Metallocenes electronic structures

Electronic Effects in Metallocenes and Certain Related Systems, 10, 79 Electronic Structure of Alkali Metal Adducts of Aromatic Hydrocarbons, 2, 115 Fast Exchange Reactions of Group I, II, and III Organometallic Compounds, 8,167 Fluorocarbon Derivatives of Metals, 1, 143 Heterocyclic Organoboranes, 2, 257... 

Table 4.52. A synopsis of localized electronic structures for simple metallocenes in terms of MLX formulation, spin-state multiplicity, nonbonding d electrons (d count) and orbitals (nd), ordinary (2c/2e) and tu (3c/4e) bonds, and nominal sd" hybridization...

All chemical transformations relevant to metal/olefin reactions occur at the three orbitals in the plane between the two Cp rings or between the Cp and the N atom in the case of the CGC catalysts. This plane is usually referred to as the wedge or belt of the catalyst. The first detailed analysis of the electronic structure of group 4 metallocenes, and the implications on their chemistry, was performed by Lauher and Hoffmann with simple... 

A rather similar situation regarding the breakdown of Koopmans theorem exists for the metallocenes. The electronic structures of these interesting compounds have attracted the attention of both theoreticians and experimentalists for several years. Ferrocene is the metallocene that has been studied the most extensively. From the UPS standpoint this stems from the diamagnetism of the neutral molecule and the stability of the ferricenium cation. 

Compare the electronic structure of group 4 and group 3 metallocene catalysts for olefin polymerisation. 

In contrast to heterogeneous Ziegler-Natta catalysts, homogeneous catalysts based on biscyclopentadienyl derivatives of group 4 transition metals, which contain cationic metallocene species of formally d° 14-electronic structure, hardly promote the polymerisation of conjugated dienes, since the diene can act as a donor of four electrons rather than of two electrons as in monoolefin polymerisation (let us recall that the polymerisation of conjugated dienes is catalysed by half-sandwich metallocene-based catalysts). However, it has been reported [162] that statistical copolymers of ethylene and butadiene were obtained with the Cp2ZrCl2— [Al(Me)0]x catalyst. 

Table 1 Electronic structure and ground-state as well as Lande g values and reduction potentials (vs. SCE) of metallocenes and decamethylmetallocenes as well as their cations, [MCp2] + ...

In addition, the electronic structure, mainly determined by the central metal and Lewis-basic or Lewis-acidic substituents at the ligands, is a considerable aspect for the polymerization activity of metallocene and half-sandwich complexes." Complexes with a sterically accessible oxygen or a nitrogen atom (Lewis bases) coordinated to the central metal are not useful for good catalytic performance. Generally, hafnium metallocene and half-sandwich complexes exhibit lower polymerization activities due to a more stable hafnium—carbon bond," which lowers the insertion rate of the coordinated monomer into the growing polymer chain. 

For the closely related pentadienyl compounds, or open metallocenes, I and II, PE spectroscopy has shown interesting differences in electronic structure from the closed analogs. 

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