Manganese(II) complexes of mononegative pentadentate N4O ligands [Mn2(mgbpen)(2)(H2O)(2)](ClO4)(2) (1), (mgbpen(-) = N-methyl-N'-glycyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine) and [Mn-2(bzgbpen)(2)(H2O)(2)](ClO4)(2) ( 2), (bzgbpen(-)=N-benzyl-N'-glycyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine) have been prepared. The crystal structure of the Mn(II)-aqua complex of 1, shows it to be dimeric via (mu-kappaO)-bridging through one carboxylate oxygen atom of each of the two ligands. The non-coordinated carboxylate oxygen atoms are H-bonded to the water ligands on the adjacent Mn ion. The magnetic coupling interaction is weak and antiferromagnetic, J = -1.3(1) cm(-1). The dimeric structures of 1 and 2 are retained in solution and can exist in the gas phase. Complexes 1 and 2 are air stable but can be oxidised by (BuOOH)-Bu-t to give unstable mononuclear Mn(III) complexes, or oxo-bridged dimanganese(III) and di-mu-oxo-dimanganese(II) complexes, depending on solvent. The [Mn(III)-OR](+), R = H or CH3 complexes are generated in water or methanol, respectively, and are potentially useful spectroscopic models for active Mn-lipoxygenases. In acetonitrile, di-mu-oxo-dimanganese(II) complexes are the highest oxidation state products detected, and these are formed via shorter-lived intermediate mu-oxo-dimanganese(III) compounds. The rate of formation of the various oxidized products is slower in the case of the bzgbpen(-) systems which contains a bulkier non-coordinating arm. The oxidised complexes were characterised by UV-visible spectroscopy, ESI mass spectrometry and cyclic voltammetry. In addition, III-IV and II-III species were electrochemically generated. Thus the new mononegative pentadentate ligand systems display significant flexibility in the range of Mn oxidation states and species of biological relevance that are accessible: A series of dinuclear compounds with different structures in the five oxidation levels between II-II and IV-IV has been identified. No solid state structures were obtained for high oxidation state species, however it is assumed that in the oxo-bridged compounds the carboxylate groups are terminally ligated in contrast to the starting Mn(II) complexes. Thus the system represents examples of limiting structures in the "carboxylate shift" mechanism proposed to be important in non-heme H2O and O-2 activation processes.