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Proton-switchable access to seven-coordinate ONNO dicarboxamide and NNNN dicarboxamidate rhenium oxo complexes provides a platform for understanding thermodynamics and bonding in pentagonal bipyramidal complexes. 

When irradiated with blue light in the presence of a Lewis base (L), [CpW(CO) 3 ] 2 undergoes metal–metal bond cleavage followed by a disproportionation reaction to form [CpW(CO) 3 L] + and [CpW(CO) 3 ] − . Here, we show that in the presence of pyridinium tetrafluoroborate, [CpW(CO) 3 ] − reacts further to form a metal hydride complex CpW(CO) 3 H. The rection was monitored through in situ photo 1 H NMR spectroscopy experiments and the mechanism of light-driven hydride formation was investigated by determining quantum yields of formation. Quantum yields of formation of CpW(CO) 3 H correlate with I −1/2 (I = photon flux on our sample tube), indicating that the net disproportionation of [CpW(CO) 3 ] 2 to form the hydride precursor [CpW(CO) 3 ] − occurs primarily through a radical chain mechanism. 

Reduction of the cobalt(II) chloride complex, Ph₂B(^tBuIm)₂Co(THF)Cl (1) in the presence of^tBuN≡C affords the diamagnetic, square planar cobalt(I) complex Ph₂B(^tBuIm)₂Co(C≡N^tBu)₂(2). This is a rare example of a 16‐electron cobalt(I) complex that is structurally related to square planar noble metal complexes. Accordingly, the electronic structure of2, as calculated by DFT, reveals that the HOMO is largely d_z²in character. Complex2is readily oxidized to its cobalt(II) congener [Ph₂B(^tBuIm)₂Co(C=N^tBu)₂]BPh₄(3‐BPh₄), whose EPR spectral parameters are characteristic of low‐spin d⁷with an unpaired electron in an orbital of d_z²parentage. This is also consistent with the results of DFT calculations. Despite its 16‐electron configuration and the d_z²parentage of the HOMO, the only tractable reactions of2involve one electron oxidation to afford3.