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A LevelChemistryCIERevision Notes6. Inorganic Chemistry (A Level Only)6.3 Transition Element Complexes: Isomers, Reactions & Stability (A Level Only)6.3.6 Stability Constants, Kstab

Stability Constants, Kstab (CIE A Level Chemistry)

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Francesca

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Francesca

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Define & Write a Stability Constant for a Complex

  • When transition element ions are in aqueous solutions, they will automatically become hydrated
    • Water molecules will surround the ion and act as ligands by forming dative covalent bonds to the central metal ion

  • When there are other potential ligands present in solution, there is a competing equilibrium in ligand exchange and the most stable complex will be formed
  • For example, a Co(II) ion in solution will form a [Co(H2O)6]2+ complex
  • Adding ammonia results in the stepwise substitution of the water ligands by ammonia ligands until a stable complex of [Co(NH3)4(H2O)2]2+ is formed

[Cu(H2O)6]2+ + 4NH3 ⇌ [Cu(NH3)4(H2O)2]2+ + 4H2O

  • For the substitution reaction above, there are four stepwise constants:

[Cu(H2O)6]2+ + NH3 ⇌ [Cu(NH3)(H2O)5]2+ + H2O              K1

[Cu(NH3)(H2O)5]2+ + NH3 ⇌ [Cu(NH3)2(H2O)4]2+ + H2O             K2

[Cu(NH3)2(H2O)4]2+ + NH3 ⇌ [Cu(NH3)3(H2O)3]2+ + H2O             K3

[Cu(NH3)3(H2O)3]2+ + NH3 ⇌ [Cu(NH3)4(H2O)2]2+ + H2O             K4

  • These stepwise constants are summarised in the overall stability constant, Kstab
  • The stability constant is the equilibrium constant for the formation of the complex ion in a solvent from its constituent ions or molecules

Expression of Kstab

  • The expression for Kstab can be deduced in a similar way as the expression for the equilibrium constant (Kc)
  • For example, the equilibrium expression for the substitution of water ligands by ammonia ligands in the Co(II) complex is:

[Cu(H2O)6]2+ + 4NH3 ⇌ [Cu(NH3)4(H2O)2]2+ + 4H2O

 

Kstab = fraction numerator stretchy left square bracket Cu open parentheses NH subscript 3 close parentheses subscript 4 open parentheses H subscript 2 O close parentheses subscript 2 stretchy right square bracket to the power of bold 2 bold plus end exponent over denominator stretchy left square bracket Cu open parentheses H subscript 2 O close parentheses 6 stretchy right square bracket to the power of bold 2 bold plus end exponent bold space stretchy left square bracket NH subscript 3 stretchy right square bracket to the power of bold 4 end fraction

  • The concentration of water is not included in the expression as the water is in excess
  • Therefore, any water produced in the reaction is negligible compared to the water that is already present
  • The units of the Kstab can be deduced from the expression in a similar way to the units of Kc
  • The stability constants can be used to compare the stability of ligands relative to the aqueous metal ion where the ligand is water
  • The larger the Kstab value, the more stable the complex formed is

Calculations Involving Stability Constants

  • If the concentrations of the transition element complex and the reacting ligands are known, the expression for the stability constant (Kstab) can be used to determine which complex is more stable
  • The greater the value of Kstab the more stable the complex is

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    Previous:6.3.5 Stereoisomerism in Transition Element ComplexesNext:6.3.7 Effect of Ligand Exchange on Stability Constant
    Francesca

    Author: Francesca

    Expertise: Head of Science

    Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.