Metal carbonyl and related complexes (dative ligands)

1. Metal carbonyl and related complexes (dative ligands) Textbook H: Chapter 2.1 – 2.2.7 Textbook A: Chapter 7.1 – 7.4

2. Stable binary carbonyls

3. Synthesis of metal carbonyl complexes • From CO • Directly from reaction with the metal • Ligand substitution • Reductive carbonylation • Deinsertion: not very common

4. Bonding in CO 13C NMR features, d: 180 – 250 ppm

5. Bonding in CO complexes • Bonding in CO • The transfer of electrons from the lone pair of O to C leads to a C--O+ polarization, which is almost cancelled out by the C+-O- polarization (O electronegativity). • HOMO of CO is the C-lone pair (O orbitals are deeper-lying). • LUMO of CO (p*) is polarized toward C. • CO binds through C • s-bond removes electron density from C. • p-backbond increases electron density both at C and O. • Polarization of CO on binding • C more positive, O more negative • Dependent on the other ligands present • C is more electrophilic when good p acceptors are present or when the complex is cationic (s-bond is enhanced, p-backbond is weakened) • O is more nucleophilic when good p donors are present or when the complex is anionic • Can be followed by IR • The more important the p-backbond contribution, the weaker the C-O bond, the lower the stretching frequency.

6. Backbonding Experimental support for backdonation: • X-ray: in (C5H5)Mo(CO)3Me, M-C is 2.38 Å, M=C is 1.99 Å (>0.07 Å) • IR, n(CO): • free CO, 2149 cm-1 • H3B-CO, 2178 cm-1 • Cr(CO)6, 2000 cm-1 • V(CO)6-, 1860 cm-1 • Mn(CO)6+, 2090 cm-1 • Cr(tren)(CO)3, 1880 cm-1 (tren = H2NCH2CH2NHCH2CH2NH2)

7. Binding modes of CO

8. CO frequency and phosphine ligands p-Acid character: PMe3 ~ P(NR2)3 < PAr3 < P(OMe)3 < (POAr)3 < PCl3 < CO ~ PF3

9. Phosphine ligands: bonding As the electronegativity of the atom attached to the phosphorus increases, the s* orbital becomes lower in energy and the phosphine is a better p-acceptor.

10. Monodentate phosphines: cone angles Reference: Tolman, C. A. Chem. Rev. 1977, 77, 313

11. Metal nitrosyl complexes

12. Nitrosyl (NO) complexes: binding • Two different coordination modes

13. Molecular Orbital Diagram of NO

14. Electronic Structure • Non-innocent ligand • M–N–O can range from 120–180° + e– – e– NO NO+ NO– Isoelectronic with O2 Isoelectronic with CO

15. Bent Linear Binding Modes and IR spectroscopy • Multiple binding modes • Effect on the NO stretching frequency 1900 1800 1700 1600 1500 1400 2000 νNO (cm-1)

16. Enemark and Feltham Notation {M(NO)n}m • m = number of electrons in the metal d orbitals and the π*(NO) orbital Allows to circumvent assigning oxidation states in difficult molecules NO0 = 1 π*(NO) electron Co0 = 9 d electrons {CoNO}10 If considered NO+ the oxidation state is Co( –1)