STEREOCHEMISTRY OF THIOCYANATE PHOTOANATION IN TRANS-CRCL(NH3)(1,3-DIAMINOPROPANE)(2)(2+)

The compound trans-CrCl(NH3)(tn)2(2+) (where tn = 1,3-diaminopropane) has been synthesized, characterized and the structure determined by X- ray single-crystal methods. The crystal is monoclinic, P2(1)/n (No. 14 ), with chemical composition trans-[CrCl(NH3)(tn)2]2(ClO4)3(NO3).H2O, M(r) = 883.8, and a = 13.516(1) angstrom, b = 9.749(2) angstrom, c = 2 6.533(3) angstrom, beta = 93.02(1)-degrees, V(cell) = 3491.3 angstrom3 , and Z = 4 (asymmetric unit was Cr2Cl2N11C12H48O16). In acidic aqueou s solution at room temperature, the compound loses chloride at a slow rate (rate constant <5 X 10(-7) s-1 at 28-degrees-C), but photolysis l eads to aquation of ammonia with a wavelength independent quantum yiel d of 0.33 +/- 0.01 to give cis-CrCl(H2O)(tn)2(2+) as the only observab le product, the total chloride yield having an upper limit of 0.003. W hen photolyzed in the presence of thiocyanate ion, cis-CrCl(NCS)(tn)2 is produced in competition with the aquation, and the quantum yield o f ammonia loss remains unchanged over the range 0-3 M NCS-. These resu lts show that photoanation competes with photoaquation and that both p rocesses occur with entry of the substituting nucleophile trans to the displaced ammonia ligand. Chromatographic studies of the products of thermal and photoaquation followed by thermal thiocyanate anation of t rans- and cis-CrCl2(tn)2+ were used to identify the photoproduct. This work additionally revealed that, thermally, direct displacement of ch loride by thiocyanate can occur in competition with anation, particula rly for cis-CrCl(H2O)(tn)22+.