The deep blue complex [Cu(NH₃)₄]²⁺ is square planar and paramagnetic…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The deep blue complex is square planar and paramagnetic. The atomic number of copper is 29. (a) How many 3d electrons does have? (b) Valence bond theory needs one empty 3d orbital for . Say how it is found. (c) Find the number of unpaired electrons and the moment. (d) What does the step in (b) show about the theory?
Need a reminder? The key ideaShow
In valence bond theory the ligands give lone pairs into empty hybrid orbitals. A strong field ligand pairs the d electrons and can free two 3d orbitals for . A weak field ligand leaves the electrons unpaired, so the ion uses . The spin-only moment is , with the number of unpaired electrons.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The 3d count. has electrons in 3d. Four orbitals hold a pair and one holds a single electron.
Step 2. Freeing an orbital. The theory moves the single 3d electron up into a 4p orbital. Now four 3d orbitals hold pairs and one is empty. That empty 3d orbital, the 4s orbital and two of the 4p orbitals make four hybrids. They point to the corners of a square.
Step 3. The unpaired electron. The electron that was moved sits alone in the third 4p orbital. So one electron is unpaired, and .
Answer: 1 unpaired electron and a moment of .
Check. The measured moment of copper(II) complexes is close to this value, so the prediction agrees.
What it shows. An electron in a 4p orbital is far from the nucleus and would be easy to remove. Yet copper(II) complexes are not easily oxidised in this way. The theory reaches the right answer only by a step added by hand.
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