PREPARATION, CRYSTAL-STRUCTURES, VIBRATIO NAL-SPECTRA, AND NORMAL-COORDINATE ANALYSIS OF [(MO6BR8I)Y-6(A)](2-) Y-A = CN, NCS

By treatment of [(Mo6Br8i)Br-6(a)](2-) with AgNO3 in acetone and addit ion of KCN or KNCS the hexacyano and hexaisothiocyanato derivates [(Mo 6Br8i)Y-6(a)](2-), Y-a = CN, NCS are formed. X-ray structure determina tions of (Ph4P)(2)[(Mo6Br8i)(CN)(6)(a)]. 4H(2)O (1) (triclinic, spaceg roup P1, a = 11.63(3), b = 11.85(1), c = 14.23(5) Angstrom, alpha = 71 .8(1)degrees, beta = 67.6(3)degrees, gamma = 62.8(1)degrees, Z = 1) an d (n-Bu4N)(2)[(Mo6B8i)(NCS)(6)(a)]. 2Et(2)O (2) (monoclinic, spacegrou p P2(1)/n, a = 11.483(3), b = 16.348(5), c = 20.059(6) Angstrom, beta = 95.44(3)degrees, Z = 2) have been performed. The via C coordinated c yano ligands of (1) reveal facial groups with (MoCN) angles of 168.0-1 71.5 degrees and 174.1 degrees-175.7 degrees. In (2) the via N coordin ated isothiocyanato groups at the apical positions show MoNC-angles of 164.4 degrees, the equatorial angles are 172.7-173.5 degrees. Using t he molecular parameters of the X-ray determinations the 10 K IR and Ra man spectra of the (n-Bu4N) cluster salts are assigned by normal coord inate analyses based on a modified valence force field. The valence fo rce constants are f(d)(MoMo) = 1.41 (CNa), 1.43 (NCSa), f(d)(MoBri) = 0.97 (CNa), 0.96 (NCSa), f(d)(MoC) = 1.62, f(d)(Mo-N) = 2.09 mdyne/Ang strom.