M-CO and M-PR 3 Bonds

1. M-CO and M-PR3 Bonds 201450105 오성진

2. CO molecular orbital diagram

3. Structure and Bonding • The highest energy occupied orbital (HOMO) has its largest lobe on carbon. • Carbon monoxide also has two empty π* orbitals (the lowest unoccupied, or LUMO); these also have larger lobes on carbon than on oxygen.

4. Structure and Bonding • Sigma and Pi Interactions Between CO and a Metal Atom.

5. Structure and Bonding • In free CO, the electrons are polarized toward the more electronegative oxygen. • The presence of a transition metal cation tends to reduce the polarization in the C-O bond by attracting the bonding electrons. • The consequence is that the electrons in the positively charged complex are more equally shared by the carbon and the oxygen, giving rise to a stronger bond and a higher-energy C-O stretch.

6. Structure and Bonding M-C bond : increases increasesincreasesincreases C-O bond : increases decreases decreasesdecreases v(CO) freq : increases decreases decreasesdecreases

7. Structures of Metal Carbonyls • Mononuclear Carbonyls • Binuclear Carbonyls • The M-C-O chains are essentially linear, and these compounds obey the 18-electron rule [except V(CO)6].

8. Structures of Metal Carbonyls • Trinuclear Carbonyls • Tetranuclear Carbonyls

9. Bridging modes of the CO ligand • Although CO is most commonly found as a terminal ligand attached to a single metal atom, many cases are known in which CO forms bridges between two or more metals.

10. Bridging mode of the CO ligand • In cases in which CO bridges two metal atoms, both metals can contribute electron density into π* orbitals of CO to weaken the C-O bond and lower the energy of the stretch. • Consequently, the C-O stretch for doubly bridging CO is at a much lower energy than for terminal COs.

11. Preparation of CO complexes • From CO gas • Metal + CO • Metal salts + reducing agent + CO (18e- complexes) • Typical reducing agents : Na, Al, H2, AlR3 ···

12. Preparation of CO complexes • Unsaturated compounds + CO • From a organic carbonyl • This can happen for aldehydes, alcohols

13. Reactions of metal carbonyls • Reactions of metal carbonyl • substitution • Nucleophilic attack at carbon

14. Reactions of metal carbonyls • Electrophilic attack at carbon • Migratory Insertion

15. Phosphine ligands • Because of the three R-groups on the phosphine ligand and the overall tetrahedral coordination geometry it is the most versatile of the neutral 2-electron donor ligands.

16. Phosphine ligands

17. Phosphine ligands • As the atom attached to the P atom becomes more electronegative, the empty P-X σ* orbital becomes more stable (lower in energy) making it a better acceptor of electron density from the metal center.

18. Phosphine ligands • Occupation of the P−R σ*orbital by back donation from the metal also implies that the P−R bonds should lengthen slightly on binding.

19. Phosphine ligands • Tolman electronic parameter • CO stretching frequencies measured for Ni(CO)3L where L are PR3 ligands of different σ-donor abilities. [ v(CO) = 2143cm-1 ] • The electronic effect of various PR3 ligands can be adjusted by changing the R group as, quantified by Tolman, who compared the v(CO) frequencies of a series of complexes of the type LNi(CO)3, containing different PR3 ligands.

20. Phosphine ligands • Tolman cone angle • To describe steric effects, Tolman has defined the cone angle as the apex angle, θ, of a cone that encompasses the van der Waals radii of the outermost atoms of a ligand. • The presence of bulky ligands, having large cone angles, can lead to more rapid ligand dissociation as a consequence of crowding around the metal.