The reaction between M(2)(NMe(2))(6) and Me(3)SiI (1 equiv) in toluene at 22 degrees C leads to an equilibrium mixture of M(2)(NMe(2))(6), M(2)(NMe(2))(5)I, and M(2)(NMe(2))(4)I-2 (ca. 1:5:1), where M = Mo and W. The reactions between M(2)(NMe(2))(6) and Me(2)SiI (2 equiv) in toluene yield the compound M(2)(NMe(2))(4)I-2 in ca. 85% isolated yield. The monoiodo compounds are also formed by the reactions between M(2)(NMe(2))(6) and M(2)(NMe(2))(4)I-2 in toluene. In solution the M(2)(NMe(2))(4)I-2 compounds exist as a mixture of gauche and anti isomers in a ca. 1:2 ratio. We have characterized by single-crystal X-ray crystallography (i) anti-W-2(NMe(2))(4)I-2, (ii) a crystalline sample containing a solid solution of Mo-2(NMe(2))(5)I (90%) and anti- and gauche-Mo-2(NMe(2))(4)I-2; and (iii) W-2(NMe(2))(5)I which contains a 5% disorder. Crystal data : for W-2(NMe(2))(4)I-2 at -179 degrees C, a = 7.838(3) Angstrom, b = 7.706(2) Angstrom, c = 15.010(5) Angstrom, beta = 103.48(1)degrees, Z = 2, d(calcd) = 3.05 g cm(-3), and space group P2(1)/n; for Mo-2(NMe(2))(5)I at -178 degrees C, a = 14.251(2) Angstrom, b = 18.031(2) Angstrom, c = 7.477(1) Angstrom, Z = 4, d(calcd) = 1.86 g cm(-3), and space group Pmnb; for W-2(NMe(2))(5)I at -171 degrees C, a = 14.220(2) Angstrom, b = 18.179(2) Angstrom, c = 7.449(1) Angstrom, Z = 4, d(calcd) = 2.47 g cm(-3), and space group Pmnb. In each of the above there is a staggered ethane-like M(2)N(5)I or M(2)N(4)I(2) core with W=W distances = 2.29(1) Angstrom, and W-I = 2.69(1) Angstrom (average) and Mo=Mo distances = 2.21(1) Angstrom and Mo-I = 2.70(1) Angstrom, with M-N in the range 1.92-1.97 Angstrom. The substitution behavior of the M(2)(NMe(2))(5)I compounds has been investigated by metathetic reactions involving ClMgCH(2)Ph, LiCH(2)SiMe(3), LiMe, KCH2C6H4-3-Me, LiPPh(2), (LiC5H4)(2)Fe, vinyl lithium, LiC=CR, where R = Bu(t) and Ph, SnMe(4), Sn(vinyl)(4), and AlMe(3). The monosubstituted compounds M(2)(NMe(2))(5)R were isolated and characterized for R = Me, CH(2)Ph, m-CH(2)C(6)H(4)Me, and PPh(2). Substitution of the M-I bond in reactions involving organolithium compounds requires the addition of Me(2)NCH(2)CH(2)NMe(2) (TMEDA). Reactions employing alkynyl- and vinyllithium reagents gave intractable products.