The syntheses, crystallography and magnetic properties of a series of compounds of formula trans-[Fe(II)(L(1))2(NCX)2] (X = S, Se, BH3 ()), cis-[Fe(II)(L(2))(NCX)2]*CH2Cl2 (X = S, Se, BH3 ()) and trans-[Fe(II)(L(3))(NCX)2]n (X = S, Se ()) are described (L(1) = 6-chloro-N(2),N(2)-diethyl-N(4),N(4)-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine, L(2) = 6,6'-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(N(2),N(2)-diethyl-N(4),N(4)-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine, L(3) = 6,6'-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(N(2),N(2),N(4),N(4)-tetra(pyridin-2-yl)-1,3,5-triazine-2,4-diamine)). The magnetostructural properties of have been probed in detail by variable temperature magnetic measurements and crystallographic methods. display mononuclear structures while and form 1-D chain structures. Complexes have the potential to form 1D-chains via L(2) bridging, but instead form mononuclear complexes. Magnetic studies show that complexes , , and remain in the high-spin (HS) state at all temperatures. An aged, dry, powdered sample of gives an abrupt HS to LS transition (T1/2 = 200 K), while a freshly prepared, powdered sample of *1.5H2O displays thermal hysteresis (Δ = 7 K). Complexes , and undergo a gradual spin transition with T1/2 values of 100 K, 150 K and 130 K, respectively. Cooperativity parameters are compared, with showing cooperativity (positive C) and and showing anticooperativity. Photomagnetic LIESST (light induced excited spin state trapping) studies were performed on complexes and and reveal T(LIESST) values lower than 60 K. An attempt has been made to understand the electronic structure of complex and its cooperativity behaviour using density functional methods, the calculations reproducing the sign and, in part, the magnitude of the cooperativity.< Réduire