Lecture 31: Reactivity of Carbonyl Complexes

Read sections 23.7 and 23.8 from your textbook.

Synthesis of Carbonyl Complexes

  1. Direct reaction between the metal and CO

    This reaction is possible for only Ni and Fe.




  2. Reaction of metal oxides with CO (high temperature and pressure)




  3. Reductive carbonylation of metal halides

    Reducing agents include Li, Na, K, AlEt3, Li[AlH4]




  4. Reaction of late transistion metal halides with methanol




Substitution reactions

  1. Substitution of other ligands by CO

    CO can substitute for other ligands but this only happens when the metal is electron rich. The reactions can proceed through associative, dissociative, or interchange mechanisms. See lecture 26 and lecture 27 for more information.

  2. Substitution of CO by other ligands (heat, light)

    CO is very strongly bonded in metal complex. In order to substitute CO by another ligand, it is necessary to add thermal energy or to weaken the M-CO bond by photolysis. Typically, mixtures of products are formed.




  3. Using amine N-oxides to facilitate substitution

    Amine N-oxides convert a coordianted CO to CO2 and a metal-amine complex. The amine is a good leaving group and will be substituted by any incoming ligand in a typical substitution reaction. The mechanism of the reaction is shown below. The oxygen of the amine N-oxide acts as a nucleophile and adds to the carbonyl carbon.




Addition of external nucleophiles

Organic carbonyl compounds add nucleophiles at the carbonyl carbon. Metal carbonyl complexes are polarized like these compounds with a partial positive charge on the carbonyl carbon and a partial negative charge on the carbonyl oxygen. They also add a variety of nucleophiles.




Addition of internal nucleophiles

A hydride or an alkyl group bonded to a transition metal that also has a carbonyl ligand can act as an internal nucleophile.




The resulting ligands (formyl, CHO; acyl, CRO) are 1 electron donors for electron counting and -1 ligands for the oxidation state formalism. Both additions are reversible but the hydride-carbonyl is generally favored over the formyl complex.

Formation of carbenes from CO

The addition of a nucleophile to coordinated CO followed by the addition of an electrophilic alkylating agent give a carbene complex. Typical electrohilic alkylating agents include CH3I, CH3OS(O)2CF3, and [Me3O][BF4].




This type of carbene ligand is a 2 electron donor for electron counting and a neutral ligand for the oxidation state formalism, just like CO itself.