The Synthesis And Structure Of The Alkaline-Earth Metal Organic-Compounds [M(Oar)2(Thf)N] [M = Ca, N = 3 (1) Or 0 - M = Ba, N = 4] And [(Ca(Nr2)(Mu-Nr2)(Thf))2], And The X-Ray Structure Of (1) (Ar = C6H2But2-2,6-Me-4 - R = Sime3 - Thf = Oc4H8)