SYNTHESIS AND STRUCTURE OF TRIPHENYLMETHYLPHOSPHONIUM TETRAKIS(TETRAHYDROBORATO)ALUMINATE, [PH(3)MEP][AL(BH4)(4)], AN EXAMPLE OF 8-COORDINATE ALUMINUM(III)

Complex cation, [(Ph(3)P)(2)N](+) and [Ph(3)MeP](+), salts of the tetr akis(tetrahydroborato) aluminate anion [Al(BH4)(4)](-) were prepared a s white crystalline solids in good yield from the reactions of [Ph(3)M eP][BH4] and [(Ph(3)P)(2)N][BH4] with Al(BH4)(3) in CH2Cl2 solutions. These salts are thermally stable above 100 degrees C. They have been c haracterized by H-1, B-11, and Al-27 NMR spectroscopy, and the structu re of [Ph(3)MeP][Al(BH4)(4)] has been; determined by single crystal X- ray analysis. Crystal data for [Ph(3)MeP][Al(BH4)(4)] (-60 degrees C): space group Pca2(1), a = 17.701(3) Angstrom, b = 9.030(2) Angstrom, c = 14.993(3) Angstrom, V = 2396.4 Angstrom(3), rho(calcd) = 1.008 g cm (-3), MW = 363.67, Z = 4. Four BH4- units form eight apparent 3c-2e Al -H-B bonds with the Al3+ center. This is the first molecular structure of an aluminum compound which contains an eight-coordinate aluminum a tom. The four BH4 units are arranged in a distorted tetrahedral fashio n around the aluminum atom. Each BH4 unit contributes two hydrogen bri dges to the aluminum. The eight bridging hydrogens form a slightly dis torted dodecahedral coordination geometry around the aluminum center. Other previously reported salts M[AlH(BH4)(3)], M[AlH2(BH4)(2)], and M [AlH3(BH4)] (M(+) = Li+, BU4N+) were studied further in solution by NM R spectroscopy.