The rate and equilibrium kinetics of the reactions of M + , M + (pyrrole) and M + (pyrrole) 2 (M=Ni, Cu) with the small diatomic ligands O 2 and CO have been investigated in the gas phase at 295+/-2K in helium buffer gas at a pressure of 0.35+/-0.01Torr. The measurements were taken with an inductively-coupled plasma/selected-ion flow tube (ICP-SIFT) tandem mass spectrometer. Only ligation was observed. While atomic Cu + was observed to bind up to two ligands of O 2 and CO, atomic Ni + was observed to bind up to three. The presence of one molecule of pyrrole dramatically increases the gas-phase rate of metal-ion ligation except for the ligation of Cu + with O 2 . Ni + (pyrrole) 2 and Cu + (pyrrole) 2 were found to be unreactive with O 2 , k<1.0x10 - 1 3 cm 3 molecule - 1 s - 1 , but both ions were observed to ligate a single molecule of CO. While equilibrium was observed to be approached in several of the ligation reactions, an absolute value for the standard free energy of ligation could be obtained only for the ligation of Ni + (pyrrole)(CO) with CO. Quantum chemical calculations using density functional theory (DFT) with the B3LYP (Becke-3 Lee-Yang-Parr) hybrid functional have provided insight into the energetics and geometries of ligation. The bonding of pyrrole to either Ni + or Cu + is much stronger than bonding of either O 2 or CO. This agrees with the failure to observe experimentally any ligand-switching reactions involving the pyrrole ligand. Also, the computations show that the ligation of pyrrole does not significantly change the ligation energy of O 2 and CO to the metal ions. The various isomers of CO-containing complexes were investigated and it was found that metal-C bonding was always thermodynamically favored over metal-O bonding. The computations also show that the addition of a ligand of O 2 or CO can skew the symmetry inherent in M + -pyrrole complexes (but less so with O 2 ) by shifting the position of the metal ion relative to the midline of the pyrrole molecule. The structures determined for the various metal ion-CO complexes were found to have a linear M CO geometry, while structures of metal ion-O 2 complexes were found to have a bent M O 2 geometry.