The complex [Co( acac )2(NO2acac)] has been isolated after fractional crystallization from a reaction mixture of [Co( acac )3] and nitrating agent. The complex crystallizes in the triclinic space group P1, with c 14.557(2) � , α 78.98(1), β 83.71(2), γ75.32(1)° and Z 2. The structure refined to a final R value of 0.074 for 2287 'observed' [I > 3 σ(I)] reflections. The structure consists of discrete complex molecules with the cobalt atom surrounded by six oxygen atoms at the corners of a distorted octahedron. The NO2 group is twisted at 50.7� to the chelate ring on which it is situated and has little influence on the geometry, with intrachelate and interchelate oxygen-oxygen separations essentially the same as those found in [Co( acac )3]. The 59Co n.m.r. spectra of the [Co( acac )2(NO2acac)] complex, as well as [Co( acac )3], [Co(NO2acac)3] and [Co( acac )(NO2acac)2], have been recorded, this resonance being particularly sensitive to substitution on they γcarbon atom of the acetylacetonato chelate ring. The 59CO relaxation rates for the [Co( acac )2(NO2acac)] and [Co( acac )(NO2acac)2] complexes are faster than that displayed by the symmetric [Co(NO2acac)3] complex, (1948 � 20, 2765 � 30 and 1696 � 20 s-1, respectively). The results obtained for the relaxation rates for these complexes are compared with those obtained for similar halide-substituted complexes [Co( acac )3-n( Xacac )n] � (X = Cl , Br, I; n = 0-3).
Two multisubstrate analogues of the transition state in the reaction catalysed by the enzyme GABA- transaminase (E.C. 2.6.1.19), sulfonic acid pyridoxal dervative , C10H16N2O5S (1) and carboxylic acid pyridoxal derivative, C13H18N2O4 (2), have been characterized by X-ray analyses of crystals of (1). HCl , (1).H2O and (2). HCl . In each structure, the nitrogen on the side chain is the donor in intramolecular hydrogen bonding. However, it is only in (2). HCl that this interaction is with the phenolic oxygen as postulated in the proposed transition state of the reaction catalysed by GABA- transaminase . For both structures of (1), on the other hand, this interaction is with the oxygen of the ring hydroxymethyl substituent, and results in a seven- membered ring. Conformational analysis indicates that both modes of hydrogen bonding may be present in the pyridoxal derivatives, although no quantitative assessment is possible at the MINDO/3 or MNDO levels. Simple classical potential energy calculations indicate significant structural differences between the lowest energy conformations of these compounds and the calculated transition state. However, conformations which match the key features of the transition state are also relatively low in energy.
The reaction of WSCl4 and Me3SiS2CNEt2 in tetrahydrofuran produces a green solid which yields WS(S2)(S2CNEt2)2, WO(S2)(S2CNEt2)2 and syn-W2O2( �-S)2(S2CNEt2)2 upon exposure to oxygen. Crystals of syn-W2O2( �-S)2(S2CNEt2)2 are monoclinic and belong to space group C2/c with a 33.880(4), b 7.012(1), c 18.072(1) � ,β 105.94(1)� and Z 8. Refinement on 2651 data measured with Cu K α: radiation converged at R 0.070. The complex possesses a syn-[W2O2(�-S)2]2+ core with a W-W bond [2.808(1) � ] and two terminal W-O bonds [1.66(2) and 1.68(2) � ]. The W-S(bridging) bond distances are in the range 2.319(5)-2.3346) � Each of the square-pyramidal tungsten atoms is further coordinated to a bidentate dithiocarbamate ligand with W-S bond distances ranging from 2.447(6) to 2.459(5) � .
The crystal structure of the acetone solvate of a mixed ligand tetranuclear nickel(II) complex involving the pyridine-2-methoxide ( pym ) and 0-ethylxanthate (exa) anions has been determined by X-ray diffraction methods. Crystals of [Ni (exa)( pym )]4.Me2CO are triclinic, space group Pī , a 13.825(2), b
15.606(3), c 12.288(2)Ǻ, α 101.16(2), β 101.38(1), γ 85.80(1)°, with Z 2; R and Rw were 0.056 and 0.057 respectively for 4,771 unique reflections for which I ≥ 3σ(I). The structure consists of tetranuclear cubane -like clusters with four nickel and four oxygen atoms arranged at alternate corners of a distorted cube. The discrete molecules of occluded acetone are located in cavities defined by the [Ni( exa )( pym )]4 units.
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