Reaction of [Ru₆C(CO)₁₆]²⁻ with an excess of AgX (X = Cl, Br or I) affords heteronuclear clusters of formula [{Ru₆C(CO)₁₆Ag₂X}₂]²⁻ in 80% yield, which for X = I and X = Br/Cl were crystallographically characterised. The formation of the cluster was followed in solution using electrospray ionisation mass spectrometry (ESI-MS), which revealed the presence of a wide range of clusters with the general formula [{Ru₆C(CO)₁₆}_<i>x</i>Ag_<i>y</i>X_<i>z</i>]^{(2<i>x−y</i>+<i>z</i>)−} where <i>x</i> = 1 or 2, <i>y</i> = 1, 2, 3 or 4 and <i>z</i> = 0, 1 or 2. The high yield of the product despite the evident complicated solution speciation is attributed to selective crystallisation of the observed compound driving the equilibrium toward this product.