Synthesis and Chemistry of the Bis(trimethylsilyl)amido Bis- tetrahydrofuranates of the Group 2 Metals Magnesium, Calcium, Strontium and Barium. X-Ray Crystal Structures of Mg(N(SiMe3)2)2× 2 THF and Related Mn(N(SiMe3)2)2×2 THF.