Chemistry of molybdenum and tungsten bis[hydrazido(2-1)] complexes. Preparations and x-ray crystal structures of [Mo(NNMePh)2(S2CNMe2)2], [Mo(NNPh2)2(S2CNMe2)2]Me2CO, [MoCl(NNMe2)2(PPh3)2]BPh4.CH2Cl2, and [Mo(NNMe2)2(bpy)2][BPh4]2.CH2Cl2