds are broken or made, the release of the larger number of smaller molecules leads to a large increase in entropy (a large positive ΔS).

    • This makes the free energy change, calculated from ΔG = ΔH - TΔS, more negative, therefore more feasible for these nickel complex reactions.

• The complex with EDTA is also readily formed.

  • EDTA is an even more powerful chelating agent with an extremely high K_stab values.

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

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

  • K_stab = 1.0 x 10¹⁹ mol^–1 dm³ [lg(K_stab) = 19.0]

  • Remember [H₂O] is not included in these equilibrium expressions.

  • Note that K_stab for the same ion tends to increase the greater the chelating power of an individual ligand in terms of the ligand bond formed – mainly due to the increase in entropy as more ligand particles are displaced by the polydentate ligands displacing the unidentate ligands.

  • e.g. for the same nickel(II) ion K_stab(EDTA) > K_stab(en) > K_stab(NH₃)

  • That is from left to right in the sequence, the entropy change decreases, multidentate > bidentate > monodentate (unidentate)

The structure of the complex ion [NiEDTA]^2- formed between the aqueous nickel(II) ion, Ni²⁺_(aq) and the EDTA anion [EDTA]^4-

The process is called a chelation of the central nickel(II) ion.

• Other complexes of nickel

  • Nickel carbonyl, Ni(CO)₄,

    • Note

    • (i) Nickel carbonyl is a neutral complex i.e. you can write it as [Ni(CO)₄]⁰

    • (ii) It is a tetrahedrally shaped covalent molecule with a OC-N-CO bond angle of 109.5^o..

    • (i) nickel is in a zero oxidation state and the compound is a colourless liquid.

    • (ii) the ligand CO also acts as ligand with haemoglobin (hemoglobin) in carbon monoxide poisoning.

      • The carbon monoxide can act as a lone pair donor ligand :C≡O

  • Ni²⁺ forms the tetrachloronickelate(II) ion, [NiCl₄]^2–, a tetrahedral anionic complex with the chloride ion ligand (Cl^–).

    • [Ni(H₂O)₆]²⁺_(aq) +  4Cl^–_(aq) [NiCl₄]^2–_(aq)  +  6H₂O_(l)

    • In this ligand exchange reaction, the nickel(II) complex ion shape changes from octahedral to tetrahedral, the co-ordination number changes from 6 to 4, but the oxidation state of nickel remains at +2. The overall electrical charge on the chloro complex is 2- (from 2+/+2 and 4x-1).

    • Its likely that the more bulky chloride ion (radius Cl > C) 'forces' the formation of the tetrahedral shape rather than a square planar shaped complex in the reaction described below.

    • K_stab = [[NiCl₄]^2–_(aq)] / [[Ni(H₂O)₆]²⁺_(aq)] [Cl^–_(aq)]⁴

    • K_stab = ? mol⁴ dm^–12 

    • lg(K_stab) = ?

  • Ni²⁺ forms the tetracyanonickelate(II) ion, [Ni(CN)₄]^2–, a square planar anionic complex with the cyanide ion (the ligand structure is :C≡N^–).

    • [Ni(H₂O)₆]²⁺_(aq) + 4CN^–_(aq) [NiCN₄]^2–_(aq) + 6H₂O_(l)

    • Similar to above, in this ligand exchange reaction, the nickel(II) complex ion shape changes from octahedral to square planar, the co-ordination number changes from 6 to 4, but the oxidation state of nickel remains at +2. The overall electrical charge on the chloro complex is 2- (from 2+/+2 and 4x-1).

    • K_stab = [[NiCN₄]^2–_(aq)] / [[Ni(H₂O)₆]²⁺_(aq)] [CN^–_(aq)]⁴

    • K_stab = 2 x 10³¹ mol⁴ dm^–12 

    • lg(K_stab) = 31.3

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

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Nickel(III) and nickel(IV) oxidation state chemistry

 Higher oxidation state compounds of nickel can ve stabilised by electronegative elements like oxygen or fluorine.

e.g. nickel(III) oxide, Ni₂O₃, a grey-black solid

nickel(III) fluoride, NiF₃, forms complex ions e.g. the hexafluoronickelate(III) ion, [NiF₆]^3- in the salt K₃NiF₆

The nickel(IV) oxidation state occurs in the salt K₂NiF₆ which contains the hexafluoronickelate(II) ion, [NiF₆]^2-

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An example of the catalytic action of nickel metal.

• 

  • An example of nickel acting as a heterogeneous catalysis is illustrated above, the hydrogenation of alkenes (e.g. ethene + hydrogen ===> ethane). 

  • Hydrogenation is an extremely important process in the food industry or converting unsaturated oils into low melting hydrogenated solid fats to make the more spreadable margarine.

  • For more details see Natural esters - triglyceride fats and oils, manufacture of margarine and biodiesel

  • Nickel is the solid phase catalyst and the reactant gases in the different gaseous phase.

  • In terms of activation energies, with reference to the reaction profile below:

    • (1) ==> (2) is represented by E_a1, the absorption of the reactant molecules onto the catalyst surface to form the intermediate state between nickel and the adsorbed gases..

    • (3) represents the minimum potential energy trough where the molecules are adsorbed onto the catalyst surface.

    • (2) ==> (3-5) is represented by E_a2 the formation of the products form the intermediate adsorbed states of the molecules.

