The preparation and characterisation of [FeIII(sal2-trien)][MnIICrIII(ox)3]·CH2Cl2 and [FeIII(sal2-trien)][MnIICrIII(ox)3]·0.75CH3OH·0.5H2O is presented in Chapter 2. The use of [FeIII(sal2-trien)]+ leads, depending on the synthetic conditions, to two different structures. Compound [FeIII(sal2-trien)][MnIICrIII(ox)3]·CH2Cl2 presents a 2D network and shows coexistence of a complete spin-crossover and a ferromagnetic ordering from the bimetallic oxalate network around 5 K. The second compound, [FeIII(sal2-trien)] [MnIICrIII(ox)3]·0.75CH3OH·0.5H2O, with a 3D achiral network, exhibits a partial spincrossover besides the ferromagnetic ordering. The formation of two different oxalate networks with the same templating cation is a very unusual fact for this type of compounds that we attribute to the flexibility of the organic ligands of [FeIII(sal2-trien)]+ complex in the presence of different solvents. The photomagnetic properties of [FeIII(sal2-trien)] [MnIICrIII(ox)3]·CH2Cl2 are studied in Chapter 3. This compound shows LIESST effect below 42 K. This is a very unusual result as this effect is rarely observed in FeIII complexes. In Chapter 4, the preparation of several analogs of [FeIII(sal2-trien)] [MnIICrIII(ox)3]·CH2Cl2 in other halogenated solvents (CHCl3, CH2Br2 y CHBr3) is described. The same 2D structure is maintained in the compounds of formula [FeIII(sal2-trien)] [MnIICrIII(ox)3]·CH2Br2, [FeIII(sal2-trien)][MnIICrIII(ox)3]·CHBr3 and [FeIII(sal2-trien)] [MnIICrIII(ox)3]·CHCl3. The change of CH2Cl2 by bulkier solvents allows tuning T1/2, which is shifted toward lower temperatures, and TLIESST, which is shifted towards higher values. The FeIII complexes of these compounds have been diluted with similar diamagnetic GaIII complexes. This has allowed to study the spin-crossover of the isolated FeIII complexes by spectroscopic methods in [Ga0.99Fe0.01(sal2-trien)][MnIICrIII(ox)3]·CH2Cl2. This compound shows a thermal spin-crossover and LIESST effect as the FeIII pure compound. Finally, a structural analysis of these compounds and other FeIII ones described in the literature showing LIESST effect has been performed. In Chapter 5, 6 and 7 the search of other oxalate-based magnetic hybrids with derivatives of [FeIII(sal2-trien)]+ complex has been continued. [FeIII(5-NO2-sal2trien)][MnIICrIII(ox)3]·MeNO2·0.5H2O and [FeIII(5-CH3O-sal2trien)][MnIICrIII(ox)3] with 2D and 3D achiral structures, respectively, are presented in Chapter 5. The two compounds show a magnetic ordering around 5 K but they do not present spin-crossover. The syntheses, structure and magnetic properties of [FeIII(5-Cl-sal2trien)][MnIICrIII(ox)3]·0.5MeNO2, [FeIII(5-Br-sal2trien)][MnIICrIII(ox)3] and [InIII(5-Cl-sal2trien)][MnIICrIII(ox)3] are shown in Chapter 6. They present a new chiral 3D structure that affords a new property to the multifunctional system, chirality, besides magnetic ordering and spin-crossover in the FeIII compounds. Finally, derivatives of [FeIII(sal2-trien)]+ with substituent in 3 or 4 positions lead to the compounds of formula [FeIII(4-Br-sal2trien)][MnIICrIII(ox)3]0.67·Cl0.33 ·MeOH, [FeIII(3-Cl-sal2trien)][MnIICrIII(ox)3]·(CH3OH)2·(CH3CN)2, [FeIII(3-Br-sal2trien)] [MnIICrIII(ox)3]·(CH3CN)2, and [FeIII(3-MeO-sal2trien)][MnIICrIII(ox)3]·(CH3OH)·(H2O)1.5 ·(CH2Cl2)0.5 with 2D structures. The magnetic ordering around 5 K of all of them coexists with a complete spin-crossover from 300 to 400 K for [FeIII(4-Br-sal2trien)] [MnIICrIII(ox)3]0.67·Cl0.33·MeOH and partial spin-crossover for [FeIII(3-Cl-sal2trien)] [MnIICrIII(ox)3]·(CH3OH)2·(CH3CN)2 and [FeIII(3-Br-sal2trien)][MnIICrIII(ox)3]·(CH3CN)2. FeIII complexes of [FeIII(3-MeO-sal2trien)][MnIICrIII(ox)3]·(CH3OH)·(H2O)1.5·(CH2Cl2)0.5 are in the High-Spin state in all the range of temperature studied. The insertion of [MnIII(salen)]+ complexes and derivatives into oxalate-based bimetallic networks has been studied in Chapters 8 and 9. These complexes give rise to a great variety of structures. The insertion of derivatives of [MnIII(salen)]+ leads to the stabilization of MnIII monomers coordinated to the tetradentate Schiff base and two solvent molecules as shown in Chapter 8. Some of these compounds of formula [Mn((R)- salmen)(CH3OH)2][MnCr(ox)3](CH2Cl2)0.375·(CH3OH)0.125·(H2O)0.375 and [Mn((S)- salmen)(CH3OH)2][MnCr(ox)3](CH2Cl2)0.375·(CH3OH)0.375·(H2O)0.125 present 3D achiral structures totally different to those obtained before. The compounds [Mn((R)- salmen)(CH3OH)(CH3CN)][MnCr(ox)3]·(CH3OH)0.5·(CH3CN)1.25 and [Mn((S)-salmen) (CH3OH)(CH3CN)][MnCr(ox)3]·(CH3OH)0.5·(CH3CN)1.25 with a chiral 2D structure constitute the most promising result, as the they can be obtained enantiomerically pure. In contrast to its derivatives, [MnIII(salen)]+ inserted into bimetallic networks maintains its dimeric structure in the compound [MnIII(salen)(H2O)]2[MnIICrIII(ox)3]2·(MeOH)3·(H2O) and the paramagnetic analog of formula [MnIII(salen)(H2O)]2[ZnIICrIII(ox)3]2·(MeOH)3 ·(H2O). They are formed by [MnIII(salen)(H2O)]2 2+ dimers inserted into an achiral 3D network very similar to that obtained with [FeIII(sal2-trien)]+, in Chapter 2. The preservation of the dimeric structure of MnIII precursor, allows a single molecule magnet (SMM) behavior in the ZnII paramagnetic compound although at very low temperatures (below 0.5 K). Magnetic characterization of the ferromagnetic [MnIII(salen)(H2O)]2[MnIICrIII(ox)3]2·(MeOH)3·(H2O) compound is consistent with coexistence of magnetic ordering and a SMM behavior with important changes in the two properties due to the interaction between the bimetallic oxalate network and the SMMs.