Preparation and properties of dinitrogen trimethylphosphine complexes of molybdenum and tungsten. 4. Synthesis, chemical properties, and x-ray structure of cis-[Mo(N2)2(PMe3)4]. The crystal and molecular structures of trans-[Mo(C2H4)2(PMe3)4] and trans,mer-[Mo(C2H4)2(CO)(PMe3)3]