The synthesis and crystallographic characterization of a new family of M(mu-CN)Ln complexes are reported. Two structural series have been prepared by reacting in water rare earth nitrates (Ln(III) = La, Pr, Nd, Sm, Eu, Gd, Dy, Ho) with K-3[M(CN)(6)] (M-IIIF = Fe, Co) in the presence of hexamethylenetetramine (hmt). The first series consists of six isomorphous heterobinuclear complexes, (CN)(5)M-CN-Ln(H2O)(8)] center dot 2hmt ([FeLa] 1, [FePr] 2, [FeNd] 3, [FeSm] 4, [FeEu] 5, [FeGd] 6), while the second series consists of four isostructural ionic complexes, [M(CN)(6)][Ln(H2O)(8)] center dot hmt aFeDy] 7, [FeHo] 8, [CoEu] 9, [CoGd] 10). The hexamethylenetetramine molecules contribute to the stabilization of the crystals by participating in an extended network of hydrogen bond interactions. In both series the aqua ligands are hydrogen bonded to the nitrogen atoms from both the terminal CN groups and the hmt molecules. The [FeGd] complex has been analyzed with Fe-57 Mossbauer spectroscopy and magnetic susceptibility measurements. We have also analyzed the [FeLal complex, in which the paramagnetic is replaced by diamagnetic Lam, with Fe-57 Mossbauer spectroscopy, electron paramagnetic resonance (EPR), and magnetic susceptibility measurements, to obtain information about the low-spin Fe-III site that is not accessible in the presence of a paramagnetic ion at the complementary site. For the same reason, the [CoGd] complex, containing diamagnetic Co-III, was studied with EPA and magnetic susceptibility measurements, which confirmed the S= 7/2 spin of Gd-III. Prior knowledge about the paramagnetic sites in [FeGd] allows a detailed analysis of the exchange interactions between them. In particular, the question of whether the exchange interaction in [FeGd] is isotropic or anisotropic has been addressed. Standard variable-temperature magnetic susceptibility measurements provide only the value for a linear combination of 4, 4, and J but contain no information about the values of the individual exchange parameters J(x), J(y), and J(z) In contrast, the spin-Hamiltonian analysis of the variable-field, variable-temperature Mossbauer spectra reveals an exquisite sensitivity on the anisotropic exchange parameters. Analysis of these dependencies in conjunction with adopting the gvalues obtained for [FeLa], yielded the values J(x) = +0.11 cm(-1), J(y) = +0.33 cm(-1), and J, = +1.20 cm-1 ((S) over cap (1) . J. S-2 convention). The consistency of these results with magnetic susceptibility data is analyzed. The exchange anisotropy is rooted in the spatial anisotropy of the low-spin Fe-II ion. The condition for anisotropic exchange is the presence of low-lying orbital excited states at the ferric site that (i) effectively interact through spin orbit coupling with the orbital ground state and (ii) have an exchange parameter with the Gd site with a value different from that for the ground state. Density functional theory (DFT) calculations, without spin orbit coupling, reveal that the unpaired electron of the t(2g)(5) ground configuration of the Fe-III ion occupies the xy orbital, that is, the orbital along the plane perpendicular to the Fe center dot center dot center dot Gd vector. The exchange-coupling constants for this orbital, j(xy), and for the other t(29) orbitals, j(xy) and j(xz) have been determined using a theoretical model that relates them to the anisotropic exchange parameters and the g-values of The resulting values, j(yz) = 5.7 cm(-1), j(xz) = 4.9 cm(-1), and j(xy) = +0.3 cm(-1) are quite different. The origin of the difference is briefly discussed.