Two metallacyclic complexes of general formula Na8[MII2L3][middle dot]xH2O [M = Ni (4) and Co (5) with x = 15 (4) and 17 (5)] have been self-assembled in aqueous solution from N,N[prime or minute]-1,3-phenylenebis(oxamic acid) (H4L) and M2+ ions in a ligand/metal molar ratio of 3 : 2 in the presence of NaOH acting as base. X-Ray structural analyses of 4 and 5 show triple-stranded, dinuclear anions of the meso-helicate-type (so-called mesocates) with C3h molecular symmetry. The two octahedral metal-tris(oxamate) moieties of opposite chiralities ([capital Delta],[capital Lambda] form) are connected by three m-phenylene spacers at intermetallic distances of 6.822(2) (4) and 6.868(2) A (5) to give a metallacryptand core. In the crystal lattice, the binding of these heterochiral dinickel(ii) and dicobalt(ii) triple mesocates to sodium(i) ions leads to oxamato-bridged heterobimetallic three-dimensional open-frameworks with a hexagonal diamond architecture having small pores of 17.566(4) (4) and 17.640(2) A (5) in diameter where the crystallization water molecules and the sodium(i) countercations are hosted. Variable temperature (2.0-300 K) magnetic susceptibility measurements reveal relatively anisotropic S = 2 NiII2 (4) and S = 3 CoII2 (5) ground states resulting from the moderate to weak intramolecular ferromagnetic coupling between the two high-spin NiII (SNi = 1) or CoII (SCo = 3/2) ions across the m-phenylenediamidate bridges [J = +3.6 (4) and +1.1 cm-1 (5); H = -JS1[middle dot]S2]. A simple molecular orbital analysis of the electron exchange interaction identifies the [small pi]-type pathways of the meta-substituted phenylene spacers involving the dz2 and dx2-y2 pairs of magnetic orbitals of the two trigonally distorted octahedral high-spin MII ions (M = Ni and Co) as responsible for the overall ferromagnetic coupling observed in 4 and 5 in agreement with a spin polarization mechanism. The decrease of the overall ferromagnetic coupling from 4 to 5 is in turn explained by the additional antiferromagnetic exchange contribution involving the dxy pair of magnetic orbitals of the two trigonally distorted octahedral high-spin CoII ions across the [sigma]-type pathway of the meta-substituted phenylene spacers.