PREPARATIONS, STRUCTURES, ELECTRONIC-STRUCTURES, AND MAGNETIC-PROPERTIES OF FACE-SHARING BIOCTAHEDRAL NIOBIUM(III) AND TANTALUM(III) COMPOUNDS

Four face-sharing bioctahedral anions containing niobium and tantalum have been obtained in crystalline compounds and structurally character ized. The following compounds and their crystal structures are reporte d. (Et4N)2[Nb2Cl8(THT)]: Pcca, a = 29.602(5) angstrom, b = 16.057(3) a ngstrom, c = 14.338(3) angstrom, V = 6815(2) angstrom3, Z = 8; the THT (tetrahydrothiophene) sulfur atom occupies a bridging position and Nb -Nb = 2.707(2) angstrom. (Et4N)3-[Nb2Cl9].5CH2Cl2: C2, a = 18.338(3) a ngstrom, b = 14.126(3) angstrom, c = 11.747(2) angstrom, beta = 114.70 (1)degrees, V = 2765(1) angstrom3, Z = 2; the anion has an Nb-Nb dista nce of 2.7413(3) angstrom. (Pr4N)3[Nb2Cl9].6CH2Cl2: Iba2, a = 17.235(4 ) angstrom, b = 45.588(7) angstrom, c = 18.604(4) angstrom, V = 14,617 (5) angstrom3, Z = 8; Nb-Nb = 2.691(2) angstrom. (Et4N)2[Ta2Cl8(THT)]: Pcca, a = 29.577(4) angstrom, b = 16.046(3) angstrom, c = 14.332(1) a ngstrom, v = 6802(2) angstrom3, Z = 8; this compound, isomorphous with its Nb analog, has Ta-Ta=2.6875(8) angstrom. Variable-temperature mag netic susceptibilities are reported for (Et4N)2[Nb2-Cl8(THT)] and a pr eviously reported compound, (Bu4N)[Nb2Cl7(PEt3)2]. In both cases, the susceptibilities were found to be temperature independent over a wide temperature range. Electronic structures of the model species [Nb2Cl9] 3-, [Nb2Cl7(PH3)2]-, and [Nb2Cl8(mu-SH2)]2- were studied by employing quantum chemical calculations in which both the SCF-Xalpha-SW molecula r orbital method and the ab initio configuration interaction method we re employed. The results of the calculations indicate that the metal-m etal interaction in each type of face-sharing compound can be formally described by a double bond although the pi component is relatively we ak. On the basis of the calculated electronic structures, a satisfacto ry explanation can be, obtained for the observed temperature-independe nt paramagnetism of the compounds.