The copper(I) coordination chemistry of tris(3-R-5-R′-pyrazolyl)borate ligands (TpR,R′) bearing substituent groups attached to the pyrazole rings has been investigated under nitrogen and carbon monoxide atmospheres [1 (TpPh,SMe) R = Ph, R′ = SCH3; 2 (TpPy) R = 2-pyridyl, R′ = H; 3 (TpPhos,Me) R = 2-(diphenylphosphinyl)phenyl, R′ = CH3]. The reactivity is strongly dependent on the coordinating potential of the substituent groups; in the absence of CO, 1 formed an insoluble complex [Cu(TpPh,SMe)]n (1b), 2 formed a multinuclear species [Cu(TpPy)]n (2a; tetrameric in solution, dimeric in the solid state) and 3 formed a discrete complex [Cu(TpPhos,Me)] (3a). Under a CO atmosphere, 1 displayed irreversible CO binding to form the complex [Cu(TpPh,SMe)CO] (1c), 2 showed reversible CO binding between 2a and the mononuclear species [Cu(TpPy)CO] (2b), and 3 did not bind to CO, but instead formed 3a. The reactivity of the copper complexes of 1 and 2 with [11C]carbon monoxide has been examined in the radiochemistry laboratory to investigate the reversibility of 11CO binding. The solid-state structures of [Tl(TpPh,SMe)] (1a), 1c and 2a were elucidated by X-ray crystallography.