Note that the H3C-Br bond is broken on the reaction oxidative addition generating two new anionic ligands: CH3 and Br. If the starting metal complex is 16e- (as shown above) both ligands will usually end up coordinated to the metal to make an 18e- complex. In the case of a starting 18e- complex (shown below) only of the one two anionic ligands (usually the strongest binding) generated from the oxidative addition will end up coordinated to the metal unless a separate substitution reaction occurs. In this case the alkyl anion is the best donor ligand and easily “beats out” the more electronegative and poorly donating Cl anion. Note that the alkyl ligand (-CH2CH=CH2) initially coordinated to the Re after the oxidative addition is an ligand and that it can convert to the 1-allyl generally more stable on ligand dissociation. 3-allyl CO General Features of Oxidative Additions Because oxidative addition involves oxidation (removal of electrons) of the metal center, the more electron-rich the metal is the easier the oxidative addition to the metal center will be. So in comparing two or more metal complexes to see which will be the most reactive towards a particular substrate for oxidative addition you would pick the metal center with the strongest donor ligands, fewest ligands, or -acceptor most negative charge. Also remember that the non-electrophillic ligands (Class A) and ligands (Class C) usually require that there is an “intact” empty orbital (16e- or lower) on the metal center in order to react. Kinetic Data for Oxidative Addition Reactions of MX(CO)(PR3)2 Rate Const H‡ S‡ M X PR3 Reactant (M1 sec1) (kcal/mol) (J/mol K) Ir Cl PPh3 H2 0.67 10.8 23
Note that the H3C-Br bond is broken on the oxidative addition reaction generating two new anionic ligands: CH3 and Br. If the starting metal complex is 16e- (as shown above) both ligands will usually end up coordinated to the metal to make an 18e- complex. In the case of a starting 18e- complex (shown below) only one of the two anionic ligands (usually the strongest binding) generated from the oxidative addition will end up coordinated to the metal unless a separate substitution reaction occurs. In this case the alkyl anion is the best donor ligand and easily “beats out” the more electronegative and poorly donating Cl anion. Note that the alkyl ligand (-CH2CH=CH2) initially coordinated to the Re after the oxidative addition is an 1-allyl ligand and that it can convert to the generally more stable 3-allyl on CO ligand dissociation. General Features of Oxidative Additions Because oxidative addition involves oxidation (removal of electrons) of the metal center, the more electron-rich the metal is the easier the oxidative addition to the metal center will be. So in comparing two or more metal complexes to see which will be the most reactive towards a particular substrate for oxidative addition you would pick the metal center with the strongest donor ligands, fewest -acceptor ligands, or most negative charge. Also remember that the non-electrophillic ligands (Class A) and “intact” ligands (Class C) usually require that there is an empty orbital (16e- or lower) on the metal center in order to react. Kinetic Data for Oxidative Addition Reactions of MX(CO)(PR3)2 M X PR3 Reactant Rate Const (M1 sec1) H‡ (kcal/mol) S‡ (J/mol K) Ir Cl PPh3 H2 0.67 10.8 23