The structural characterization of a series of complexes of formula [M(dca)(2)L](n), where dca) dicyanamide, L) 1,10-phenanthroline (phen) [ 1 - 4] and 2,9-dimethylphenanthroline (2,9-dmphen) [ 9 - 12], and M) Mn ( 1 and 9), Fe ( 2 and 10), Co ( 3 and 11), and Ni ( 4 and 12), has revealed the effect of the presence of the methyl substituents of L on the resulting network. The structure of [ Mn( dca) 2( phen)] n ( 1), which is identical to those of 2 - 4, together with the investigation of its magnetic properties in the temperature range of 77 - 300 K were reported elsewhere. The use of the 4,7-dimethylphenanthroline (4,7-dmphen) as the co-ligand yielded a series of compounds of formula [M(dca)(2)(4,7- dmphen)](n) [M = Mn ( 5), Fe ( 6), Co ( 7), and Ni ( 8)], which are isostructural with 1 - 4. Compounds containing phen ( 1 - 4) and 4,7- dmphen ( 5 - 8) are made of two-dimensional grids of metal atoms, each metal atom being linked to three other metal centers through single ( three metal atoms involved) and double ( two metal atoms involved) dca bridges exhibiting the mu-1,5 coordination mode. The isostructural complexes [M(dca)(2)(2,9-dmphen)] n ( 9 - 12) also have a sheetlike structure, the metal atoms in each layer being linked by two single and one double mu-1,5-dca units, as in 1 - 8. However, the topology of the network in 9 - 12 is different from that in 1 - 8 because of the different arrangement of the two single mu-1,5 dca bridges: cis in 1 - 8 versus trans in 9 - 12. The magnetic study of compounds 1 - 12 in the temperature range of 1.9 - 290 K has revealed the occurrence of weak ferromagnetic (M = Ni) and antiferromagnetic interactions (M = Mn, Fe, and Co). The different magnetic behavior in 1 - 12 was analyzed in the light of their structures, and the values of the magnetic interactions were compared to those of related systems.