The reaction of [Pt3(µ-CO)3(PCy3)3] (1) with one mole-equivalent of iodo-acetonitrile was quantitative at â 70 °C giving the oxidative addition product [Pt3(µ-CO)3(PCy3)3(I)(CH2CN)] (2). Fragmentation of 2 was observed in solution giving [Pt2I(CH2CN)(CO)2(PCy3)2] (3) which is the major product at room temperature if the starting cluster/reactant ratio is equal to or less than 1 to 1.5. Dimer 3 decomposes slowly in solution giving [Pt2I2(CO)2(PCy3)2] ( 4a) and succinonitrile. Monomer [PtI(CH2CN)(CO)(PCy3)] was the final product of the reaction when using excess of iodo-acetonitrile. The reactions of 1 with one mole-equivalent of halogens X2 gave the new 44-electron clusters [Pt3X(µ-CO)2(µ-X)(PCy3)3] (X = I2 ( 7a) or Br2 ( 7b)) by oxidative addition followed by substitution of CO by X^â. Fragmentation of 7a and 7b took place in solution when using one and a half mole-equivalents of X2 giving dimers 4a and [Pt2Br2(CO)2(PCy3)2] ( 4b) as well as [Pt2X2(µ-X)2(CO)2(PCy3)2]. Monomers cis-[PtX2(CO)(PCy3)] were the final products of the reaction of 1 with excess of halogens. Insertion of SnCl2 was observed into the Ptâ Pt bond but not into the Ptâ X bond, when equimolar amounts of SnCl2·2H2O were added to a solution of 4a or its chloro-analogue giving [Pt2X2(µ-SnCl2)(CO)2(PCy3)2]. The Pt(i) dimers have unusually small J(Ptâ Pt) values as observed by ¹⁹⁵Pt NMR and calculated by DFT. These values showed periodic changes comparing 4a and its analogues with other halides and mixed halide dimers.