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 characterized. The following compounds and their crystal structures are reported. (Et4N)2[Nb2Cl8(THT)]: Pcca, a = 29.602(5) angstrom, b = 16.057(3) angstrom, 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) angstrom, 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 distance 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) angstrom, v = 6802(2) angstrom3, Z = 8; this compound, isomorphous with its Nb analog, has Ta-Ta=2.6875(8) angstrom. Variable-temperature magnetic susceptibilities are reported for (Et4N)2[Nb2-Cl8(THT)] and a previously 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 molecular orbital method and the ab initio configuration interaction method were employed. The results of the calculations indicate that the metal-metal interaction in each type of face-sharing compound can be formally described by a double bond although the pi component is relatively weak. On the basis of the calculated electronic structures, a satisfactory explanation can be, obtained for the observed temperature-independent paramagnetism of the compounds.