Four dinuclear Co II complexes have been synthesized and structurally characterized with the dinucleating ligand susan (susan = 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10tetraazadecane) varying in the exogeneous ligands:-{Co II Br} 2 ](ClO 4 ) 2 , and [(susan){Co II (μ-OH)Co II }](ClO 4 ) 3 . The Co II ions are six-coordinate with CH 3 CN ligands and five-coordinate with anionic ligands. The electronic absorption spectra reflect the differences in the electronic structures, not only between the six-and five-coordinate complexes, but also between the fivecoordinate complexes. These variations are also reflected in the magnetic properties with the highest orbital angular momen-tum contribution in six-coordinate [(susan){Co II (CH 3 CN) 2 } 2 ](PF 6 ) 4 . The lower symmetry in the five-coordinate complexes reduces the orbital angular momentum contribution. The hydroxobridged complex [(susan){Co II (μ-OH)Co II }](ClO 4 ) 3 exhibits an additional antiferromagnetic interaction. The electrochemical characterization reveals that the π-acceptor ligand CH 3 CN facilitates not only reduction from Co II to Co I but also oxidation to Co III , while the π-donor ligands Br À , Cl À , and μ-OH À impede both reduction to Co I and oxidation to Co III . [(susan){Co II -(CH 3 CN) 2 } 2 ](PF 6 ) 4 and [(susan){Co II Cl} 2 ](ClO 4 ) 2 show electrocatalytic proton reduction at a potential of � À 1.9 V vs Fc + /Fc that is associated with a ligand-centered reduction.
Two new silicon/phosphorus-based frustrated Lewis pairs (FLP), F 3 SiCH 2 PtBu 2 (1) and Cl 3 SiCH 2 PtBu 2 (2) were prepared from lithiated di-tert-butylmethylphosphane, LiCH 2 PtBu 2 , and the corresponding silicon tetrahalides. They were characterised by NMR spectroscopy and by single-crystal X-ray diffraction. A gas phase electron diffraction study of 1 identified two conformers of similar energy in the vapor. The reactivity of both, 1 and 2, towards a series of small molecules was investi-
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