The structures of three linear-chain platinum(II) diimine complexes have been determined [Pt ... Pt, Angstrom] : Pt(bpm)Cl-2 . 0.5(nmp) (3) [3.411(1), 3.371(1)], Pt(phen)(CN)(2) (6) [3.338(1), 3.332(1)], and Pt(bpy)(NCS)(2) (7) [3.299(2)] (bpm = 2,2’-bipyrimidine, phen = 1,10-phenanthroline, bpy = 2,2’-bipyridine, nmp = 1-methyl-2-pyrrolidinone). The Pt Pt distances in these and in seven related compounds range from 3.24 to 3.49 Angstrom. While we find evidence of interligand interactions influencing these structures, the Pt ... Pt bonds are the most important of the stacking forces. The metal-metal distances are generally consistent with an electronic structural model in which sigma-donor/pi-acceptor ligands strengthen Pt ... Pt bonding interactions (for example, the Pt ... Pt distances in 3 are 0.04 and 0.08 Angstrom shorter than in the bpy analogue). We have also found that the yellow form of Pt(dmbpy)(NCO)(2) (1b) (4,4’-dimethyl-2,2’-bipyridine) has a columnar structure; however, in contrast to the linear-chain form (1), which is orange, the Pt atoms are well separated (>4.9 Angstrom). Interestingly, the yellow form is 7% denser than the orange form; this result is consistent with the concept that directed intermolecular interactions give rise to lower density polymorphs. Crystal data : (3) monoclinic, C2/m (No. 12), a = 12.668(4) Angstrom, b = 15.618(6) Angstrom, c = 6.704(3) Angstrom, beta = 93.43(3)degrees, Z = 4; (6) orthorhombic, Pbca (No. 61), a 38.731(13) Angstrom, b = 18.569(3) Angstrom, c = 6.628(1) Angstrom, Z = 16; (7) orthorhombic, Pbcm (No. 57), a = 10.349(3) Angstrom, b = 19.927(5) Angstrom, c = 6.572(3) Angstrom, Z = 4; (Ib) monoclinic, C2/c (No. 15), a = 17.313(4) Angstrom, b = 12.263(3) Angstrom, c = 14.291(4) Angstrom, beta = 114.00(2)degrees, Z = 8.