Preparation of Tetraammonium Octakis (isothiocyanato)dimoiybdenum (II) and Structural Characterization of Two Crystalline Hydrates

By a ligand replacement reaction in aqueous solution the [Mo₂(NCS)₈]_4- ion has been prepared and isolated in two compounds: (NH4)₄Mo₂(NCS)₈. 4H₂O (1) and (NH4)4Mo₂(NCS)₈-6H₂O (2). The structures of these deep blue-green substances have been determined by X-ray crystallography and the electronic absorption spectrum of the [Mo₂(NCS)₈]_4- ion has been measured. Compound 1 crystallizes in space group Pbca with a=13.859 (3) υ, b=13.012 (4) υ, c=16.937 (4) υ, V=3054 υ3, and Z=4. Compound 2 crystallizes in space group P with a=10.706 (2) υ, b=11.708 (3) υ, c=14.346 (3) υ, a=105.79 (2)°, β=93.37 (2)°, γ=105.71 (2)°, V=1649 (1) υ3, and Z=2. In 2 there are two crystallographically independent [Mo2(NCS)8]4- ions, each on an inversion center, and the [Mo₂(NCS)₈]_4- in 1 also resides on an inversion center. All three crystallographically distinct anions have virtual D4h symmetry with approximately linear MoNCS chains. The Mo-Mo distances are 2.162 (1) υ in 1 and 2.177 (1) and 2.174 (1) υ in 2. These distances are slightly longer than Mo-Mo quadrupole bond distances previously observed in any compound except the allyl, Mo₂(allyl)4. The small differences between the three Mo-Mo bond lengths appear to be due to differences in the intermolecular interactions. The NH4+ions in each case are hydrogen bonded to water molecules. The visible spectrum contains a band at about 14 500 cm̜1 which may be assigned to the δ5→δ* transition.