Organometallics other along the (a + c)/2 direction. A diagram of the unit cell is available."The structure is consistent with that of Mg(SiMe3)?. DME in which the Si-C distance is 2.630 (2) A and Si-Mg-Si angle is 125.2 ( 1 ) O . The smaller Si-Mg-Si angle in the present result (115') may be explained by VSEPR theory on the basis that the weaker base, TMEDA, will form less polar Mg-N bonds than the Mg-0 bonds in the DME adduct. Alternatively the methyl groups on TME-DA may cause interference with those on the trimethylsilyl group forcing the smaller angle separation between these units. Further, the N-Mg bond distances and angles are similar to those observed for organomagnesium compounds. Comparison with the three complexes MgMe2.TMEDA,22 which have average N-Mg distances of 2.24,2.20, and 2.26 A and N-Mg-N angles of 81.5,82.5, and 82.5, respectively, are clearly within the range of those observed in the present system. MgPh2.TMEDA,B and q5-C5H5MgBr.(Et2N(CH2)2NEt2),24 (22) Greiser, T.; Kopf, J.; Thoennes, D.; Weiss, E. J. Organomet.1983,2, 750-755

No abstract

The crystal structures of the series (j75-C9H7)2M(CH3)2, where M = Ti, Zr, and Hf, have been determined from three-dimensional X-ray data measured by counter methods. The compounds are isostructural and crystallize in the orthorhombic space group P2i2i2 [Z>23; No. 18], For bis(r;5-indenyl)dimethyltitanium(IV) the unit cell dimensions are a = 14.124 (7) Á, b = 8.073 (5) Á, c = 6.844 (5) Á, and Z = 2. Full-matrix least-squares refinement has led to a final R value of 0.070 based on 280 independent observed reflections. For bis(Tj5-indenyl)dimethylzirconium(IV) the cell dimensions are a -14.248 (4), b = 8.244 (3), and c = 6.929 (3) A. The refinement led to a final R value of 0.025 based on 904 reflections. For bis(ij5-indenyl)dimethylhafnium(IV) the cell dimensions are a = 14.243 (6), b = 8.215 (4), and c = 6.918 (4) Á. The refinement led to a final R value of 0.031 based on 965 reflections. The effect of the lanthanide contraction is evident even with the cell parameters: that of hafnium has a volume of 5 Á3 less than that of zirconium. The molecules have crystallographically imposed twofold symmetry. In each case the metal-carbon -bond length is 0.2-0.3 Á shorter than the metal-carbon distances for the 5 ligands. Although the Hf-C(n5) approach (2.53 Á average) is closer than with the Zr analog (2.55 A average), the Hf-C(a) length (2.332 (12) Á) is significantly longer than the Zr-C(a) distance (2.251 (6) Á). The values for the titanium complex are Ti-C(?;5) = 2.44 A (average) and Tí-C(ct) = 2.21 (2) A.

The biogeochemical processes controlling the speciation and transport of manganese in a Colorado mountain stream were studied using a conservative tracer approach combined with laboratory experiments. The study stream, Lake Fork Creek, receives manganese-rich inflows from a wetland contaminated by acid mine drainage. A conservative tracer experiment was conducted on a 1300-m reach of the stream. Results indicate that manganese was progressively removed from the stream, resulting in a loss of 64 +/- 17 micromol day(-1) m(-1). Laboratory experiments using streamwater, mine effluent, and rocks from the stream showed the importance of surface-catalyzed oxidation and photoreduction on the speciation of manganese. The field and modeling results indicate that light generally promotes oxidation and removal of manganese from the stream, presumably through a photosynthetically enhanced oxidation process. Differences in Mn speciation within the stream suggest that reductive processes affect Mn speciation within the water column. These results identify the rapid oxidation and precipitation of MnOx as a dominant process within this freshwater stream.

No abstract

An annual study of a natural wetland receiving drainage with high concentrations of iron, zinc, and manganese from an abandoned mine tunnel was conducted. During summer, the wetland reduced the mass flow from the tunnel to a receiving stream by more than 90% for iron, by 65% for zinc, and by 25% for manganese. Plant uptake accounted for less than 1% of the total retention. Zinc and manganese mass flows to the stream were greater than mass flows from the tunnel during autumn and winter, indicating a seasonal cycle of the wetland acting as a net sink in the summer and then a net source in the winter. Iron mass flow to the stream was less than the iron mass flow from the tunnel for the majority of the year, with the high retention of iron in the wetland associated with large deposits of iron oxides, especially thick in the upper reaches of the wetland. Spring snowmelt saturated the otherwise dry material in the mine tailings above the wetland, creating an additional source of inflow, with metal concentrations two times higher and three pH units lower than water flowing from the mine tunnel. This period of runoff from the tailings, with the associated high concentration of metals in the stream below the wetland, represented a critical time for stream biota.

Monomers consisting of N-alkyl-su bstituted-2,5-bis(2-thienyl)pyrroles and their electro-oxidatively generated polymers display electrochemical and optical properties which reflect the steric demands of the alkyl substituents.-

A n electroactive polymer with an energy gap of 1.4 eV has been obtained from the monomer 2,5-di(2-t hienyl)pyridino[c]thiophene.