We report herein two new nonlinear optical (NLO) crystals, [Et4N][Cd(XCN)(3)], where X = S (1) and Se (2), that are transparent from 220 to 3300 nm, covering the entire near-ultraviolet, the visible, and the near-infrared spectral regions and giving rise to a very wide and continuous optical window, which is useful for many frequency conversion applications. Both 1 and 2 exhibit highly efficient second harmonic generation (SHG) as measured via the Kurtz-Perry method. The corresponding [Me4N](+) salts, [Me4N][Cd(XCN)(3) where X = S (3) and Se (4), show no SHG effects. All four structures adopt noncentrosymmetric space groups (Cmc2(1) for 1 and 2 and Pna2(1) for 3 and presumably 4) and are based on one-dimensional anionic [Cd(XCN)(3)(-)](infinity) zigzag chains. However, a detailed analysis of the structures of [R4N][Cd(XCN)(3)], where R = Et, Me, and X = S, Se revealed that the difference in the second-order nonlinear responses of the Et4N+ salts (1 and 2) and the Me4N+ salts (3 and 4) is attributable to the relative alignment of the [Cd(XCN)(3)(-)](infinity) zigzag chains, being parallel in the crystals of 1 and 2 but antiparallel in the crystals of 3 and 4. Also reported, for the first time, are the synthesis and crystal structure of [Et4N][Cd(SeCN)(3)] (2). Compound 2 crystallizes in an orthorhombic unit cell of Cmc2(1) space group symmetry with lattice parameters of 9.938(1) Angstrom, 16.868(2) Angstrom, 11.054(1) Angstrom, and Z = 4. Other issues related to the molecular and crystal engineering of this class of NLO materials are also discussed.