IntroductionIn previous papers, we have reported how crystal structures of transition metal complex salts vary with stoichiometry and solvation. The formation of stable crystal structures of such salts requires an effective combination of topologically dissimilar molecules or ions. Consequently, the salts containing [M(mnt)2]z−anions (M = Ni, Pd or Pt;z= 1 or 2; mnt = [S2C2(CN)2]2−), and either square planar [Pt(CNMe)4]2+cations1or redox-active cubane cluster cations [Fe4(μ3-S)4Cp4]z+(z= 1 or 2)2exhibit a variety of structural motifs. Investigating the series of 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] showed a range of energetically accessible structural types (polymorphs andpseudo-polymorphs), with the lattice formed dependent on the crystallisation conditions.3We have now extended these studies to investigate the effect of ion size and shape on the structure of transition metal complex salts by combiningpseudo-spherical metallocenium cations with square planar perhalophenylplatinate anions.Metallocenes, in particular the decamethylferrocenium ion, have been widely incorporated in low dimensional solids. Moreover linear chain organometallic electron-transfer salts based on the donor [FeCp*2] (Cp* = η-C5Me5) and a variety of planar organic electron acceptors comprise a well characterised class of molecular solids exhibiting ferromagnetic behaviour.4The use of ferrocenium ions in low dimensional compounds is limited in comparison with the equivalent decamethylferrocenium salts mainly due to the higher oxidation potential of [FeCp2].Although many platinum aryl complexes are known, very few have been used in the study of charge-transfer salts or anisotropic solids. Usón and Forniés have prepared a comprehensive series of tetrakis- and tris(perhalophenyl) derivatives of Ni,5Pd6and Pt6,7of the general form [NR4]x[M(C6X5)y] (x= 1 or 2, X = F or Cl,y= 3 or 4) by the reaction of metal chloride salts with aryl lithium or Grignard reagents.The studies described in this paper investigate the effect of combining the metallocenium cations, [ML2]+[M = Co, L = Cp, Cp* or Ind (Ind = η-C9H7); M = Fe, L = Cp*], with perhalophenyl platinum complex anions, [Pt(C6F5)4]2−, [Pt(C6Cl5)4]2−, [Pt(C6Cl5)4]−and [PtCl2(C6Cl5)2]2−. A comparison of salts containing iron metallocenes with those containing cobalt shows the effect of the metal centre of the cation on the crystal structure, whilst oxidation of the Pt(ii) complex [Pt(C6Cl5)4]2−to the Pt(iii) monoanion shows the effect of the charge of the anion (and hence stoichiometry) on the crystal structure. Changing the carbocyclic ligand of the metallocenium cation (e.g.Cpvs.Cp*vs.Ind) shows the effect of cation size and shape on the crystal structure, whilst a comparison of perfluorophenyl platinum complexes with perchlorophenyl analogues demonstrates the effect of anion size.