Reactions of Ln(BH4)(3)(THF)(3) (Ln = Nd, Ce) and M(2)dddt (M = Na, K; dddt = 5,6-dihydro-1,4-dithiine-2,3-dithiolate) in THF or pyridine gave, after addition of 18c6 (18-crown-6), several crystalline compounds which all contain the tris(dithiolene) Ln(dddt)(3) unit. Crystals of [Na(18c6)(py)(2)](2)[Na(18c6)(py)][Nd(dddt)(3)(py)].3py (1.3py) are built up from discrete mononuclear cationic and anionic species whereas crystals of {[Na(18c6)(py)(2)](0.5)[Na(18c6)(py)(1.5)][Na1.5Nd(dddt)(3)]}(infinity) (2) are composed of discrete [Na(18c6)(py)(x)](+) cations and polymeric anionic two-dimensional layers in which the Nd(dddt)(3) units are linked to three neighbors by sodium atoms to form a honeycomb network. Analysis of the temperature dependence of the molar magnetic susceptibility of 2 shows that chi(M)Tdecreases from 1.63 cm(3) K mol(-1) at 300 K down to 0.6 cm(3) K mol(-1) at 5 K, due to the crystal-field splitting of the I-4(9/2) free-ion state. Complexes {[Na-3(18c6)(1.5)Nd(dddt)(3)(THF)].3THF}(infinity) (3.3THF) and {[K-3(18c6)(1.5)Nd(dddt)(3)(py)].3py}(infinity) (4-3py) exhibit neutral polymeric layers with the Nd(dddt)3 units linked by M-2(18c6) fragments. In the cerium compound {[Na-2(18c6)Na(py)(2)Ce(dddt)(3)(PY)].3py}(infinity) (5-3py), each Ce(dddt)(3) unit is linked to two neighbors only by Na-2(18c6) moieties, giving infinite zigzag chains.