The synthesis and structures of a series of [tris(pyrazolyl)hydroborato]cadmium complexes, {eta3-HB(3,5-Pri2pz)3}-CdI, {eta3-HB(3-Butpz)3}CdI, {eta3-HB(3-But-5-Mepz)3}CdCH3, and {eta3-HB(3-But-5-Mepz)3}Cd(eta2-O2NO), are reported. The complex {eta3-HB(3-But-5-Mepz)3}Cd(eta2-O2NO) exhibits bidentate coordination of the nitrate ligand [2.272(6) and 2.295(7) angstrom], in contrast to the unidentate coordination mode that is observed in the related zinc derivative [eta3-HB(3-Butpz)3}Zn(eta1-ONO2). Cadmium-substituted carbonic anhydrase is less active than the zinc enzyme, and although many factors, such as the higher pK(a) of the coordinated water, may be responsible for the lower activity, the observation of bidentate coordination in {eta3-HB(3-But-5-Mepz)3}Cd(eta2-O2NO) supports the notion that facile access to a unidentate bicarbonate intermediate may be an important requirement for carbonic anhydrase activity. {eta3-HB(3,5-Pri2pz)3}CdI is monoclinic, P2(1)/m (No. 11), with a = 9.840(2) angstrom, b = 16.449(9) angstrom, c = 10.633(2) angstrom, beta = 103.04(2)-degrees, V = 1677(1) angstrom3, and Z = 2. {eta3-HB(3-Butpz)3}CdI is tetragonal, P42(1)mBAR (No. 113), with a = b = 16.938 (2) angstrom, c = 9.915(2) angstrom, V = 2845(1) angstrom3, and Z = 4. {eta3-HB(3-But-5-Mepz)3}CdCH3 is cubic, I43dBAR (No. 220), with a = 22.747(2) angstrom, V = 11770(2) angstrom3, and Z = 16. {eta3-HB(3-But-5-Mepz)3}Cd(eta2-O2NO) is monoclinic, P2(1)/n (No. 14), with a = 10.690(1) angstrom, b = 17.233(3) angstrom, c = 16.272(2) angstrom, beta = 91.70(1)-degrees, V = 2996(1) angstrom3, and Z = 4.