    • 

    • The red line represents the catalysed reaction profile (the catalysed pathway)

    • The blue line represents the uncatalysed reaction profile (the uncatalysed pathway)

    • A two stage reaction profile for a catalytic cycle (E_a = activation energy)

      • This sort of diagram is most applicable to homogeneous catalysis where definite intermediates are formed, but in general principle it applies to heterogeneous catalysis too where the adsorption (particularly chemical) is equivalent to forming a transition state or complex.

      • E_a1 is the activation energy leading to the formation of an intermediate complex between nickel and the adsorbed gases.

      • E_a2 is the activation energy for the change of the intermediate complex into product (ethane).

      • E_a3 is the activation energy of the uncatalysed reaction between nickel and hydrogen.

physical and chemical properties of the 3d block transition metal nickel, oxidation and reduction reactions of nickel ions, outer electronic configurations of nickel, principal oxidation states of nickel, shapes of nickel's complexes, octahedral complexes of nickel, tetrahedral complexes of nickel, square planar complexes of nickel, stability data for nickel's complexes, aqueous chemistry of nickel ions, redox reactions of nickel ions, physical properties of nickel, melting point of nickel, boiling point of nickel, electronegativity of nickel, density of nickel, atomic radius of nickel, ion radius of nickel, ionic radii of nickel's ions, common oxidation states of nickel, standard electrode potential data for nickel, ionisation energies of nickel, polarising power of nickel ions, industrial applications of nickel compounds, chemical properties of nickel compounds, why are nickel complexes coloured?, isomerism in the complexes of nickel, formulae of nickel compounds, tests for nickel ions keywords redox reactions ligand substitution displacement balanced equations formula complex ions complexes ligand exchange reactions redox reactions ligands colours oxidation states: nickel ions Ni(0) Ni2+ Ni(+2) Ni(II) NiCl2 NiSO4 Ni2+ + 2OH– ==> Ni(OH)2 Ni2+ + CO32– ==> NiCO3 Ni2+ + 2HCO3– ==> NiCO3 + 4H2O + CO2 [Ni(H2O)6]2+ + 6 NH3 [Ni(NH3)6]2+ + 6H2O [Ni(H2O)6]2+ + 6NH3 ==> [Ni(NH3)6]2+ + 6 H2O Kstab = [[Ni(NH3)6]2+] / [[Ni(H2O)6] 2+] [NH3]6 Ni(OH)2 + 6NH3 [Ni(NH3)6]2+ + 2OH– [Ni(H2O)6]2+ + 3en ===> [Ni(en)3]2+ + 6H2O Kstab = [[Ni (en)3]2+] / [[Ni(H2O)6]2+] [[en]3] [Ni(H2O)6]2+ + EDTA4– ===> [Ni(EDTA)]2– + 6H2O Kstab = [[Ni(EDTA)3]2–] / [[Ni (H2O)6]2+] [[EDTA4–]][Ni(H2O)6]2+ + 4 Cl– ==> [NiCl4]2– + 6H2O Kstab = [[NiCl4]2–] / [[Ni(H2O)6]2+] [Cl–]4 [Ni(H2O)6]2+ + 4 CN– ==> [NiCN4]2– + 6H2O Kstab = [[NiCN4]2–] / [[Ni(H2O)6]2+] [CN–]4 Kstab = 2 x 1031 mol4 dm–12 [lg(Kstab) = 31.3] oxidation states of nickel, redox reactions of nickel, ligand substitution displacement reactions of nickel, balanced equations of nickel chemistry, formula of nickel complex ions, shapes colours of nickel complexes  Na2CO3 NaOH NH3 nickel chemistry for AQA AS chemistry, nickel chemistry for Edexcel A level AS chemistry, nickel chemistry for A level OCR AS chemistry A, nickel chemistry for OCR Salters AS chemistry B, nickel chemistry for AQA A level chemistry, nickel chemistry for A level Edexcel A level chemistry, nickel chemistry for OCR A level chemistry A, nickel chemistry for A level OCR Salters A level chemistry B nickel chemistry for US Honours grade 11 grade 12 nickel chemistry for pre-university chemistry courses pre-university A level revision notes for nickel chemistry  A level guide notes on nickel chemistry for schools colleges academies science course tutors images pictures diagrams for nickel chemistry A level chemistry revision notes on nickel chemistry for revising module topics notes to help on understanding of nickel 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 nickel chemistry Edexcel A level chemistry notes on nickel chemistry for OCR A level chemistry notes WJEC A level chemistry notes on nickel chemistry CCEA/CEA A level chemistry notes on nickel chemistry for university entrance examinations biological role of cobalt nickel is unknown, nickel(II) chemistry, shape and formula of complexes of nickel(II) Ni2+, complexes of nickel with ammonia, oxidation of nickel(II) ion Ni2+ to the nickel(III) ion Ni3+, reactions of the nickel(II) ion Ni2+ with hydroxide ion, structure formula and shape of nickel carbonyl, colour and structure of nickel(III) Ni3+ complexes, formula of EDTA complexes of nickel, tetrahedral complexes of nickel with chloride ion ligands, octahedral complexes of nickel(II) ion Ni2+ with cyanide and water ligands, octahedral complexes of nickel(III) Ni3+