Olefin reorientation has been detected in a series of Pt acac Cl(olefin) complexes by n.m.r. Rates have been measured, and the temperature-dependence indicates energy barriers in the range 10–15 kcal./mole. The observed n.m.r. spectra and calculated energy barriers are consistent with an intramolecular rotation of the olefin about the platinum–olefin axis. The stable orientation of the olefin is shown to be perpendicular to the square plane, which is consistent with some metal d_π–p_π hybridization.