A theoretical study of the cyclodimerization of (Cy3P)(2)Pt(BO)Br (1Br) and [(Cy3P)(2)Pt(BO)](+) (1) (Cy = cyclohexyl) suggests that the reactivity of the BO ligand is primarily controlled by M BO sigma donation. Therefore, increasing the electron density at the metal center through strong sigma-donor and weak pi-acceptor ancillary ligands and a low formal metal oxidation state are suggested to reduce the polarity of the boronyl ligand and thus lower its reactivity toward cyclodimerization. The stable 1Br has lower Pt BO sigma donation and thus a less electrophilic boron atom, leading to a less polarized BO ligand. However, 1 is unstable in dichloromethane, since the dicationic dimer and transition state are highly stabilized by strong electrostatic interactions.