The red, five-coordinate complexes Ru(CO)Cl(PPh3)(2)(CH=CHPh) and [Ru(CO)Cl(PPh3)(2)](2)(mu-CH=CHC6H4CH=CH) undergo reversible coordination of PPh3 at low temperature to produce the pale yellow, six-coordinate complexes Ru(CO)Cl(PPh3)(3)(CH=CHPh) and [Ru(CO)Cl(PPh3)(3)](2)(mu-CH=CHC6H4CH=CH). X-ray crystal structures of the latter complex and of the hydride complex RuH(CO)Cl(PPh3)(3) were obtained. H-1 and P-31 NMR spectra between 20 and -70 degrees C exhibit large changes in both equilibrium constants and dynamic effects. Thermodynamic parameters, Delta H = -17.5 +/- 2.0 kcal/mol and Delta S = -57.5 +/- 7.6 eu, were obtained for PPh3 coordination to the monoruthenium complex, and activation parameters, Delta H = 20.6 +/-0.7 kcal/mol and Delta S = 41.6 +/- 2.0 eu, were obtained for the reverse decoordination. Coordination of PPh3 was not observed upon cooling of the shorter bridged complex, [Ru(CO)Cl(PPh3)(2)](2)(mu-CH=CHCH=CH).