Mercury(II) exhibits a strong preference for linear coordination which has been attributed to relativistic effects splitting the 6p orbitals and promoting sp hybridization. If the two ligands attached to the mercury(II) ion are weak donors, the metal ion can act as a good Lewis acid and expand its coordination number. Moreover, mercury has a special affinity for softer bases, such as S and N atoms, and has much less affinity for hard bases, such as those including an O atom. The asymmetric unit of dichlorido[tris(piperidin-1-yl)phosphane oxide-κO]mercury(II)-dichloridomercury(II) (2/1), [HgCl2{(C5H10N)3PO}]2·[HgCl2], is composed of one HgCl2{(C5H10N)3PO} complex and one half of a discrete HgCl2 entity located on an inversion centre. The coordination environment around the Hg(II) centre in the complex component is a distorted T-shape. Bond-valence-sum calculations confirm the three-coordination mode of the Hg(II) atom of the complex molecule. The noncovalent nature of the Hg...Cl and Hg...O interactions in the structure are discussed.
Keywords: bond-valence model; crystal structure; geometry index; linear coordination; mercury(II) chloride; three-coordinate complex.