in n-heptane a mixt. of (C6Me6)Ru(C2H4)2 and an excess of (C5Me5)Rh(CO)2 was refluxed in heptane for 4 h under N2;; after evapd. in vacuo, the residue was subjected to column chromy. on SiO2 at -35°C; elem. anal. for the tetranuclear cluster;;