Electron Counting and the Eighteen-Electron Rule · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
En anglais

Counting to eighteen at a metal centre

Science · Chemistry · ages 20-21
Name ______________________   Date ____________

  1. Four terminal carbonyl ligands each donate 2 electrons. How many electrons do the ligands donate altogether?

    Answer: ______________

  2. What is the orbital basis of the eighteen-electron guideline?

    • Five d, one s and three p orbitals each hold two electrons
    • Only five d and three p orbitals contribute to the count
    • Five d orbitals each hold three electrons rather than two

  3. Correct neutral and ionic counts of the same complex, with the same binding modes, give the same total.

    Circle one:   True   False

  4. For a neutral mononuclear ruthenium carbonyl with only terminal CO, how many CO ligands give 18 electrons? Ru is in group 8.

    • Four CO ligands, giving a total of 16 electrons
    • Five CO ligands, giving a total of 18 electrons
    • Six CO ligands, giving a total of 20 electrons

  5. Count [Mn(CO)6]+ by the neutral method. Mn is in group 7 and all six CO ligands are terminal.

    Answer: ______________

  6. Neutral counting gives 16 and ionic counting gives 18 for the same complex. What should be checked?

    • Average the two counts rather than rechecking ligand classifications
    • Keep the ionic count rather than checking the complex charge
    • Check charge, ligand classification, binding modes and arithmetic

  7. In the stated simple symmetric two-metal model, one CO bridges both metals and supplies 2 electrons in total. What is its contribution to each metal?

    • Two per metal: count the complete CO donation at each centre
    • Zero per metal: omit the donation when CO bridges two centres
    • One per metal: share the CO donation equally between centres

  8. A strong-field square-planar d8 complex has 16 electrons. Which judgment is justified by the count?

    • Sixteen can be stable; the count alone does not establish stability
    • Sixteen is unstable; the count must reach eighteen before stability
    • Sixteen proves stability; square-planar geometry removes any doubt
LightMySky · lightmysky.comW1-mt_uab5c5pODY-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Counting to eighteen at a metal centre W1-mt_uab5c5pODY-s1

  1. 8 · Count only the ligands: 4 × 2 = 8 electrons.
  2. Five d, one s and three p orbitals each hold two electrons · Five d, one s and three p orbitals total nine. Each can hold two electrons, but their energies limit the guideline.
  3. True · The methods assign contributions differently but count the same metal-centred valence electrons.
  4. Five CO ligands, giving a total of 18 electrons · Ru contributes 8. Ten more electrons are needed, so five 2-electron CO ligands give 18.
  5. 18 · The neutral count is 7 + 6 × 2 - 1 = 18. The positive charge removes one electron.
  6. Check charge, ligand classification, binding modes and arithmetic · Correct methods agree. Disagreement can be a classification or charge error, not just an arithmetic slip.
  7. One per metal: share the CO donation equally between centres · In this bridge model, the 2-electron CO donation is shared between two metal centres: 1 electron per centre.
  8. Sixteen can be stable; the count alone does not establish stability · Sixteen-electron square-planar d8 complexes are a familiar exception. The count permits stability but does not prove it.
Worksheet · LightMySky