Yamamoto and Yama~aki.~~,~ Varying yields of iminoacyl and acyl products were observed with the maximum yield of iminbacyl when X = C1. Moreover, it was demonstrated that the isocyanide insertion product arose via some route not involving alkyl migration from CO to isocyanide since it could not be obtained by heating the acyl. This behavior is in contrast to the usual mode of reaction of molybdenum alkyls with nucleophilic regents L (L = phosphines,loa*b t-C4H9NC,*OC 2,6-(CH3)2C6H3NC)'oC leading exclusively to $-(C5H5)Mo(CO),LC(O)R and resembles (at least superficially) the reaction with electrophilic sulfur dioxide which leads exclusively to v~-(C~H,)MO(CO)~S(O)~R,~~ the product of direct insertion into the metal-alkyl bond. As part of a program of research involving isocyanides and their reactions with transition-metal species and because of our interest in insertion reactions we have investigated the behavior of some alkyl manganese pentacarbonyl complexes with isocyanides in order to ascertain whether direct insertion to afford iminoacyls or carbonylation to afford isocyanidesubstituted acyls would occur. Similar reactions were previously investigated by Joshi, Pauson and Stubbs12 who found that methyl- and phenylpentacarbonylmanganese were reduced to substitution products of dimanganese decacarbonyl by phenyl isocyanide in refluxing tetrahydrofuran. Our results, which differ considerably from theirs, are reported here. In particular, acyl complexes are found to be reaction products at room temperature. We also report some preliminary studies on attempts to achieve alkyl migration onto isocyanides coordinated to Mn.