The period 1951–1954 was a golden one for structural chemistry; proteins, DNA, Ferrocene (1952) and the one I discuss here, a bonding model for Zeise’s salt (3).
In “A review of π Complex Theory”, Bull. Soc. Chim. Fr., 1951, 1 8 , C79 (it is not online) M. J. S. Dewar sets out his theory of the role of π-complexes in (mostly) organic chemistry. The paper derives from an international colloquium held in Montpellier, in which audience responses to the presentation are included as an annex to the article itself. It is as a footnoted response (to P. Bartlett) that Dewar presents his theory of the alkene-metal π-complex, of which the best known example is Zeise’s salt (3).
This diagram illustrates the binding of a silver cation Ag+ to ethene (1). Dewar uses group theory to show how the molecular orbitals from ethene can be combined with the atomic orbitals on the metal. Two filled and two empty orbitals combine to give two new combinations, with a total occupancy of four electrons defining the interaction between alkene and metal. Dewar regards this four-electron-three-centre interaction as distinctive from simply the formation of two single metal-C bonds (a metallacyclopropane).
Zeise’s salt itself derives from Pt2+ by addition of three chloride anions to give PtCl3–. To compare this with Dewar’s Ag+ example, I use here just the naked metal cations 1-2.‡ I went about this analysis as follows:
Ag | Pt |
Ag | Pt |
Ag | Pt |
|
The famous Dewar π-complex model of alkene-metal interaction as applied to the Ag+ cation describes one “normal molecular bond” and a second bond “opposite in direction to the first”, what we now call a back-bond. What has emerged however is that two “normal molecular bonds” can be identified for Ag+ based purely on their symmetry but only one for Pt2+ (which of course has two valence electrons less) and both exhibit one back-bond. The diagram above must absorb a further pair of electrons from a formally non-bonding filled dz2 orbital, whilst recognising that hybridisation may allow it too to take on some bonding role.
You might ask what the missing orbitals 12, 14-16 are? Well, formally they derive from the other occupied four metal d-orbitals, but in fact mixed heavily with the C-H bonds of the ethene. I have to conclude that a molecular orbital analysis of e.g. Zeise’s salt (with additional orbital mixing from the three chlorides) ends up being pretty complex! But despite this complexity, Dewar’s original hypothesis, produced in response to a question from the audience, certainly started something. It is worth reminding that the 1952 Nobel-prize winning suggestion for the structure of Ferrocene[2] includes no group theoretical orbital analysis of the bonding on a par with Dewar’s 1951 insights.
‡ In fact, the MOs turn out to be pretty sensitive to the ligands surrounding the metal, and so those presented here for the naked cations will differ from those for “real molecules” such as Zeise’s salt.
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