REACTIVITY OF THE M-M MULTIPLE BOND IN METAL-CARBONYL DERIVATIVES .12. SYNTHESIS AND STRUCTURE OF [(C5ME5)2M2SE5] - FORMATION OF A MU,ETA-2-TETRASELENIDE LIGAND AT THE M-M DOUBLE-BOND OF [(C5ME5)2CO2(MU-CO)2], [(C5ME5)2RH2(MU-CO)2] Journal Article