Ethylene in Zeise’s salt

Olefin exchange has been used for the preparation of certain plati-num(II) complexes (67). Thus, trimethylvinylsilane replaces ethylene in Zeise s salt to give the compound K[Cl3Pt(H2C=CHSiMe3)]. The SiMe3 group in this complex is readily cleaved by water (wet acetone) with elimination of hexamethyldisiloxane and regeneration of Zeise s salt, but resists attack by pyridine N-oxide, which instead replaces a chlorine atom (67). 

The simplest alkyne complexes, the metal acetylenes, resemble those of ethylene. For example, there are analogues of Zeise s salt in which an acetylene molecule ts bound to platinum(l)) and occupies a position like that of ethylene in Zeise s salt. In addition, there are L,Pt(RC=CR) complexes that have structures paralleling that of LjWHjC CHj) (Fig. 15.24). For both of these PtfO) complexes, an approximate square planar arrangement around the metal is found. Alkynes are more electronegative than ulkenes and are therefore better it acceptors. Thus it is appropriate to view them as metal lacyclopropenes 79... 

An MO treatment of the platinum-olefin p/ -bond was applied by Chatt and Dun-canson in 1953 131>. It is similar to Dewar s explanation of bonding in silver complexes but o-bond overlap is between a filled ligand 7r-orbital and an empty 5d6s6p2 hybrid orbital of the metal. Back co-ordination from the filled metal 5d6p hybrid orbital to the empty olefin jt orbital assists in restoring a more favourable charge distribution as demonstrated in E.II. A result of the mixing in of a 6p with the Sd orbital of the metal is that the C=C bond will be perpendicular to the co-ordination plane of the metal atom if overlaps are to be maximised. The orientation of ethylene in Zeise s salt and related compounds... 

As an illustrative example, the bonding between the platinum(II) ion and ethylene in Zeise s salt (Fig. 3-8) involves the following ... 

Consider the bonding of ethylene in Zeise s salt K [C2H4PtCl3]. X-ray diffraction studies have shown that the anion has the structure given in Figure 27. The platinum is in a square planar configuration with the axis... 

Consider the bonding of the ethylene in Zeise s salt, K+[C2H4PtCl3] , which X-ray studies have shown the structure of the anion to be that given... 

FIGURE 21.8 The 7r bonding that occurs between ethylene and Pt in Zeise s salt. 

A structural investif tion of the anion in Zeise s salt has shown that the ethylene occupies the fourth coordination site of the square planar complex wth the C—C axis perpendicular to the platinum-ligand plane (Fig. 15.22).75 Relative to free ethylene, the C—C bond is lengthened slightly (from 133.7 pm to 137.5 pm), and the hydregens are slightly tilled back from a planar arrangement. 

In this molecule ethylene lies in the same plane as the other ligands, unlike the case in Zeise s salt where planarity is sterically prevented. By analogy to a three-membered ring of carbon atoms, this compound may be viewed as a mctallacyclopropane. 

Fig The structure of the anion in Zeise s salt, trichloro (ethylene) platinate(II) ion. 

Figure 3 Early proposal by J. Chatt of bonding between ethylene and platinum ii) in Zeise s salt taken from ref 13)...

When an alkene acts as a ligand, as ethylene does in Zeise s salt, it binds to the metal in a sideways manner, where the C-C T(, MO donates electron density to an empty orbital on the metal, as shown in Figure 18.15, and the m MO accepts electron density back from a filled d-orbital on the metal. As a result, the C=C bond is weakened by both the r-donation and the a -backbonding interaction. The C=C bond length in free ethylene is 133.7 pm in Zeise s salt it increases to 137.5 pm and it is as lar ge as 143 pm in [F (PPh3)2(C2H4)], where the metal is electron-rich as a result of the low oxidation number and donor ligands. 

One of the typical examples of the transition metal tt complexes is the plat-inum(II) complex with ethylene, [PtCl3(C2H4)], which is known as the anion in Zeise s salt ... 

In this complex, the molecule of coordinated ethylene occupies a single coordination site and is oriented perpendicularly to the plane of the square anion. In both complexes mentioned above (i.e., the complexes with two-center ligands and the olefin rr complexes), the bonding of ligands to the central atom occurs via the dihapto type. Moreover, the rotation of olefin about the metal-ligand bond was observed both in Zeise s salt and in some other ir complexes. Thus, the free energies for the activation of olefin (L) rotation in complexes (PR3)PtCl2 -L and PtCl(acac) - L depend on the nature of R and L, and vary within 44.4-67 kJ mol (10.6 16 kcal mol ) [80]. 

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