The crystal structure of a new magnesium(II) complex, [Mg(dmso)₆][BPh₄]₂ (1) (dmso: dimethylsulfoxide), was determined, and the reason for the observed structure was clarified by conformational analysis. For a dmso-ligand arm, three conformations, α, β, and γ, are possible. The α-arm is the most energetically favourable and is suitable for reducing the steric repulsion between the arms; the β-arm is less energetically favourable, but can be stabilized by interaction with surroundings (e.g. CH⋯π interaction); the γ-arm is not energetically favourable, but is effective in reducing the size of the complex cation. From the conformational analysis, the most stable conformer of the [Mg(dmso)₆]²⁺ complex cation was found to be the α₆ conformer, and the complex cation in dmso solution was predicted to exist as a mixture of α₆, α₅β, and trans-α₄β₂ species. On the contrary, in the crystal structure, the trans-β₂γ₄ species, considered to be unstable, was observed. From the conformational analysis in the tetraphenylborate surroundings, the trans-β₂γ₄ structure was found to become more stable, due to its small size suitable for crystal packing with bulky tetraphenylborate anions.