Silyl complexes: M-SiR 3 (R = alkyl, aryl, OR) First complex: CpFe(CO) 2 (SiMe 3 ), Wilkinson 1956 Trimethylsilyl (TMS) complexes are more numerous than t-Bu complexes (rare)   -elimination inhibited due to instability of the Si=C bonds  Sterically less congested because M-Si is longer than M-C  M-SiR 3 bonds are stronger than M-C bonds due to  -interaction between M and SiR 3 fragment Most common synthesis method: oxidative addition of Si-H bonds (in contrast to C-H activation) Rich chemistry  1

Amide, oxides, and halide ligands Extra lone pairs present on the heteroatom  Late TM: when 18e, repulsion between lone pairs and filled d orbitals; weakening of the M- heteroatom bond  Early TM: when d 0, empty orbitals available for  interaction: strengthening of the M-heteroatom bond 2

Complexes of Anionic, Multiple-Carbon Ligands Textbook H: Chapter 3.5 – 3.7 Textbook A: Chapter 10.4 – 10.5

Allyl complexes Two binding modes:  1 and  3 The plane of the allyl is canted at an angle (  = °) such that orbital interaction is improved. Fluxionality: syn and anti substituents exchange through an  1 - intermediate. 1 H NMR: , 1 – 3 ppm (H anti ), 2 – 5 ppm (H syn ), 4 – 6 ppm (H meso ) 13 C NMR: , 80 – 90 ppm (C terminal ), 110 – 130 ppm (C terminal ) Fluxional on the NMR time scale:

Discovery of ferrocene 1948: Miller, Tebboth, Tremain report that olefins in the presence of Fe salts form FeC 10 H : Keally, Pauson, and Miller oxidized C 5 H 5 MgBr with FeCl 3 : Wilkinson/Woodward (Harvard) and Fischer (Munich) report the correct structure: Fe(  5 -C 5 H 5 ) Nobel prize Geoffrey Wilkinson and Ernst Otto Fischer on sandwich compoundsNobel prizeGeoffrey WilkinsonErnst Otto Fischer 5

Sandwich complexes They usually follow the 18 e rule 1 st row TMs: electron counts from 15 to 20 are known because of the favorable metallocene structure 18 e rule does not apply to lanthanides and actinides 6

Synthesis of metallocenes 7

Cyclopentadienyl ligands

Cyclopentadienyl (Cp) complexes: orbitals in Cp - Bonding is covalent, there is very little charge separation.

Cp: orbital combinations for two ligands

Metallocenes, MCp 2 HOMO of FeCp 2, d z 2 (nonbonding) Most important interaction: e 1g C-C bond in Cp is 1.44 Ǻ (C 6 H 6, 1.35 Ǻ); H atoms of Cp are bent downwards.

Reactivity Electrophilic aromatic substitution: faster than for benzene Reactions of paramagnetic metallocenes

Other metallocenes Bent metallocenes Ansa-metallocenes Cp* = C 5 Me 5 Higher field, more bulky, more-electron donating than Cp Indenyl complexes Better  -acceptor than Cp

Ligands similar to Cp Tris(pyrazolyl)borate (Tp) Sometimes these ligands show interesting reactivity because one arm can open and allow substrate binding.

Uranocene: Streitwieser, 1968 U-C: 2.647(10); C-C: 1.392(13) Å K. N. Raymond et al. Inorg. Chem. 1972, 11, 1083

COT 2- vs. benzene Benzene LUMO COT 2- HOMO

 bonding in uranocene UV-PES studies show that bonding in uranocene has 5f and 6d contributions. e 2u interaction shown can only occur via f orbitals. C. Elschenbroich; A. Salzer Organometallics, 2 nd ed., VCH, Weinheim, 1992, p. 363

Reactions of organometallic complexes Textbook H: Chapter 5.1 – 5.5 Textbook A: Chapter 5

Classification of organometallic reactions Substitution: addition or removal of a ligand Electron transfer: change in the oxidation state of the metal Activation of ligands: attack at a ligand Combination of processes  Oxidative addition / reductive elimination  Migratory-insertion reactions  Redox-catalyzed substitution  Redox-catalyzed insertion reactions 19

Transition state vs intermediate 20