Transmetalation reactions of metallic ytterbium with Hg[Co(CO)(4)](2) in the coordinating solvents pyridine and THF yield the solvent-separated ion pairs [Yb(L)(6)] [Co(CO)(4)](2) (1a, L = Pyr; 2a, L = THF). The IR spectrum of 1a in pyridine indicates that the tetracarbonylcobaltate anion is not directly bonded to the divalent Yb cation owing to the strong coordinating ability of pyridine. On the other hand, IR spectra of 2a in THF are concentration dependent. In dilute solutions there is an equilibrium between the solvent-separated ion pair and a weak contact ion pair. Higher concentrations of 2a facilitate the formation of a tight ion pair that has a low-frequency isocarbonyl absorption. Remarkably, complexes la and 2a are easily transformed in toluene into the two-dimensional sheetlike arrays [(Pyr)(4)Yb{(mu-CO)(2)Co(CO)(2)}(2)](infinity) (1b) and [(THF)(2)Yb{(mu-CO)(3)Co(CO)}(2).Tol](infinity) (2b). The two-dimensional frameworks are supported by isocarbonyl linkages. Infrared spectra of toluene solutions substantiate the existence of the isocarbonyl bridges with low-frequency absorptions at 1780 cm(-1). Compounds 1b and 2b belong to a rare class of lanthanide-transition-metal carbonyl extended arrays, only three others of which have been structurally established. Dissolving 1b in pyridine regenerates la, but the complete conversion of 2b into 2a cannot be achieved. Crystal data: 1a.Pyr is monoclinic, P2(1)/c, a = 11.171(1) Angstrom, b = 11.925(1) Angstrom, c = 33.978(1) Angstrom, beta = 95.10(1)degrees, Z = 4; 2a is monoclinic, C2/c, a = 17.724(1) Angstrom, b = 12.468(1) Angstrom, c = 18.413(1) Angstrom, beta = 100.34(1)degrees, Z = 4; 1b is monoclinic, C2/c, a = 11.047(1) Angstrom, b = 13.423(1) Angstrom, c = 21.933(1) Angstrom, beta = 103.49(1)degrees, Z = 4; 2b is monoclinic, C2/c, a = 28.589(1) Angstrom, b = 7.223(1) Angstrom, c = 14.983(1) Angstrom, beta = 118.90(1)degrees, Z = 4.