For the first time a basic transition-metal hydrate, Ni3Cl2.1(OH)(3.9)center dot 4H(2)O, is found to be isostructural to a maingroup metal phase, Mg3Cl2.0(OH)(4.0)center dot 4H(2)O. The Ni phase was found as crystalline solid in the course of investigations into the formation of basic nickel(II) chloride phases at 25 and 40 degrees C in alkaline, concentrated nickel(II) chloride solutions. Ni3Cl2.1(OH)(3.9)center dot 4H(2)O was characterized by thermal analysis, IR spectroscopy, scanning electron microscopy, and X-ray powder diffraction. The crystal structure was determined from high-resolution laboratory X-ray powder diffraction data. Ni3Cl2.1(OH)(3.9)center dot 4H(2)O crystallizes in space group C2/m (12) with Z = 2, a = 14.9575(4) angstrom, b = 3.1413(1) angstrom, c = 10.4818(5) angstrom, beta = 101.482(1)degrees, and V = 482.50(3) angstrom(3). The main building unit of the structure is an infinite triple chain of edge-linked distorted NiO6 octahedra. These chains are separated by interstitial one-dimensional zigzag chains of disordered Cl- ions and H2O molecules. The crystal structures of Ni3Cl2.1(OH)(3.9)-4H(2)O and the isostructural magnesium salt hydrate Mg3Cl2(OH)(4)center dot 4H(2)O (2-1-4 phase) are compared in detail.