1-Phenyl-2,3,4,5-tetramethylphosphole reacts with [Mo(CO)(6)] to give the mono- and bis(phosphole) complexes [Mo(CO)(5)L] and cis- and trans-[Mo(CO)(4)L-2], where L=C4Me5PPh (tmpPh). The new complexes have been characterised by spectroscopic methods, supplemented by single-crystal X-ray analyses in the case of the bis(phosphole) isomers. The isomeric bis-tmpPh complexes show little evidence of the overcrowding which is thought to lengthen bonds and distort the metal coordination in the corresponding bis-PPh3 and bis-PCy3 isomers. The structural basis for the greater stability of the trans isomers is briefly considered: the cis –> trans isomerisation typically involves little change in the mean Mo-CO distance, whereas there is a marked shortening of the Mo-P bonds. (C) 2001 Elsevier Science B.V. All rights reserved.