A molecular mechanics analysis of [Co(medien)(subscript n)(dien)₂₋(subscript n)]³⁺ {n=0-2} (dien = diethylenetriamine or 1,5-diamino-3-azapentane; medien = 3-methyl-1,5-diamino-3 azapentane), and X-ray structural studies of the mer- and s-fac-[Co(medien)(dien)]³⁺ cations

Crystals of mer-[Co(medien)(dien)]Br₂(ClO₄)·H₂O (1 crystallize in the monoclinic space group P21/n with unit cell dimensions a=12.206(2), b=12.744(1), c=12.712(2) Å, β=94.99(1)°, V=1969.9 ų, Z=4; crystals of s-fac- [Co(medien)(dien)] (S₂O₆)₁.₅·0.5H₂O (2) are monoclinic, space group P2₁ with a=9.772(3), b=10.512(2), c=19.263(4) Å, β=92.24(2)°, V=2014.2 ų, Z=4. The structures were refined by a full-matrix least-squares procedures in each case. At convergence, final R0.065, Rw0.067 for 2578 reflections with I⩾2.5σ (I for (1) and R0.061, Rw0.061 for 2845 reflections for (2). A molecular mechanics study of the isomers of the bis (tridentate)cobalt(III) complexes [Co(medien)(subscript n)(dien)₂₋(subscript n)]³⁺,n=0−2, (dien = diethylenetriamine or 1,5-diamino-3-azapentane; medien = 3-methyl-1,5-diamino-3-azapentane) is reported. For the [Co(dien)₂]³⁺ isomers, and the mer and s-fac isomers of [Co(medien)(dien)]³⁺, the structural details are satisfactorily modelled by MM2. For both the [Co(dien)₂]³⁺ and [Co(medien)(dien)]³⁺ systems, the MM2 procedure sbustantially overestimates the stability of the mer geometric isomers relative to the fac forms, although the relative stabilities of the s-fac/ u-fac forms correspond well with the observed isomer ratios. The calculated isomers ratios for the [Co(medien)₂]³⁺ system agree with those found experimentally. The apparent variance in the capability of the MM2 procedure to predict the relative stabilities of the isomers in the three analogous systems is discussed