Transmetalation

Transmetalation (alt. spelling: transmetallation) is a type of organometallic reaction that involves the transfer of ligands from one metal to another. It has the general form:
M1–R + M2–R′ → M1–R′ + M2–R
where R and R′ can be, but are not limited to, an alkyl, aryl, alkynyl, allyl, halogen, or pseudohalogen group. The reaction is usually an irreversible process due to thermodynamic and kinetic reasons. Thermodynamics will favor the reaction based on the electronegativities of the metals and kinetics will favor the reaction if there are empty orbitals on both metals. There are different types of transmetalation including redox transmetalation and redox transmetalation/ligand exchange. During transmetalation the metal carbon bond is activated, leading to the formation of new metal carbon bonds. Transmetalation is commonly used in catalysis, synthesis of main group complexes, and synthesis of transition metal complexes.

There are two main types of transmetalation, redox transmetalation (RT) and redox transmetalation/ligand exchange (RTLE). Below, M1 is usually a 4d or 5d transition metal and M2 is usually a main group or 3d transition metal. By looking at the electronegativities of the metals and ligands, one can predict whether the RT or RTLE reaction will proceed and what products the reaction will yield. For example, one can predict that the addition of 3 HgPh2 to 2 Al will yield 3 Hg and 2 AlPh3 because Hg is a more electronegative element than Al.

M1n+–R + M2 → M1 + M2n+–R. In redox transmetalation a ligand is transferred from one metal to the other through an intermolecular mechanism. During the reaction one of the metal centers is oxidized and the other is reduced. The electronegativities of the metals and ligands is what causes the reaction to go forward. If M1 is more electronegative than M2, it is thermodynamically favorable for the R group to coordinate to the less electronegative M2.

M1–R + M2–X → M1–X + M2–R. In redox transmetalation/ligand exchange the ligands of two metal complexes switch places with each other, bonding with the other metal center. The R ligand can be an alkyl, aryl, alkynyl, or allyl group and the X ligand can be a halogen, pseudo halogen, alkyl, or aryl group. The reaction can proceed by two possible intermediate steps. The first is an associative intermediate, where the R and X ligands bridge the two metals, stabilizing the transition state. The second and less common intermediate is the formation of a cation where R is bridging the two metals and X is anionic. The RTLE reaction proceeds in a concerted manner. Like in RT reactions, the reaction is driven by electronegativity values. The X ligand is attracted to highly electropositive metals. If M1 is a more electropositive metal than M2, it is thermodynamically favorable for the exchange of the R and X ligands to occur.

Alkaline earth metal complexes have been synthesized by RTLE, employing the same methodology used in synthesizing lanthanide complexes. The use of diphenylmercury in alkaline earth metal reactions leads to the production of elemental mercury. The handling and disposal of elemental mercury is challenging due to its toxicity to humans and the environment. This led to the desire for an alternative RTLE reagent that would be less toxic and still very effective. Triphenylbismuth, BiPh3, was discovered to be a suitable alternative. Mercury and bismuth have similar electronegativity values and behave similarly in RTLE reactions. BiPh3 has been used to synthesize alkaline earth metal amides and alkaline earth metal cyclopentadienides. The difference between HgPh2 and BiPh3 in these syntheses was that the reaction time was longer when using BiPh3.