Alkaline-earth metallocenes

The alkaline-earth metallocenes exhibit various structures, depending on the sizes of the metal atoms. In Cp2Be, the Be2+ ion is if /if coordinated between two Cp rings, as shown in Fig. 12.7.3(a). The mean Be-C distances are 193 and 183 pm for rj5 and rj1 Cp rings, respectively. In contrast, Cp2Mg adopts a typical sandwich structure, as shown in Fig. 12.7.3(b). The mean Mg-C distance is 230 pm. In the crystalline state, the two parallel rings have a staggered conformation. 

The compounds Cp2Ca, Cp Sr and Cp Ba exhibit a different coordination mode, as the two Cp rings are not aligned in a parallel fashion like Cp2Mg, but 

Structures of some alkaline-earth metallocenes (a) Cp2Be, (b) Cp2Mg, (c) CP2Ca, (d) Sr[C5H3(SiMe3)2]2-THF, 

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Metallocenes with substituted cyclopentadienyl rings. Metallocenes with methylated rings were among the first heavy alkaline earth metallocenes to be structurally characterized, but many other substituents have been incorporated into bis(cyclopentadienyl) complexes. Under this classification are included compounds with indenyl ligands, which in... 

Applications involving ring transfer or loss. The kinetic lability, volatility, and Lewis acidity of heavy alkaline earth metallocenes have been the properties most important to their applications. The gas-phase decomposition of volatile metallocenes is useful in the preparation of thin films of alkaline earth-containing materials and in doping semiconductors. Reviews are available on the use of group 2 organometallic compounds as precursors for chemical-vapour deposition (CVD).2 3... 

D. Stalke, The lithocene anion and open calcocene — new impulses in the chemistry of alkali and alkaline earth metallocenes. Angew. Chem. Int. Ed. 33, 2168-71(1994). 

Group 2 metallocenes have the general formula M(Cp)2L , where Cp is a derivatized or underivatized cyclopentadienyl ring (see Table II) and L is a two- or four-electron Lewis base donor such as THF, PR3, Et20, or DME (n = 0-2). Several methods have been used to prepare alkaline-earth metallocene complexes. 

There have also been a few reports of the synthesis of alkaline-earth metallocenes by other methods. For example, Ca(C5Me5),(THF)2, Ca( 1,3-f-Bu2C5H3)2(THF), Ca((SiMe3)C5H4)2(THF), and Ca(l,3-(SiMe3)2C5H3)2... 

In ab initio calculations, there are many different possible choices of theoretical levels and basis sets. Since HF calculations fail to reproduce the bent structure and C(Cp)-E bond distances in alkaline earth and Group 14 metallocenes (30,39,40), it is probably safe to assume that electron correlation must be included in geometry optimizations. Thus, electron correlation is required to reproduce the donor-acceptor nature of the main group-Cp interaction as well as the structural consequence of the main group lone pairs. For instance, previous DFT calculations (which include electron correlation) predicted reasonable C(Cp)-E distances and the bent structures for heavier alkaline-earth metallocenes... 

Table 13.10 shows the result of the energy partitioning analysis of the alkaline-earth metallocenes with symmetry. For all molecules the major contribution to... 

Cyclopentadienide derivatives appear both as simple complexes, reminiscent of ferrocene, or as heteroleptic systems. Because of the primarily electrostatic bonding models attributed to alkaline earth organometallics, the range of geometries available even for simple homoleptic alkaline earth metallocenes Cp2M (M = Be-Ba) is surprising. 

Alkaline earth metallocenes have been featured in a Highlight article. ... 

Figure 5.9 Synthesis of NHC-alkaline earth metallocene complexes 55-57 reported by Arduengo.

Li H, Zhang WX, Xi Z (2013) Alkaline-earth metallocenes coordinated with ester pendants synthesis, structural characterization, and application in metathesis reaction. Chem Eur J 19 12859-12866... 

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