Insertion Reactions of Carbon Disulfide into Mg-C and Mg-N Bonds: Syntheses and Structural Determinations of Mg(S2CY)2(THF)(subscript n) (Y=NEt2, N(superscript i)Pr2, (superscript i)Pr, Ph) and BrMg(S2CZ)(THF)3 (Z=Ph, (superscript i)Pr)