Synthesis, crystal structures, and properties of the chromium(II) phosphate halides Cr-2(PO4)Br and Cr-2(PO4)I

The new compounds Cr-2(PO4)Br and Cr-2(PO4)I have been obtained by reaction of CrPO4, Cr and Br-2 or I-2 in evacuated silica tubes at elevated tempera tures (Cr-2(PO4)Br: 900 degrees C, Cr-2(PO4)I: 700 degrees C). Single cryst als of deep blue Cr-2(PO4)Br and turquoise Cr-2(PO4)I with edgelengths up t o 2 mm and 0.3 mm, respectively, have been grown in experiments involving t he gaseous phase. Single crystal data have been used for structure determin ation and refinement. Though being not isotypic, the two crystal structures are closely related. Two crystallographically independent Cr2+, in polyhed ra [Cr1O(3)X(3)] and [Cr2O5X], form dimers [Cr1(2)O(2)O(2/2)X(4)] and [Cr2( 2)O(8)X(2)] Distances are 1.978 Angstrom less than or equal to d(Cr-O) less than or equal to 2.096 Angstrom (for the iodide: 1.959 Angstrom less than or equal to d(Cr-O) less than or equal to 2.105 Angstrom), 2.587 Angstrom l ess than or equal to d(Cr-Br) less than or equal to 3.158 Angstrom and 2.86 7 Angstrom less than or equal to d(Cr-I) less than or equal to 3.327 Angstr om. The structures of bromide and iodide can be distinguished by the differ ent way of connection of the Crl containing dimers. The phosphate group shows slightly distorted tetrahedral geometry with 1.49 1 Angstrom less than or equal to d(P-O) less than or equal to 1.559 Angstro m (1.486 Angstrom less than or equal to. d(P-O) less than or equal to 1.567 Angstrom) and angles of 106.48 degrees less than or equal to L(O-P-O) less than or equal to 111.69 degrees (106.57 degrees less than or equal to L(O- P-O) less than or equal to 111.72 degrees. IR-spectra of Cr-2(PO4)Br and Cr-2(PO4)I, the Raman-spectrum of Cr-2(PO4)Br and electronic spectra of the two compounds in the UV/vis region at low te mperature are reported and discussed.