Tetraheptylammonium salts of various transition-metal-substituted heteropolyanions with alpha-Keggin ([XW11O39M](n-)), alpha-Wells-Dawson ([P2W17O61M](m-)), and Weakley and Finke structures ([P2W18O68Co4](10-)) were investigated with respect to their reactivity with CO2 in nonpolar solvents. It was found that copper(II)- and manganese(III)substituted heteropolyanions do not react with CO2. Germano- and silicotungstates with the alpha-Keggin structure do form complexes with CO2 when substituted with Co(II), Ni(II), and Mn(II). In contrast, boro- and phosphotungstates substituted with Co(II), Ni(II), and Mn(II) are unreactive. The alpha(2) isomers of Wells-Dawson phosphotungstates show reactivity similar to that of alpha-Keggin silicotungstates-i.e., Co(II), Ni(II), and Mn(II) derivatives do react with CO2. On the other hand, the alpha(1) isomer of the Co(II)-substituted Wells-Dawson anion does not react with CO2, and neither does the Weakley and Finke cobaltotungstate. When reactions do occur, they are completely reversible. An excess of water decomposes the complexes. Traces of water are, however, necessary for the reactions to take place. The CO2 adducts were characterized by UV/vis, IR, and C-13 NMR. The IR data could be explained as originating either from CO2 complexes with a direct eta(1) metal-carbon bond or from bicarbonato complexes. IR spectra with isotopically enriched (CO2)-C-13 and (CO2)-O-18 support the presence of a eta(1) metal-carbon bond. The C-13 NMR spectra indicate the presence of two different kinds of paramagnetic CO2 complexes after the reaction of alpha-[SiW11O39Co](6-) with CO2 (chemical shifts 792 and 596 ppm at 26 degrees C). The variable-temperature experiments are consistent with the chemical exchange between these two species. UV/vis, IR, and NMR studies in the presence of controlled amounts of water or ethanol suggest the existence of H-bonding in the CO2 complexes, similar to that reported in the past for complexes between heteropolyanions and dioxygen.