The new rhodium-dinitrogen complex Rh(N2)[HC(CH2CH2P(t-Bu)2) 2] (2) was prepared by elimination of HCl with sodium hydride from the hydrido chloride HRh(Cl)[HC(CH2CH2P(t-Bu)2)2] (1). Complex 2 reacts with various small gaseous molecules, giving rise to the new complexes Rh(X)[HC(CH2CH2P(t-Bu)2)2], X = H2 (3), C2H4 (4), CO2 (5). The first 16 electron rhodium-carbon dioxide complex 5 is spectroscopically characterized. All the transformations are reversible in the presence of free nitrogen. The thermodynamic parameters for these equilibria reactions have been evaluated. It is found that at 25°C formation of the dihydrogen complex 3 is about 1.24 kcal/mol more favorable than formation of its dinitrogen analogue 2, whereas formation of the carbon dioxide (5) and, surprisingly, ethylene (4) complexes is less favorable than 2 by 2.97 and 1.57 kcal/mol, respectively, yielding the ligating ability to the Rh(I) T-shaped core L = H2 > N2 > C2H4 > (CO2). The new hydrido formate HRh(O2CH)[HC(CH2CH2P(t-Bu)2) 2] (7) can be obtained either by reaction of the dihydrogen complex 3 with CO2 or from the carbon dioxide complex 5 with hydrogen.