Transition metal allyl complex
Transition-metal allyl complexes are coordination complexes with allyl and its derivatives as ligands. Allyl is the radical with the connectivity CH2CHCH2, although as a ligand it is usually viewed as an allyl anion CH2=CH−CH2−, which is usually described as two equivalent resonance structures.
Examples
The allyl ligand is commonly in organometallic chemistry. Usually, allyl ligands bind to metals via all three carbon atoms, the η3-binding mode. The η3-allyl group is classified as an LX-type ligand in the Green LXZ ligand classification scheme, serving as a 3e– donor using neutral electron counting and 4e– donor using ionic electron counting.Scope
Commonly, allyl ligands occur in mixed ligand complexes. Examples include Mn4 and CpPd.Substituents on the allyl group are also common, e.g. 2-methallyl.
Homoleptic complexes
- bisnickel
- bispalladium
- bisplatinum
- trischromium
- trisrhodium
- trisiridium
Chelating bis(allyl) complexes
Allyl σ ligands
Complexes with η1-allyl ligands are also known. One example is CpFe2, in which only the methylene group is attached to the Fe centre. As is the case for many other η1-allyl complexes, the monohapticity of the allyl ligand in this species is enforced by the 18-electron rule, since CpFe2 is already an 18-electron complex, while an η3-allyl ligand would result in an electron count of 20 and violate the 18-electron rule. Such complexes can convert to the η3-allyl derivatives by dissociation of a neutral ligand L. For CpFe2, dissociation of L = CO occurs under photochemical conditions:Synthetic methods
Allyl complexes are often generated by oxidative addition of allylic halides to low-valent metal complexes. This route is used to prepare 2Ni2Cl2:A similar oxidative addition involves the reaction of allyl bromide to diiron nonacarbonyl. The oxidative addition route has also been used to prepared Mo allyl complexes:
Other methods of synthesis involve addition of nucleophiles to η4-diene complexes and hydride abstraction from alkene complexes. For example, palladium chloride attacks alkenes to give first an alkene complex, but then abstracts hydrogen to give a dichlorohydridopalladium alkene complex, and then eliminates hydrogen chloride:
One allyl complex can transfer an allyl ligand to another complex. An anionic metal complex can displace a halide, to give an allyl complex. However, if the metal center is coordinated to 6 or more other ligands, the allyl may end up "trapped" as a σ ligand. In such circumstances, heating or irradiation can dislocate another ligand to free up space for the alkene-metal bond.
In principle, salt metathesis reactions can adjoin an allyl ligand from an allylmagnesium bromide or related allyl lithium reagent. However, the carbanion salt precursors require careful synthesis, as allyl halides readily undergo Wurtz coupling. Mercury and tin allyl halides appear to avoid this side-reaction.
Benzyl complexes
Benzyl and allyl ligands often exhibit similar chemical properties. Benzyl ligands commonly adopt either η1 or η3 bonding modes. The interconversion reactions parallel those of η1- or η3-allyl ligands:In all bonding modes, the benzylic carbon atom is more strongly attached to the metal as indicated by M-C bond distances, which differ by ca. 0.2 Å in η3-bonded complexes. X-ray crystallography demonstrate that the benzyl ligands in tetrabenzylzirconium are highly flexible. One polymorph features four η2-benzyl ligands, whereas another polymorph has two η1- and two η2-benzyl ligands.
Applications
Allyl complexes are often discussed in academic research, but few have commercial applications. A popular allyl complex is allyl palladium chloride.The reactivity of allyl ligands depends on the overall complex, although the influence of the metal center can be roughly summarized as
Such complexes are usually electrophilic, but nickel allyl complexes are usually nucleophilic. In the former case, the addition may occur at unusual locations, and can be useful in organic synthesis.