THE IMPORTANCE OF INTERMOLECULAR FORCES IN CRYSTAL-STRUCTURE ANALYSIS- SOLID-STATE STUDY OF THE DIAG-TO-LAT ISOMERIZATION OF (ETA(5)-C5H4ME)RE(CO)(2)BR-2

Single crystal X-ray structures of diag and lat-(eta(5)-C5H4Me)Re(CO)( 2)Br-2 have been determined. The diag form crystallizes in the triclin ic space group P (1) over bar, a = 6.751(2), b = 8.537(1), c = 9.758(1 ) Angstrom; alpha = 96.70(1), beta = 93.15(2), gamma = 104.96(2)degree s V= 534.8(2) Angstrom(3), Z= 2. The lat form is monoclinic, P2(1)/c, a =11.820(1), b = 7.133(1), c = 12.924(1) Angstrom; beta = 98.278(8)de grees V = 1078.2(2) Angstrom(3), Z = 4. The unit cell volumes (per Z) of the two isomers differ by ca 1%. Molecular modelling reveals an ene rgy difference between the two isomers of 1 kJ mol(-1) or less. The XR D powder diffraction patterns for the lat isomer produced by crystalli zation from solution or prepared by thermal isomerization are identica l. The evidence thus suggests that the solid state isomerization react ion is a novel example of a reaction which yields a molecular structur e determined by crystal packing forces.