A series of Cu(I) complexes of sterically hindered tris(pyrazolyl)hydroborates [TpRR′Cu]2 (R and R′ are the 3-and 5-pyrazolyl substituents, respectively; R = t-Bu, R′ = H or R = R′ = Ph) and TpRR′CuL (R = t-Bu, R′ = H, L = CH3CN; R = R′ = Ph, L = CH3CN; R = R′ = Ph, L = 3,5-Ph2pz = 3,5-diphenylpyrazole) were isolated and characterized by elemental analysis, variable-temperature 1H NMR spectroscopy, osmometry, cyclic voltammetry, and, in the case of [Tpt-BuCu]2, [TpPh2Cu]2, and TpPh2Cu(3,5-Ph2pz), by X-ray crystallography. ([Tpt-BuCu]2-Et2O: C46H78N12B2Cu2O, space group P21/n (No. 14) at−101 °C with a = 19.001(8) Å, b = 13.629(8) Å, c = 21.490(7) Å, β = 113.01(3)°, V = 5122(8) Å3, Z = 4, R = 0.082 and Rw = 0.075 for 3749 unique, observed reflections with I > 2σ(I) and 541 variable parameters. [TpPh2Cu]2·2CH2Cl2: C47H38N6CuCl4B, space group P21/n (No. 14) at −101 °C with a = 17.801(8) Å, b = 13.063(8) Å, c = 18.551(9) Å, β = 99.42(4)°, V= 4255(7) Å3, Z = 4, R = 0.062 and Rw = 0.061 for 5552 unique, observed reflections with I > 2σ(I) and 532 variable parameters. Tpph2Cu(3,5-Ph2pz)·CH2Cl2: C61H48N8BCuCl2, space group P21/n (No. 14) at−101 °C with a = 10.308(7) Å, b = 29.105(9) Å, c = 17.158(8) Å, β = 93.54(5)°, V= 5138(8) Å3, Z = 4, R = 0.069 and Rw = 0.061 for 5136 unique, observed reflections with I > 2σ(I) and 658 variable parameters). Solution molecular weight data and 1H NMR spectra of [Tpt-BuCu]2, which contains linear 2-coordinate Cu(I) ions bridged by η2-Tpt-Bu ligands in the solid state, indicated that it retains its dimeric structure in solution, although the presence of a trace amount of a monomeric species could not be ruled out. A pyrazolyl ring exchange process occurs with ΔH* = 11.7(5) kcal mol−1 and ΔS* = −9(2) eu, as determined by line shape analysis of variable-temperature 1H NMR spectra. [TpPh2Cu]2, in which η3-TpPh2 ligands bridge between 3-coordinate Cu(I) ions in the solid state, apparently dissociates in solution to ill-defined species of lower molecular weight. 1H NMR spectroscopy indicated that the pyrazole ligand in the monomeric, distorted tetrahedral TpPh2Cu(3,5-Ph2pz) is labile. Cyclic voltammograms of the [TpRR′Cu]2 and TpRR′Cu(CH3CN) complexes in 0.1 M (TBA)PF6/CH2C12 showed irreversible oxidations at high potentials (>+0.6 V vs SCE), but TpPh2Cu(3,5-Ph2pz) (same conditions) and Tpt-BuCu(CH3CN) (in 10% CH3CN/CH2Cl2 with 0.1 M (TBA)OTf) exhibited chemically reversible redox processes attributed to TpPh2Cu(3,5-Ph2pz)/TpPh2Cu(3,5-Ph2pz)+ and Tpt-BuCu(CH3CN)/Tpt-BuCu(CH3CN)+ couples with E1/2 = +0.69 and +0.93 V vs SCE, respectively.