Abstract From the reactions between 1,4-dilithiobutane and 1,2-M2Cl2(NMe2)4 compounds in hydrocarbon solvents, the 1,2-dimetallacyclo-1,2-hexyne compounds M2(CH2)4(NMe2)4(MM), where M = Mo and W, have been obtained as yellow—orange, volatile microcrystalline compounds. Spectroscopic data are consistent with expectations based on previous studies of gauche 1,2-M2R2(NMe2)4 compounds in solution and a structural model based on the observed molecular structure of 1,2-Mo2Et2(NMe2)4 is presented and compared with the observed molecular structure of Mo2(CH2)4(NMe2)4 found in the solid state. Pertinent bond distances (A) and angles (°) are: Mo—Mo = 2.200(1), Mo—N = 1.96(1) (averaged), Mo—C = 2.165(8) (averaged), Mo—Mo—N = 105(1), Mo—Mo—C = 97(1).