IntroductionOne of the objectives of crystal engineering is to order molecules into controllable and reproducible crystal lattices. If, during the crystallisation process more than one crystal lattice is energetically accessible, more than one structure, or polymorph, may form. The formation of one structural type rather than another can result in markedly different physical and/or chemical properties. This is particularly relevant for compounds with biological activity, hence the extensive research into polymorphism in pharmacologically active organic compounds.1This research has shown that the conditions of crystallisation may determine the polymorph formed. Polymorphs may be obtained independently, employing the same conditions, or two different structural types may obtained from the same crystallisation. In the latter case, these are termed ‘concomitant polymorphs’.2Where the molecular structure of two forms is identical,but the crystal lattices differ by the nature and/or degree of solvation, then the structures may be termed ‘pseudo-polymorphs’.3Polymorphism is also prevalent in organometallic systems, with a higher proportion of polymorphic organometallic structures in the Cambridge Structural Database than for purely organic crystals.4This statistic may be attributed to the flexibility in metal–ligand bonding modes (e.g.conformational variability). In addition, many organometallic complexes are charged so that their crystal structures must also accommodate one or more counterions. Previous studies on the geometric characteristics of organometallic and inorganic ions concluded that solid salts are more readily isolated if the component ions have complementary size and shape.5Here we report the series of organometallic salts [ML2]2[{M′(mnt)2}2] {M = Co, L = Cp; M = Fe or Co, L = Cp* (where Cp* = η-C5Me5); M′ = Fe or Co; mnt = [S2C2(CN)2]2−}. This study investigates the effects of cation size, metal d-electron configuration and the packing ofpseudo-spherical cations and ellipsoidal dianions. In addition, by comparing salts of [CoCp2]2[{M′(mnt)2}2] (M′ = Fe1or Co2) with those of [FeCp*2]2[{M′(mnt)2}2] [M′ = Fe (whichgives the polymorphs α-3and β-3), or Co (which gives the polymorph α-4and the dichloromethane-solvatedpseudo-polymorph4S2)], the effect of cation metal and size on the crystal lattice is investigated. Three further crystal forms of the salts studied here, namely [FeCp*2]2[{Co(mnt)2}2] γ-4, and the twopseudo-polymorphs [FeCp*2]2[{M′(mnt)2}2]·MeCN (M′ = Fe3S1or Co4S1) are known.6