The synthesis and structural and magnetic characterization of 16 compounds AM(II)Fe(lII)(C2O4)(3) (A = N(n-C3H7)4, N(n-C4H9)(4), N(n-C5H11)(4), P(n-C4H9)(4), P(C6H5)(4), N(n-C4H9)(3)(C6H5CH2), (C6H5)(3)PNP(C6H5)(3), As(C6H5)(4); M(II) = Mn, Fe) are reported. X-ray powder diffraction profiles are indexed in R3c or its subgroup P6(5)22 or P6/mmm to derive unit cell constants. The structures of all the compounds consist of two-dimensional honeycomb networks [M(II)Fe(III)(C2O4)(3)(-)]infinity. The M(II) = Fe compounds behave as ferrimagnets with T-c between 33 and 48 K, but five exhibit a crossover from positive to negative magnetization near 30 K when cooled in a field of 10 mT. The compounds exhibiting this unusual magnetic behavior are those that have the highest T-c. Within the set N(nC(n)H(2n+1))(4)(FeFeIII)-Fe-II(C2O4)(3)(n=3-5), T-c increases with interlayer separation and the low-temperature magnetization changes from positive (n = 3) to negative (n = 4, 5). In the M = Mn-II compounds, the in-plane cell parameter a(0) is similar to 0.03 Angstrom greater than in the corresponding M = Fe-II ones while the interlayer separation (c(0)/6) is on average 0.08 Angstrom smaller. All members of the M(II) = Mn series have magnetic susceptibilities showing broad maxima at 55 K characteristic of two-dimensional antiferromagnetism, but the magnetization of several of the salts increases sharply below 27 K due to the onset of spin canting, the magnitude of which varies significantly with A.