The thermodynamics of ligand photorelease from Ru(II)bis(2,2'-bipyridine)(6,6'-dimethy1-2,2'-bipyridine) ( [Ru(bpy)2(dmbpy)](2+)) complex in aqueous solution have been examined using photoacoustic calorimetry (PAC). Photolysis of the [Ru(bpy)2(dmbpy)](2+) complex with a 5 ns laser pulse (FWHM) results in enthalpy changes of 42 +/- 4 kcal mol(-1), with a corresponding volume change of 6 +/- 1 mL mol(-1). Density functional theory calculations also demonstrate that the positive enthalpy values arise from N a-donation to the Ru ion. It is noted that the LUMO-Ligand Field gap fails to track with photoaquation quantum yield. (C) 2014 Elsevier B.V. All rights reserved.