In these reactions an acceptor species adds a generic XY, where X and Y represent two atoms or groups linked by a single bond. The addition product can be:
A + X-Y ⇄ X-A-Y
A + X-Y ⇄ [X-A]+Y-
Oxidative additions are very common in the chemistry of coordination compounds of transition elements (complexes) especially in organometallic compounds. These processes occur in coordinatively unsaturated compounds with oxidation numbers that can increase one or two units. Some saturated complex react after removal of a ligand. Different reagents can participate, frequently H 2 sub>, halogens, hydrogen halides and alkyl halides. The mechanisms, not always been well-characterized, are highly dependent on the substrates and the solvent. They can be concerted or radicalary.
The PBr3 is a nucleophile through the HOMO orbital centered on the phosphorus atom. Therefore it will attack the bromine breaking the Br-Br bond heterolytically and generating Br+ (which is attached to PBr 3) and Br - (which remains as counterion).
Cl2, in the gas phase, is added to PCl3 in an axial position, forming PCl5 that in these conditions is monomer. The reaction is rapid because the concerted approach (axial) of chorine parallel to the C3 symmetry axis of PCl3 is allowed by symmetry. In polar solvents the mechanism would be different.
The HOMO of PCl3 (the donor orbital) is slightly bonding in P-Cl bonds and the LUMO of chlorine (orbital acceptor) is antibonding. Therefore, the addition breaks the Cl-Cl bond and weakens the P-Cl bonds. Note that the PCl5 is "hypervalent" and the axial and equatorial P-Cl bonds (2.14Å and 2.02Å respectively) are, on average, less robust than in the PCl3 (2.04 Å). The same is true of the equatorial approach (see animation). P>
In this reaction the chorine molecule is added to PCl3 in a equatorial position. This is the alternative possible mechanism for the addition of Cl2 to PCl3 gas phase (compare the above reaction). All the comments on both mechanisms are the same.
En fase gaseosa la aproximación de la molécula de cloro perpendicular al eje C2 de la molécula de SCl2 (C2v) es permitida por la simetría, por lo que la reacción es rápida. Ocurre a -78ºC y, si se calienta la reacción se invierte. Nótese la estructura del SCl4, que tiene un par no enlazante en el azufre orientado en una posición ecuatorial de la molécula.
The attack of the fluorine molecule parallel to the C4 axis of XeF4 sub> molecule is permitted by symmetry. However, it requires 300°C, 60 atmospheres and large excess of fluoride. Note that the structure of XeF6 cannot be octahedral perfect, but octahedral pointed on one face (symmetry C3V) due to nonbonding pair of Xe. In this reaction the fluorine molecule is added to XeF 2 in an axial position.
The addition of H2 to the Ir (I) complex IrCl (CO) (PPh 3)2, named Vaska´s compound: IrCl(CO)(PPh3)2 + H 2 & # x21C4; IrClH2(CO)(PPh3)2, produces a Ir(III) dihydride complex. It is an oxidative addition. Observe the approximation of the H2 molecule parallel to the OC-Ir-Cl axis.
The addition of O2 to the Vaska´s Ir(I) complex : [IrCl(CO)(PPh3) 2] + O2 & # x21C4; [IrCl(O2)(CO)(PPh3)2] seems a simple addition (compare with the addition of CO or SO2). However, the O-O distance in the final product (1.25a), is intermediate between that of the free O2 (1.21Å) and that of the O22- ion (1.49Å). This and other spectroscopic data allow the description of this the product as a complex with the ligand O22- (peroxide). As the resulting product is a Ir (III) complex, the reaction is classified as an oxidative addition.