The structures of two binary metal carbonyls containing a metal-metal bond Mn2(CO)10 and Re2(CO)10 will be studied as a function of pressure. Both these molecules undergo a phase transition at relatively modest pressures. In Mn2(CO)10 the two Mn(CO)5 pyramids are displaced to produce a short Mn-Mn bond. Our objectives are: 1 To determine accurate metrical parameters for the high pressure phase of Mn2(CO)10, and how the unusual ?displaced? metal ?metal bond responds to pressure. 2 To determine the crystal structure (presently unknown) and behaviour the high pressure phase of Re2(CO)10. 3 Using our accurately determined structures characterize the relative strengths of unsupported (no bridging carbonyl groups) for first and third row transition metals as a function of pressure and hence their relative reactivities. 4 .Obtain compressibility data on the high and low pressure phases.