Calcium, strontium and barium

   The heavier group 2 metals are highly electropositive, and metal–ligand bonding is generally considered to be predominantly ionic. Nonetheless, this remains a topic for debate and theoretical investigation. While Cp₂Be and Cp₂Mg are monomeric and are soluble in hydrocarbon solvents, Cp₂Ca, Cp₂Sr and Cp₂Ba are polymeric and are insoluble in ethers and hydrocarbons. Increasing the steric demands of the substituents on the C₅-rings leads to structural changes in the solid state and to changes in solution properties, e.g. (C₅Me₅)₂Ba is polymeric, {1,2,4- (SiMe₃)₃C₅H₂}₂Ba is dimeric and (ⁱPr₅C₅)₂Ba is monomeric. Oligomeric metallocene derivatives of Ca²⁺‏, Sr²⁺‏ and Ba^2+‏ typically exhibit bent C₅^_M^_C₅ units, but in (ⁱPr₅C₅)₂Ba, the C₅-rings are coparallel. The iPr₅C₅-rings are very bulky, and sandwich the Ba^2‏+ ion protectively, making (ⁱPr₅C₅)₂Ba airstable.

Fig. 1.1 Part of a chain in the polymeric structure (X-ray diffraction 118 K) of (η⁵-C₅Me₅)₂Ba illustrating the bent metallocene units [R.A. Williams et al. (1988) J. Chem. Soc., Chem Commun., p. 1045]. Hydrogen atoms have been omitted; colour code: Ba, orange; C, grey.

   The 1990s saw significant development of the organometallic chemistry of the heavier group 2 metals, with one driving force being the search for precursors for use in chemical vapour deposition. Some representative synthetic methodologies are given in equations 1.1–1.4, where M= Ca, Sr or Ba.

                                    (1.1)

                                 (1.2)

             (1.3)

       (1.4)