In the active site of the enzymes manganese superoxide dismutase and manganese lipoxygenase, the Mn-IIIOH/Mn-IIOH2 redox couple enables proton-coupled electron transfer (PCET) reactions. Amino acids hydrogen bond to the hydroxo ligand, but the functional importance of this interaction is unclear. Previously, we studied the role that hydrogen bonding plays in PCET reactions for two synthetic model complexes, [MnIII (OH)(PaPy2N)] + and [MnIII(OH)(PaPy2Q)] +. In the [MnIII (OH)(PaPy2N)] + complex the hydroxo ligand hydrogen bonds with a naphthyridine while the [MnIII (OH)(PaPy2Q)] + lacks this hydrogen bond. We found that hydrogen bonding enhances the rate of reaction for PCET between MnIII-hydroxo complexes and different substrates. In this study we explore the effect of hydrogen bonding on the basicity of the MnIII-hydroxo complexes. Titrations of MnIII-aqua complexes revealed that [MnIII(OH)(PaPy2N)] + is more basic than [MnIII(OH)(PaPy2Q)] +, which leads to a larger driving force for PCET reactions. A thermochemical analysis showed that for less acidic substrates there is a linear correlation between the driving force and rate of reaction, while more acidic substrates deviate from the trend. We propose that the deviation from the trends is caused by a change in mechanism from a concerted proton electron transfer (CPET) to a proton transfer (PT).