Polymorphism and pseudo-polymorphism in the crystal structures of [ML2]2[{M (mnt)2}2] (M = Fe or Co, L = Cp or Cp*)

The salts [ML2 ] (2) [{M' (mnt) (2) } (2) ] {M = Co, L = Cp; M = Fe or Co, L = Cp* (Cp* = (eta)-C5Me5); M-7 = Fe or Co; mnt = [S2C2 (CN) (2) ] (2-)} have been prepared and structurally characterised to investigate the effects of cation size, metal d-electron configuration and the packing of pseudo-spherical cations with ellipsoidal dianions. By comparing the structures of [CoCp2] (2) [{M-7 (mnt) (2) } (2) ] (M-7 = Fe 1 or Co 2) with those of [FeCp*(2)] (2) [{M-7 (mnt) 2} 2] (M = Fe 3 or Co 4) the effect of cation size and magnetic character on the crystal lattice is investigated. Two polymorphs of 3 were identified, alpha-3 and beta-3, both of which crystallise in the space group P2(1)/n. The structures of 1, 2, and alpha-3 are similar with columns of anion dimers separated by cations lying perpendicular to the anion stacks, whereas beta-3 comprises mixed, face-to-face cation cation anion stacks. Two forms of 4 were characterised, namely the polymorph alpha-4, which is isostructural with alpha-3, and the dichloromethane-solvated pseudo-polymorph 4S2. The structure of 4S2 is analogous to that of beta-3 in comprising face-to-face stacks of mixed ions. Three related structures are known, namely [FeCp*(2)] (2) [{Co( mnt) (2) } (2) ] [denoted gamma-4 (CSD refcode ZAXJAJ)] and the two pseudo-polymorphs [FeCp*(2) ] (2) [{M' (mnt) (2) } (2) ] MeCN [M' = Fe 3S1 (ZAXJEN) or Co 4S1 (ZAXHUB)]. These are included for comparison with the structures determined here. The packing coefficients, k, varied in the range 0.59(3) (for 4S2) to 0.73(1) (for beta-3).