Crystal structures of trans-acetyl-dicarbon-yl(η(5)-cyclo-penta-dien-yl)(di-methyl-phenyl-phosphane)molybdenum(II) and trans-acetyl-dicarbon-yl(η(5)-cyclo-penta-dien-yl)(ethyl-diphenyl-phosphane)molybdenum(II).

The title compounds, [Mo(C5H5)(COCH3)P(CH3)2(C6H5)(CO)2], (1), and [Mo(C5H5)(COCH3)P(C2H5)(C6H5)2)(CO)2], (2), have been prepared by phosphine-induced migratory insertion from [Mo(C5H5)(CO)3(CH3)]. Both complex mol-ecules exhibit a four-legged piano-stool geometry with trans-disposed carbonyl ligands along with Mo-P bond lengths and C-Mo-P angles that reflect the relative steric pressure of the respective phosphine ligand. The structure of compound (1) exhibits a layered arrangement parallel to (100).