New type isomers that differ in metal-metal bond type are isolated. One is the Pt-Hg-Pt complex, which has two metal-metal covalent bonds, Pt-Hg = 2.5459(8) and 2.5483(7) angstrom, and the other is the Pt -> Hg -> Pt complex, which has one metal-metal covalent bond, Pt-Hg = 2.5744(6) angstrom, and one metal-metal dative bond, Pt -> Hg = 2.6635(7) angstrom.