Unique edge-sharing sulfate-transition metal coordination in Na sub(2)M(SO sub(4)) sub(2) (M=Ni and Co)

Two compounds, Na sub(2)Ni(SO sub(4)) sub(2) and Na sub(2)Co(SO sub(4)) sub(2), were synthesized and their structure and properties were characterized. They adopt a structure that contains a bidentate coordination of sulfate to the transition metal center, which was determined via single crystal X-ray diffraction combined with model refinements to both laboratory X-ray and time-of-flight neutron powder diffraction data. The compounds were both found to crystallize in the C2/c space group with Z=24 and a unit cell of a=23.3461(3) Aa, b=10.3004(1) Aa, c=17.4115(2) Aa, beta =98.8659(9) degree , and V=4136.99(8) Aa super(3) for the cobalt analog and a=23.2253(1) Aa, b=10.26155(6) Aa, c=17.3353(1) Aa, beta =99.0376(5) degree , and V=4080.20(5) Aa super(3) for the nickel analog. Magnetization measurements show that the transition metal centers have negligible interactions with neighboring sites. Infrared and Raman spectroscopies were used to further probe the unique sulfate-transition metal coordination, and confirm the bidentate binding motif. The resulting pseudo-trigonal bipyramidal coordination produces vivid violet, Na sub(2)Co(SO sub(4)) sub(2), and yellow, Na sub(2)Ni(SO sub(4)) sub(2), colors that were probed by diffuse reflectance. Two blue distorted transition metal octahedra (oxygen in red) bridged by two sulfate tetrahedra are shown here. Each bridging sulfate tetrahedra shares an edge with one octahedron and a corner with the other. All of the remaining corners of the octahedra are corner sharing with four tetrahedra forming a polyhedral network.