Metallocenes: Structural Features and Properties

Cyclopentadienyl ligands Cps CH3 H3C 6e- strong donor 6e- stronger donor bulky ligand CH3 H3C CH3 Cp Cp* H M M M 1 5 3

Free neutral cyclopentadiene, which is deprotonated with a strong base to generate the Cp , is unstable and reacts with itself via a Diels-Alder reaction to make the dicyclopentadiene. One typically regenerates cyclopentadiene by distilling ( cracking ) it from the high boiling dimer solution and storing it in a refrigerator, but it slowly re-dimerizes to make dicyclopentadiene. +

Brief History of Ferrocene: 1901 Synthesis of KC5H5 from K and C5H6 Miller, Tebboth & Tremaine Sythesis of Fe(C5H5)2 from the reaction of C5H6 with freshly reduced Fe at 300 C 1951 Kealy & Pauson 3C5H5MgBr + FeCl3 Cp2Fe + fulvalene + 3MgBrCl They were trying to make fulvalene! They proposed that they had made: 1951 Fe E. O. Fischer proposes a Double-cone structure X-ray structural data Diamagnetism Chemical behavior 1952 Geoffrey Wilkinson & Robert Woodward: Sandwich Structure IR spectroscopy Diamagnetism Dipole moment = 0 1952 Woodward noted that the Cp rings were susceptible towards electrophillic substitutions, similar to the aromatic behavior of benzene. Thus the common name: ferrocene Fischer & Wilkinson receive the Nobel Prize in Chemistry for their discovery of ferrocene, which played a key role in opening up the new area of organometallic chemistry. 1973

Some Properties of Metallocenes Complex Color mp/ C 200 (decomp.) Miscellaneous bimetallic with two -H bridges and a fulvalene bridging ligand (structure shown later) Ti(C5H5)2 green purple 167 very air-sensitive, paramagnetic bimetallic with 1, 5-C5H4 bridges and terminal hydrides (structure shown later). very air-sensitive several bimetallic isomers with fulvalene and 1, 5 bridges and terminal hydrides (structures shown later), diamagnetic, air- sensitive. same as Mo air-sensitive and easily hydrolyzed, interesting high-spin to low-spin interconversion air-stable, can be oxidized to blue-green [Fe(C5H5)2]+which, in turn, is a good inert oxidizing agent. V(C5H5)2 Nb(C5H5)2 yellow - scarlet 173 Cr(C5H5)2 Mo(C5H5)2 Black - W(C5H5)2 yellow-green - brown 173 Mn(C5H5)2 orange 173 Fe(C5H5)2 air-sensitive, paramagnetic 19e- complex, can be oxidized to the air-stable 18e- yellow [Co(C5H5)2]+ 20e- complex, slow oxidation in air to the labile, orange cation [Ni(C5H5)2]+ purple-black 174 Co(C5H5)2 green 173 Ni(C5H5)2

Structural Features M M-C C-C Cp Cp Fe [Fe]+ 2.04 3.29 1.42 2.07 3.40 1.40 M-C Cp Cp M Ru 2.19 3.64 1.43 Os 2.19 3.61 1.45 Co [Co]+ 2.10 3.44 1.41 C-C 2.03 3.24 1.42 Ni 2.18 3.63 1.41 The changes in the neutral Fe, Co, Ni metallocenes are a direct result of going from 18e- (Fe) to 19e- (Co) to 20e- (Ni) counts. The extra electrons for the Co and Ni complexes are going into M-Cp antibonding orbitals, which are delocalized and progressively weaken the M-Cp bonding, leading to the increase in bond distances. This in spite of the fact that the metal s covalent radius is decreasing as one goes from Fe to Ni. Problem: Explain why the Fe-C distance lengthens for [Cp2Fe]+, while the Co-C distance shortens for [Cp2Co]+.

Oxidation of Cp2Os does not produce a simple cationic monomer as seen for Co and Fe. Instead one gets dimerization to produce the following bimetallic complex that has an Os-Os bond (3.04 ). Problem: Electron-count this complex. Is it para- or diamagnetic?

Bis-Cp Early TM Complexes The simple neutral bis-Cp complexes of the early transition metals are quite different because they are in very low +2 oxidation states (very electron-rich) and quite unsaturated. Thus, they are very reactive towards C-H oxidative additions and other reactions. Me Me Me Me Me CH2 Me Me Me Me H Ti Ti Ti Ti H Me Me H Me Me Me Me Me Me Me Me Problem: Electron-count this Ti2 complex. Is it para- or diamagnetic? H Nb Nb H

Mo-Mo = 3.19 Mo Mo THF H 50 C H Mo Mo H2O 100 C H h H Mo-Mo = 3.36 toluene 70 C Mo Mo H

Cp Variants these have special bonding properties important in substitution reactions (see that chapter) 2- indenyl (-) fluorenyl (-) fulvalenediyl (2-) 6e- donor Mo 2.45 azulene Behrens, Angew. Chem., 1987

MO Comparison of Cp vs. Arene Ligands Benzene-Metal Complex Cyclopentadienyl-Metal Complex metal d orbitals metal d orbitals M M