The com pou nds [ Os, ( CO ) ,,, ( q4 -scis-C, H J] ( C, H = b uta -1,3 -d iene) , [ Os, ( CO) ( p-s-trans-C, H J 3, and [Os,(CO),,( MeCN),] react with PMe,Ph at room temperature t o give [Os,(CO),,( PMe,Ph),] and [Os,(CO),( PMe,Ph),] as mixtures of isomers. The decacarbonyl compound separates on silica into a 1,land a 1,2-isomer, which d o not interconvert at room temperature, although the 1,2isomer exists as t w o rapidly interconverting isomers of C, and Czv symmetry respectively. Likewise