Isomerism of Disubstituted Manganese Pentacarbonyl Bromide

Abstract

THE manganese pentacarbonyl halides react with a variety of ligands of the type PR₃ and P(OR)₃ to give mono- and di-substituted products, Mn(CO)₄LX and Mn(CO)₃L₂X ^1,2. The reaction of 0.2 g Mn(CO)₅Br with 1.1 g P(OC₆H₅)₃ in chloroform at 37° for 12 h yields what is believed to be 1,2-bis(triphenylphosphite)-4,5,6-tricarbonyl-3-bromo-manganese(I)³, in which the P(OC₆H₅)₃ groups are cis to the Br as well as to each other (the ‘cis-isomer’). However, reaction at 55° produces 1,6-bis(triphenylphosphite)-2,4,5-tricarbonyl-3-bromomanganese(I), in which the P(OC₆H₅)₃ groups are cis to the Br but trans to each other (the ‘trans-isomer’). Both isomers can be precipitated from chloroform with n-hexane and analyse correctly for the above compounds. The dipole moments obtained in benzene by an approximate method⁴ were 4.8D for the cis-isomer and 3.5Dfor the trans, while Mn(CO)⁵Br itself had a moment of 3.4D. These measurements confirm the assignment of the trans structure, but do not distinguish between the possible isomers in which the phosphine ligands are (a) both cis to the bromide, or (b) one cis and one trans to the bromide. An attempt has been made to distinguish these by means of the nuclear magnetic resonance of phosphorus which should show splittings in case (b) owing to the different environments of the two ligands. Although resonance was observed, the resolution was poor and no conclusion could be reached on these grounds. (We thank Dr. J. Verkade and Dr. R. King, Iowa State University, for making these measurements.) However, in view of the known inertness⁵ of the CO trans to the bromide in Mn(CO)₅Br, we feel that alternative (b) is unlikely.