Organometallic compounds play an important role in catalysis, and compounds with metal carbonyl groups have been studied at elevated pressure by vibrational spectroscopy, but corresponding diffraction studies are rare. We will investigate as a function of pressure the octahedral complexes Mn(CO)5R ( R = CH3 or Br) which are known to undergo pressure dependent phase transitions. Our objectives are: 1 to determine the structures of both compounds in their high pressure phases, 2 To show how these metrical parameters in these molecules change with pressure. 3. To obtain information of possible mechanistic pathways of the reactions of these molecules, by observing how the molecule deforms with pressure 4. To compare the effect of changing the electronic demand at these metals centres affects their response to pressure. 5. To obtain compressibility data on the high and low pressure phases.