The Organometallic Chemistry of the Transition Metals (Crabtree/The Organometallic Chemistry) || Complexes of π-Bound Ligands

✍ Scribed by Crabtree, Robert H.

• Publisher: John Wiley & Sons, Inc.
• Year: 2005
• Weight: 338 KB
• Edition: 4
• Category: Article
• ISBN: 0471662569
• DOI: 10.1002/0471718769.ch5

✦ Synopsis

In this chapter we continue our survey of the different types of ligand by looking at cases in which the π electrons of an unsaturated organic fragment, rather than a lone pair, are donated to the metal to form the M-L bond.

5.1 ALKENE AND ALKYNE COMPLEXES

In 1827, the Danish chemist Zeise obtained a new compound he took to be KCl•PtCl 2 •EtOH from the reaction of K 2 PtCl 4 with EtOH. Only in the 1950s was it established that Zeise's salt, 5.1, is really K[PtCl 3 (C 2 H 4 )]•H 2 O, containing a coordinated ethylene, formed by dehydration of the ethanol, and a water of crystallization. The metal is bonded to both carbons of the ethylene, but the four C-H bonds bend slightly away from the metal, as shown in 5.4; this allows the metal to bind efficiently to the π electrons of the alkene. For Zeise's salt, the best bonding picture 1 is given by the Dewar-Chatt model. This involves donation of the C=C π electrons to an empty d σ orbital on the metal, so this electron pair is now delocalized over three centers: M, C, and C . This is accompanied by back donation from a metal d π orbital into the ligand LUMO, the C=C π * level, as shown in 5.2 (occupied orbitals shaded). By analogy with the bonding in CO, we will refer to the former as the "σ bond" and the latter as the "π bond." As is the case for CO, a σ bond is insufficient for tight binding, and so only metals capable of back donation, and not d 0 metals such as Ti(IV), bind alkenes well.