SYNTHESIS, CHARACTERIZATION AND REACTIONS OF THE TETRAHEDRAL MCoFeS CLUSTERS CONTAINING A FORMYLCYCLOPENTADIENYL LIGAND η 5 -CHOC 5 H 4 MCoFeS(CO) 8 (M=Mo, W). THE SINGLE CRYSTAL STRUCTURE OF η 5 -CHOC 5 H 4 WCoFeS(CO) 8