Two types of Ln(II)-Co-4 isocarbonyl polymeric arrays, [(Et2O)(3-x)(THF)(x)Ln[Co-4(CO)(11)]}(infinity) (1-3; x = 0, 1) and {(THF)(5)Eu[Co-4(CO)(11)]}(infinity) (4), were prepared and structurally characterized. Transmetalation involving Ln(0) and Hg[Co(CO)(4)](2) in Et2O yields {(Et2O)(3)Ln[Co-4(CO)(11)]}(infinity) (1, Ln = Yb; 2, Ln = Eu). Dissolution of the solvent-separated ion pairs [Ln(THF)(x)][Co(CO)(4)](2) (Ln = Yb, x = 6; Ln = Eu) in Et2O affords (Et2O)(2)(THF)Yb[Co-4(CO)(11)]}(infinity) (3) and [(THF)(5)Eu[CO4(CO)(11)](2-) (4). In these reactions, oxidation and condensation of the (Co(CO)(4)](-) anions result in formation of the new tetrahedral cluster [Co-4(CO)(11)](2-). The two types of Ln(II)-Co-4 compounds contain different isomers of [Co-4(CO)(11)](2-), and, consequently, the structures of the infinite isocarbonyl networks are distinct. The cluster in t(Et2O)(3-x)(THF)(x)Ln[Co-4(CO)(11)]}(infinity) (1-3) possesses pseudo C-3v symmetry (an apical Co, three basal Co atoms; one face-bridging, three edge-bridging, seven terminal carbonyls) and connects to Ln(II) centers through eta(2),mu(4-) and eta(2),mu(3)-carbonyls to generate a 2-D puckered sheet. In contrast, [(THF)(5)Eu[Co-4(CO)(11)]}(infinity) (4) incorporates a C-2v symmetric cluster (two unique Co environments; two face-bridging, one edge-bridging, eight terminal carbonyls), and isocarbonyl linkages (eta(2),mu(4)-carbonyls) to Eu(II) atoms create a 1-D zigzag chain. Complexes 1-4 contain the first reported eta(2),mu(4)-CO bridges between a Ln and a transition-metal carbonyl cluster. Infrared spectroscopic studies revealed that the isocarbonyl associations to Ln(II) persist in solution. The solution structure and dynamic behavior of the [Co-4(CO)(11)](2-) cluster in 1 was investigated by variable-temperature Co-59 and C-13 NMR spectroscopies.