¼ Cl, Br, and I) in equimolar ratios in MeOH as solvent leads to the binuclear products 1 -3 (Scheme 1). The bridgesplitting reaction of the binuclear complex [{HgI 2 (bbtppy)} 2 ] (3c) by DMSO yields the mononuclear complex [HgI 2 · (bbtppy) (DMSO)] (3d) (Scheme 2). This bridge-splitting reaction can also be a method for the synthesis of mononuclear products. C-Coordination of the ylide and O-coordination of DMSO are demonstrated by a single-crystal X-ray-analysis of the mononuclear complex 3d. Characterization of the obtained compounds was also performed by means of elemental analysis and IR and 1 H-, 31 P-, and 13 C-NMR spectroscopy. A theoretical study of some Hg II complexes with phosphonium ylides is also reported.Introduction. -The coordination chemistry of the phosphonium ylides (¼ phosphoranes) of the type R 3 P þ ÀCH 2 À , is well known [1 -4]. Resonance-stabilized phosphonium ylides, particularly, the keto ylides, are also successfully used as ligands in organometallic and coordination chemistry owing to their accessibility and stability towards air and moisture [5] [6]. Juxtaposition of the keto group and the carbanion in the phosphonium ylides allows for the resonance delocalization of the ylide electron density providing additional stabilization to the ylide species. This so-called astabilization provides ylides with the potential to act as an ambidentate ligand and thus bond to an Hg II center through either the C-atom (see B) or O-atom (see A 1 and A 2 ). Although many bonding modes are possible for keto ylides [7], coordination through the C-atom is more predominant and observed with soft metal ions, e.g., Pd II , Pt II , Ag I , Hg II , Au I , and Au III [8] [9 -12], whereas O-coordination dominates when the metals involved are hard, e.g., Ti IV , Zr IV , and Hf IV [13].