Transition metal sandwiches

For transition metal sandwich complexes the possibility of a non-degenerate ground level, in C v symmetry, arises for several dx configurations, namely... 

In conclusion though, it may reasonably be maintained that the ligand field model has contributed significantly to the advances in knowledge concerning transition metal sandwich complexes over the last ten years, and that it offers a valid and convenient framework for further experimental and theoretical progress. 

A tricyclic aromatic system closely related to borole, 9-boratafluorene, might be expected to form ( -coordinated transition metal sandwich complexes, but so far only the lithium complex has been characterized.19 Monohapto aluminum adducts of neutral 9-borafluorene, in which A1 is bound only to the boron atom, have been prepared.20... 

Metallocenes have a transition metal sandwiched between two cyclopentadienyl rings. The cyclopentadienyl rings may not necessarily be parallel because of bridging. They may be further substituted to restrict access to the metal. The catalyst structure can be changed to make different polymeric properties. A typical example would be the zirconium metallocene shown below. 

Astruc has developed the concept of transition metal sandwiches acting as electron reservoir complexes. The characteristic of an electron reservoir is that the reduced form is easily generated and does not decompose to increase stability, the radical center can be sterically protected in the heart of a bulky molecular framework. The [FeCp(arene)] series of complexes, e.g. 7, are prime examples, for which variation of the arene structure modulates the redox potential. 

A very interesting series of dendrimers containing 24 transition metal sandwich units has been synthesized from the 24-nitrile dendrimer by reduction of the nitrile groups to primary amines followed by reaction of the 24-amine dendrimer with chlorocarbonylferrocene or with [Fe( 5-C5Me5)( 6-C6H5F)][PF6] (Scheme 15) [54]. Both 24-branch metallodendrimers proved very useful and complementary for molecular recognition, as will be discussed later in this chapter. 

Recent advances in the techniques of photoelectron spectroscopy (7) are making it possible to observe ionization from incompletely filled shells of valence elctrons, such as the 3d shell in compounds of first-transition-series elements (2—4) and the 4/ shell in lanthanides (5, 6). It is certain that the study of such ionisations will give much information of interest to chemists. Unfortunately, however, the interpretation of spectra from open-shell molecules is more difficult than for closed-shell species, since, even in the simple one-electron approach to photoelectron spectra, each orbital shell may give rise to several states on ionisation (7). This phenomenon has been particularly studied in the ionisation of core electrons, where for example a molecule (or complex ion in the solid state) with initial spin Si can generate two distinct states, with spin S2=Si — or Si + on ionisation from a non-degenerate core level (8). The analogous effect in valence-shell ionisation was seen by Wertheim et al. in the 4/ band of lanthanide tri-fluorides, LnF3 (9). More recent spectra of lanthanide elements and compounds (6, 9), show a partial resolution of different orbital states, in addition to spin-multiplicity effects. Different orbital states have also been resolved in gas-phase photoelectron spectra of transition-metal sandwich compounds, such as bis-(rr-cyclo-pentadienyl) complexes (3, 4). 

The compound (C5Et5)2Co (CP2C0) (38) was one of the earhest examples of transition metal sandwich compounds to... 

Almlof, J., K. Faegri, Jr., B. E. R. Schilling, and H. P. Luthi (1984). An investigation of correlation effects in transition-metal sandwich complexes. Har-tree-Fock studies on a series of metallocenes. Chem. Phys. Lett. 106, 266-70. 

A notable example is the wheel-shaped cationic cluster [Er36([X3-OH)3o([i3-0)6(BDC)6] " " (Figure 6.41) present in the lanthanide-transition metal sandwich framework comprising (Cus) cluster pillars and layered networks of (Erse) wheels [25, 77]. The Er + ions are linked by hydroxo and oxo bridges to give two types of smaller cluster cores cubic [Er4([i3-0)([i3-0H)3] + (Ere) and dimeric [Er2([i3-OH)2]" + (Er2) cores (Figure 6.41a). Different from the familiar cubane [Ln4([X3-OH)4] + which contains four [X3-OH groups, the present (Er4) cluster... 

Table 1. 1/2 values detennined for the transition-metal sandwich complexes (V relative to the SCE). 

The redox potential of sandwich complexes can be finely tuned by modification of the number of ring substituents. (Only approximate values are given for the redox potentials). For a review of redox potential values, see [21] for values of organometallic complexes, see [22] for NAr3, pP values range from 0.52 V to 1.72 V vs. SCE depending on the nature of Ar, see [23] for transition-metal sandwich compounds, see [24]. 

Fig. 2. Examples of paramagnetic polyphosphametallocene transition metal sandwich com pounds [37,41-43]...

In addition a full ligand field treatment of all the d configurations, and the d-d spectra to be anticipated therein, assuming effectively pseudoaxial symmetry, was given by Warren - ), who also gave a complete account of their magnetic proper-ties ), and a detailed survey of the results of the ligand field approach for transition metal sandwich systems ) (quod vide for further details). 

MP systems, the ligand levels lie well below those of the metal so that the two components (t and eg) of the d-orbitals in Of, symmetry will generally be juxtaposed. Moreover, it must always be borne in mind that the actual energies of the virtual SCF levels have no real physical significance, and will in any case be appreciably lowered upon occupation, whilst to complicate matters still further it is now realised that all transition metal sandwich systems are prone to show wide deviations from the predictions of Koopmans theorem. 

To date, a great number of experimental and theoretical investigations have been carried out to elucidate the structures and chemical reactivities of transition metal sandwich complexes (11). The parallel interest in investigations of actinide sandwich complexes has revived again recently (4,12). We have found relativistic density functional theory (DFT) to be a superb tool for the elucidation of the... 

Table 2. Ionization energy data for transition metal sandwich compounds... 

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