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Fe(0) has its electronic configuration $\ce{3d^6 4s^2}$ in the ground state. In this case multiplicity will be either 1 or 3 during excitation. If multiplicity is 1, the vacant excited orbitals are $\ce{dsp^3}$. So, in this case it should be trigonal bipyramidal structure with the attachment of 5 ligands.

But in the case of multiplicity 3, the vacant excited orbitals will be $\ce{sp^3}$, excluding 2 singly occupied $\ce{3d}$ orbitals. If 4 ligands are there in attachment, what will be the structure -- tetrahedral or square planner?

Another additional information is also needed, is there any possibility to increase the number of ligand in this structure where multiplicity is 3? If yes, can you explain please?

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    $\begingroup$ You'll have hard time finding hi-spin iron (0) complexes. I'm unaware of any hi-spin metal (0) complexes. I suspect this is because hi-spin mean little covalent interaction with ligands, resulting in formation of pure metal. $\endgroup$ – permeakra Jul 11 '16 at 12:41
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    $\begingroup$ Fe(0) can have at least six ligands. See diiron nonacarbonyl. It does not answer your question(s), but then the answer to "What will be the coordination number of Fe(0)?" is It depends. $\endgroup$ – Ben Norris Jul 11 '16 at 12:43
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This is another data point for my constant mantra that using hybridisation to explain coordination chemistry is not helpful in the slightest. And even ‘worse’: you are trying to invoke some strong meaning of the 18-electron guideline even if not explicitly. The 18-electron guideline is a guideline not a rule. Even the octet guideline (that I personally like to refer to as a rule) is not as strict a rule as proper physical rules.

As Ben points out, in diiron nonacarbonyl iron(0) has a coordination number of six, which immediately voids your ‘excitation hybridisation’ argument. I am unaware of any fully characterised tetracoordinate iron(0) species. However, I guess they exist due to bulky ligand design and that they will be either square planar or tetrahedral in nature depending on the type of ligand in question. The type of ligand and its interaction is much more important than a hypothetical ‘excitation hybridisation’ scheme.

Please forget any and all of the wrong things you learnt about hybridisation with respect to transition metal complexes. Please refer to molecular orbital theory to explain these instead.

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