Complexes of group 2 metals with amido or alkoxy ligands

 The number of complexes of the group 2 metals with N- or O-donor ligands continues to grow, notably those incorporating sterically demanding amido or alkoxy ligands.

  With the bulky bis(trimethylsilyl)amido ligand, each of the M²⁺‏ ions forms at least one type of complex. In the gas phase, monomeric [Be{N(SiMe₃)₂}₂] contains a linear N^_Be^_N unit; in the solid state structure of [Mg{N(SiMePh₂)₂}₂], ∟N^_Mg^_N =162.8⁰, the deviation from linearity being attributed to weak dipolar interactions between the electropositive metal centre and the electron density of the aromatic rings. Coordination numbers of 3 and 4 for Mg(II), Ca(II), Sr(II) and Ba(II) are seen in dimers [M{N(SiMe₃)₂}₂]₂ or solvated monomers, e.g. tetrahedral [Ba{N(SiMe₃)₂}₂(THF)₂]. The structure of [Ca{N(SiMe₃)₂}₂]₂ is shown in Figure 1.1a, and similar structures have been confirmed crystallographically for the analogous Mg, Sr and Ba compounds as well as for [Mg{N(CH₂Ph)₂}₂]₂.

   While alkoxy derivatives of the alkaline earth metals have been known for many years, the area has undergone significant expansion since 1990. Much of this interest stems from the fact that calcium, strontium and barium alkoxides are potential precursors for high-temperature superconductors and volatile compounds suitable for chemical vapour deposition (CVD) studies are being sought.

   Mononuclear complexes include several of the type [M(OR)₂(THF)₃], e.g. [Ca(OC₆H₂-2,6-^tBu₂-4-Me)₂(THF)₃]. Some interesting high nuclearity species have also been isolated, including [Ba₄(µ4-O)( µ -OC₆H₂(CH₂NMe₂₎₃-2,4,6)₆], formed by treating BaI₂ with K[OC₆H₂(CH₂NMe2)₃-2,4,6] in THF, and [Ca₉(OCH₂CH₂OMe)₁₈(HOCH₂CH₂OMe)₂] (Figure 1.1b), produced by reacting Ca metal with 2-methoxyethanol in hexane.