Synthesis, crystal structure, and spectroscopic properties of the cluster anions [(Mo(6)X(8)(i))(N-3)(6)(a)](2-); X(i)-Cl, Br

By treatment of [(Mo(6)X(8)(i))Y-6(a)](2-); X(i), Y-a = Cl, Br; with AgNO3 in ethanol/dichloromethane and addition of NaN3 the octa-mu(3)-halogeno-hexazido-octahedro-hexamolybdate(2-) anions [(Mo(6)X(8)(i))(N-3)(6)(a)](2-) are formed. X-ray structure determinations have been performed on single crystals of (n-Bu(4)N)(2)[(Mo6Cl8i)(N-3)(6)(a)] (1) (monoclinic, space group P2(1)/n, a=14.592(5) Angstrom, b=12.314(5) Angstrom, c=17.039(5) Angstrom, beta = 101.183(5)degrees, Z=2), (n-Bu(4)N)(2)[(Mo6Br8i)(N-3)(6)(a)] (2) (monoclinic, space group P2(1)/n, a=14.595(6) Angstrom, b= 12.416(5) Angstrom, c=16.960(3) Angstrom, beta=100.74(3)degrees, Z=2) and (Ph(3)P=N=PPh(3))(2)[(Mo6Br8i)(N-3)(6)(a)] (3) (triclinic, space group P (1) over bar, a=11.092(4) Angstrom, b=13.161(8) Angstrom, c=14.341(11) Angstrom, alpha=78.085(6)degrees, beta=82.821(5)degrees, gamma=84.379(4)degrees, Z=1). Whereas the cluster anions of the (n-Bu(4)N) salts reveal a regular geo-metry, the (Ph(3)P=N=PPh(3)) cation causes strong distortions of the cluster kernel with variation of the Mo-Mo and Mo-Br-i distances up to 8%. Using the molecular parameters of the X-ray determinations the low temperature (10 K) IR and Raman spectra of the (n-Bu(4)N) cluster salts are assigned by normal coordinate analyses based on a modified valence force field. The valence force constants are f(d)(Mo-Mo) = 1.53 (Cl-i), 1.46 (Br-i), f(d)(Mo-Cl-i) = 1.09, f(d)(Mo-Br-i) = 0.96, f(d)(Mo-N-1) = 2.09 (Cl-i), 2.36 (Br-i) mdyne/Angstrom, respectively.