Standard oxidation-reduction potentials for the copper complexes of 2,9dimethyl-and 2-chloro-1,lO-phenanthroline have been determined, and the relative contributions of enthalpy and entropy to these potentials have been assessed. Thermodynamic stability constants for the 1 : 1 and 1 : 2 copper(I1) and zinc complexes, and the 1 : 1 iron(r1) complex, with 2,9-dimethyl-l,10phenanthroline suggest that in 1 : 1 complexes the 2-and thc 9-methyl groups interfere sterically with water molecules occupying ahy co-ordination sites lying in the same plane around the metal ion.
The crystal structure of tetraammine{(�)-pentane-2,4-diamine}cobalt(III) dithionate dihydrate [Co{(�)-ptn}(NH3)4]
(S2O6)1.5,2H2O has been determined
by X-ray structure analysis. The crystals are triclinic with the space group Pī, with a
11.43, b 11.65, c 7.78 �, α 100.48�, β 105.95�, and γ 75.95�. The
structure has been refined by least-squares methods with anisotropic
temperature factors to an R value of
0.043 for 2654 reflections. The six-membered diamine
chelate ring has a skew-boat conformation in which the two methyl groups are in
equatorial orientations. A strain- energy-minimization calculation for the
complex reveals an energy difference of 8.3 kJ mol-l between the
preferred skew-boat conformation and the chair conformation. The calculated
minimum-energy geometry for the skew-boat conformation is in good agreement
with the observed structure for the chelate ring in the dithionate salt. The lH
N.M.R. spectra of this complex and other complexes of the chiral isomer of
pentane-2,4-diamine are discussed in terms of the observed structures and the
conformational analysis.
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