A NOVEL REACTION BETWEEN [μ-PhC(O)S]-(μ-RS)Fe 2 (CO) 6 AND (μ-R 1 S)(μ-XMgS)Fe 2 (CO) 6 ——THE SYNTHESIS, CHARACTERIZATION AND FORMATION MECHANISM OF TWIN CLUSTER COMPLEXES [(μ-RS)Fe 2 (CO) 6 ](μ 4 -S)[(μ-R 1 S)Fe 2 (CO) 6 ]