The diamagnetic complexes [Ru(tpm)(bqdi)(Cl)]ClO4 ([1]ClO4) (tpm = tris(1-pyrazolyl)methane, bqdi = o-benzoquinonediimine) and [Ru(tpm)(bqdi)(H2O)](ClO4)2 ([2](ClO4)2) have been synthesized. The valence state-sensitive bond distances of coordinated bqdi [C–N: 1.311(5)/1.322(5) Å in [1]ClO4; 1.316(7)/1.314(7) Å in molecule A and 1.315(6)/1.299(7) Å in molecule B of [2](ClO4)2] imply its fully oxidised quinonediimine (bqdi0) character. DFT calculations of 1 + confirm the (RuII–bqdi0) versus the antiferromagnetically coupled (RuIII–bqdiz−) alternative. The 1H NMR spectra of [1]ClO4 in different solvents show variations in chemical shift positions of the NH (bqdi) and CH (tpm) proton resonances due to their different degrees of acidity in different solvents. In CH3CN/0.1 mol dm−3 Et4NClO4, [1]ClO4 undergoes one reversible RuII ⇌ RuIII oxidation and two reductions, the reversible first electron uptake being bqdi based (bqdi0/bqdiz−). The electrogenerated paramagnetic species (RuIII–bqdi0)(1 2+) and (RuII–Qz−)(1) exhibit RuIII-type (1 2+: <g> = 2.211/Δg = 0.580) and radical-type (1: g = 1.988) EPR signals, respectively, as is confirmed by calculated spin densities (Ru: 0.767 in 1 2+, bqdi: 0.857 in 1). The aqua complex [2](ClO4)2 exhibits two one-electron oxidations at pH = 7, suggesting the formation of (RuIVzO) species. The electronic spectral features of 1 n (n = charge associated with the different redox states of the chloro complex: 2+, 1+, 0) in CH3CN and of 2 2+ in H2O have been interpreted based on the TD-DFT calculations. The application potential of the aqua complex 2 2+ as a pre-catalyst towards the epoxidation of olefins has been explored in the presence of the sacrificial oxidant PhI(OAc)2 in CH2Cl2 at 298 K, showing the desired selectivity with a wide variety of alkenes. DFT calculations based on styrene as the model substrate predict that the epoxidation reaction proceeds through a concerted transition state pathway. © 2013 The Royal Society of Chemistry.