Synthesis, crystal structure, NMR, vibrational spectra, and normal coordinate analysis of the cluster anions [(Mo6I8i)Y-6(a)](2-), Y-a=F,Cl,Br,I

By reaction of Mo6I12 with (n-Bu(4)N); Y = Cl, Br, I in organic solvents the octa-mu 3-iodo-hexahalogeno-octahedro-hexamolybdate(2-) anions [(Mo6I8i)Yd(6)(a)](2-) are formed. X-ray structure determinations on single crystals of (n-Bu(4)N)(2)[(Mo6I8i)F-6(a)] (1) (orthorhombic, space group Pcnb, a=15.218(2), b=18.640(2), c=20.653(8) Angstrom, Z=4), (n-Bu(4)N)(2)[(Mo6I8i)Cl-6(a)] (2) (monoclinic, space group P2(1)/n, a=12.272(2), b=13.117(1), c=18.699(2) Angstrom, beta=93.72(1)degrees, Z = 2), (n-Bu(4)N)(2)[(Mo6I8i)Br-6(a)] (3) (monoclinic, space group P2(1)/c, a=12.490(8), b=13.010(2), c=19.141(1) Angstrom, beta= 92.00(1)degrees, Z=2), (n-Bu(4)N)(2)[(Mo6I8i)I-6(a)] (4) (monoclinic, space group P2(1)/n, a=11.556(2), b=11.567(8), c=24.539(4) Angstrom, beta=96.69(2)degrees, Z=2) and (Ph(3)P=N=PPh(3))(2)[(Mo6I8i)Cl-6(a)] (5) (triclinic, space group P1, a=13.095(1), b=13.475(1), c=13.470(2) Angstrom, alpha=81.392(9)degrees, beta=81.165(8)degrees, gamma=60.589(6)degrees, Z=1) show regular geometry of the cluster anions with systematic stretchings in the (Mo-Mo) bond length and a slight compression of the I-8(i) cube in the F-a to I-a series. The i.r. and Raman spectra reveal at 10 K almost constant frequencies of the (Mo6I8i) cluster vibrations, whereas all modes with Y-a ligand contribution are characteristically shifted. The A(1g) vibrations are assigned by polarization measurements on saturated solutions in propylencarbonate/acetonitrile (2:1). The valence force constants determined by normal coordination analysis are for f(d)(Mo-Mo) in the range from 1.4 (F-a) to 1.2 (I-a), for f(d)(Mo-I-i) ca. 0.80 and for f(d)(Mo-Y-a) from 2.0 (F-a) to 1.0 (I-a) mdyn/Angstrom. The Mo-95 nmr signal is shifted to higher field with decreasing electronegativity of the Y-a ligands, 4368 (F-a), 3757 (Cl-a), 3569 (Br-a) and 3262 ppm (I-a).