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 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.

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.