REDUCED POLYOXOMOLYBDATES WITH THE KEGGIN AND DAWSON STRUCTURES - PREPARATION AND CRYSTAL-STRUCTURES OF 2-ELECTRON REDUCED [K(18-CROWN-6)](2)[N(PPH(3))(2)](2)[HPMO12O40]CENTER-DOT-8MECN-CENTER-DOT-18-CROWN-6 AND 4-ELECTRON REDUCED [NBU(4)(N)](5)[H3S2MO18O62]CENTER-DOT-4MECN (18-CROWN-6=1,4,7,10,13,16-HEXAOXACYCLOOCTADECANE)

By reaction of [K(18-crown-6)][N(PPh(3))(2)](2)[Mo12O40]. 2MeCN 1 and [NBu(4)(n)](4)[S2Mo18O62] with PPh(3) in acetonitrile the new compounds [K(18-crown-6)](2)[N(PPh(3))(2)]2[HPMo12O40]. 8MeCN . 18-crown-6 2 (1 8-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane) and [NBu(4)(n)](5)[H3S2Mo18O62]. 4MeCN 3 were prepared. Compound 2 contains the two-electron reduced [HPMo12O40](4-) ion having the alpha-Keggin structure, 3 the four-electron reduced [H3S2Mo18O62](5-) ion having the alpha-Dawson structure. Compounds 1-3 were characterised by means of IR and NMR spectroscopy and X-ray diffraction. The structures of 1 and 2 are disordered in space group P (1) over bar. The heteropolyanions show apparent T-h symmetry; their actual symmetry is T. Upon reduction the Mo ... Mo distances increase slightly. The strongly alternating 'short' (mean 1.814 Angstrom) and 'long' (1.990 Angstrom) Mo-O-Mo bonds in the unreduced compound 1 become more equal in 2. The structural changes upon reduction observed in 3 were analysed in detail. The most significant alterations are an increase of the Mo ... Mo distances between corner-sharing MoO6 octahedra iri the equatorial belt by 0.066 Angstrom, and a decrease of the Mo-O-Mo bond lengths connecting the two halves of the anion. The alternation of the Mo-O(bridge) bonds is nearly evened out after reduction. All structural changes observed in 2 and 3 are consistent with the description that the additional electrons occupy delocalised molecular orbitals extending over the Mo-O-b framework.