The enantiomeric complexes ( + ) - and ( - )-C5H5Mn(COOCH3) (NO)P(C6H5)3 were prepared, firstly by a transesterification of the diastereomeric menthyl derivates ( + )- and (-) -C5H5Mn (COOC10H19) (NO)P(C6H5)3, secondly by OCH3⊖-addition to the cations of ( + )-and (-) - [C5H5Mn (CO) (NO) P (C6H5)3] [PF6]. The kinetics of the exchange of triphenylphosphine in C5H5Mn (COOCH3) (NO) P(CeH5) 3 were investigated by deuteration and monitored by IR spectroscopy. The rate of this exchange in benzene solution is equal to the rate of racemisation of (+) - and (-)-C5H5Mn (COOCH3) (NO) P(C6H5)3 in the same solvent. It is concluded that the rate determining step for the change in configuration at the asymmetric manganese atom is the dissociation of triphenylphosphine. In this reaction an achiral intermediate of relatively high stability is formed.