Preparation, crystal structures, vibrational spectra, and normal coordinate analysis of [(Mo6Br8i)Y6a]2-; Ya = CN, NCS

By treatment of [(Mo6Br8i)Br-6(a)](2-) with AgNO3 in acetone and addition of KCN or KNCS the hexacyano and hexaisothiocyanato derivates [(Mo6Br8i)Y-6(a)](2-), Y-a = CN, NCS are formed. X-ray structure determinations of (Ph4P)(2)[(Mo6Br8i)(CN)(6)(a)]. 4H(2)O (1) (triclinic, spacegroup 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) and (n-Bu4N)(2)[(Mo6B8i)(NCS)(6)(a)]. 2Et(2)O (2) (monoclinic, spacegroup 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 cyano ligands of (1) reveal facial groups with (MoCN) angles of 168.0-171.5 degrees and 174.1 degrees-175.7 degrees. In (2) the via N coordinated isothiocyanato groups at the apical positions show MoNC-angles of 164.4 degrees, the equatorial angles are 172.7-173.5 degrees. Using the molecular parameters of the X-ray determinations the 10 K IR and Raman spectra of the (n-Bu4N) cluster salts are assigned by normal coordinate analyses based on a modified valence force field. The valence force 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/Angstrom.