A series of quasi-two-dimensional molecular conductors with formula [TMTSF]2[M(tds)2] (TMTSF = tetra- methyltetraselenafulvalene; tds = [bis(tnfluoromethyl)ethylene]diselenolato; M = Ni, Pt, Cu) are described. All haveσRT= 20–100 Ω-1cm-1with metal-like character near room temperature. When M = Ni or Pt, there is a novel first-order structural transition (at Tc = 275 and 245 K, respectively) in which the low-temperature state exhibits enhanced conductivity; no corresponding transition occurs when M = Cu. At room temperature the three compounds are isostructural. The structure of M = Ni at 293 K consists of segregated stacks (along c) of TMTSF cations that are flanked by parallel chains of M(tds)2 anions so as to form a two-dimensional anion-cation network parallel to the (010) plane. Crystal data: triclinic, space group Ci1-P1; a = 11.503 (18), b = 12.465 (20), c = 7.885 (13) Å;α = 96.39 (4), β = 109.57 (4), γ= 77.92 (4)°; V = 1040.7 Å3; Z = 1. ESR and magnetic susceptibility measurements are consistent with the formulation [(TMTSF)0.5+]2[[M(tds)2]-], where the anions have an open-shell, paramagnetic (S = 1/2) configuration when M = Ni or Pt but are closed-shell and diamagnetic when M = Cu. Thus, the existence of a transition correlates with the electronic structure of the anion. The conductivity arises from a three-fourths-filled band associated with the TMTSF stacks; for M = Ni or Pt, the carrier spins are exchange-coupled to the localized anion moments. Interestingly, the structure of the low-temperature phase of M = Ni, determined at 116 K, is disordered; it is derived from the room-temperature structure by uncorrelated translations of the individual stacks and chains by ±1/8c along c. Crystal data: triclinic, space group Ci1-P1; a = 11.346 (21), b = 12.171 (34), c = 7.719 (11) Å; α = 95.30 (34), β= 108.75 (13), γ = 79.90 (25)°; V = 993.0 A3; Z = 1. The altered disposition of the stacks and chains in the disordered low-temperature structure enhances the intermolecular contacts within the two-dimensional anion-cation network.