14.1.2: Oxidative Addition
- Page ID
- 392520
An oxidative addition is a reaction in which the coordination number and the oxidation state of the metal ion both increase by 2. In other words, the metal becomes oxidized by two electrons as the coordination number increases by 2 ligands. The microscopic reverse of oxidative addition is reductive elimination (discussed in the next section).
Oxidative addition can occur in a cis- or trans-fashion. A cis-addition means that two ligands are added so that they have cis-orientation to each other after the reaction is complete. A trans-addition implies that two new ligands are in opposite, trans-position after they have been added. We can further distinguish between mononuclear and dinuclear additions. Mononuclear addition involves a complex with a single metal. Dinuclear additions are additions to a metal-metal bond. The addition cleaves the metal-metal bond and adds one ligand to each fragment.
Let's explore a few examples. The first example is a mononuclear cis-addition (Fig. \(\PageIndex{1}\)). In the reaction we add dihydrogen to a square planar iridium complex. After the reaction there are two additional hydrido ligands, and the coordination number has increased from 4 to 6. We can see that the two hydrido ligands are oriented in cis-fashion relative to each other. We can verify that the addition of the hydrido ligands oxidized the metal by determining the oxidation number of the metal before and after the reaction. There are three neutral ligands and one negatively charged chloro ligand. There is no overall charge at the complex. This means that the oxidation number of Ir is +1. After the reaction, there are the two hydrido ligands that have both a -1 charge. Remember, we get the charge at a ligand by cleaving the metal-ligand bond so that all electrons in the bond get assigned to the ligand. This means the oxidation number must be increased by +2, and is +3 after the reaction.
We can rationalize why the addition occurs in cis-fashion and not in trans-fashion. The two hydrido ligands are the smallest ligands, therefore, steric repulsion is minimized in the complex is minimized when the two hydrido ligands are in cis-position.
The second example is a mononuclear trans-addition (Fig. \(\PageIndex{2}\)). In this reaction CH3Br is added to the iridium complex. In this case the two new ligands, a CH3 and the Br group are in opposite position. Again, the coordination number increases from four to six. We can again verify that an oxidation has occurred by analyzing the oxidation number of Ir. As previously determined, the oxidation number of iridium in the reactant complex is +1. Let us analyze the product complex. The chloro ligand and the bromo ligand have a 1- charge. In addition to that also the CH3 ligand has a 1- charge assuming that we treat the Ir-C bond as a dative bond. Thus, the oxidation number of Ir must be +3 to give a neutral complex.
The third example is a dinuclear addition (Fig. \(\PageIndex{3}\)). This addition is neither cis nor trans because it cleaves a metal-metal bond, and the one new ligand is added to each fragment. In the example the Co-Co bond of the dicobalt octacarbonyl gets cleaved upon the reaction with dihydrogen, and two HCo(CO)4 complexes with Co-H bonds form. In this case the coordination number does not increase. Both the reactant and the products have the coordination number 5. We can see that an oxidation of Co has occurred by comparing the oxidation numbers. In the reactant the oxidation number of Co is zero because all CO ligands are neutral and the complex is overall neutral. In the carbonyl hydride product, Co has the oxidation number +1, because the hydrido ligand has a 1- charge.
Oxidative Additions with Intact Ligands
In the previous examples, the ligands that reacted with the complexes lost their integrity. The H-H bond in the H2 molecule cleaved to form the hydrido ligands, and the C-Br bond in CH3Br was cleaved to form separate Br and methyl ligands. Not all ligands lose their integrity upon their addition, in particular when they contain multiple bonds. These ligands are called intact ligands. In these cases the bond order between two atoms in the ligand is decreased, but the bond order is still at least 1. For example, the alkyne ligand shown belo decreases from a carbon-carbon bond order of three to a bond order of two after it binds to Pt.
Examples for intact ligands are are alkenes, alkynes, O2, etc. In an oxidative addition with an alkyne for example, the alkyne binds side-on to the transition metal under the formation of two metal-C single bonds, and the reduction of the bond order within the alkyne from 3 to 2 (Fig. \(\PageIndex{4}\)). In the shown reaction, a diphenyl ethine molecule adds to tris(triphenyl phosphine) platinum under the formation of two Pt-C single bonds. The C-C triple bond in the ligand becomes a double bond. This reaction occurs under loss of one of the three triphenyl phosphine ligands. The coordination number at the metal increases from 3 to 4. We can again show that the reaction is oxidative by comparing the oxidation numbers of the metal. In the reactant, Pt has the oxidation number 0 because the three triphenyl phosphine ligands have no charge and the complex has overall no charge. After the reaction, the oxidation number of Pt is +2. This is because when we assign the four electrons of the two Pt-C single bonds to the ligand, the ligand becomes a (Ph-C=C-Ph)2- ligand with each C atom carrying a 1- charge. Because the product complex is overall neutral, the oxidation number of Pt is +2.
Nucleophilic Displacement Reactions
Another important reaction is the nucleophilic displacement reaction. This type of reaction can be classified as an oxidative addition although it does not strictly meet the definnition. In these reactions a metal complex (typically an anion) acts as a nucleophile. A ligand is added, but no electrons are added to the complex. The reactions can be extremely useful for the synthesis of organic compounds. The utility results mostly from the fact that an organic electrophile is turned into a nucleophile.
For example, tetracarbonyl ferrate (2-), also known as Collman’s reagent reacts with alkyl halides to form alkyl tetracarbonyl ferrate (-) (Fig. \(\PageIndex{5}\)). We can check that the addition of the alkyl group did not add any electrons by counting the electrons before and after the reaction. The Fe(CO)42- anion is an 18e complex. Applying the neutral atom method to the electron counting in [R-Fe(CO)4]- means that Fe contributes 8e, the 1- charge adds 1e, the CO ligands 4x2=8 electrons and the alkyl ligand 1e. This gives 8+1+8+1=18 electrons. Also the oxidation number of Fe has not changed upon the addition of the ligand. It is -2 before and after the reaction.
In the alkyltetracarbonyl ferrate (1-) anion, the group R which was formerly an electrophile in R-X, can now act as a nucleophile due to the fact that the Fe-C bond is polarized toward the C-atom. As a nucleophile it can for instance react with protons to form an alkane. It can also react with oxygen to form a carboxylic acid. In the presence of another alkyl halide R’-X it can form a ketone R-C(O)-R’. It can also insert CO into the Fe-C bonding in a so-called migratory insertion reaction, and the reaction product can react with acid to form an aldehyde. With dihalogens X2 it can form acyl chloride. Collman’s reagent cannot only add alkyl halides by also acyl halides to form acyl tetracarbonyl ferrates (-).