Unique edge-sharing sulfate-transition metal coordination in Na2M(SO4)2 (M=Ni and Co)

Two compounds, Na2Ni(SO4)(2) and Na2Co(SO4)(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) angstrom, 6=10.3004(1) angstrom, c= 17.4115(2) angstrom, beta=98.8659(9), and V=4136.99(8) angstrom(3) for the cobalt analog and a=23.2253(1)angstrom, b=10.26155(6) angstrom, c=17.3353(1) angstrom, beta=99.0376(5), and V=4080.20(5) angstrom(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, Na2Co(SO4)(2), and yellow, Na2Ni(SO4)(2), colors that were probed by diffuse reflectance. (C) 2014 Elsevier Inc. All rights reserved.