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Cobalt(II) acetylacetonate complexes bearing a phosphine ligand can be key intermediates or precursors to cobalt‐based catalysts; however, they have been rarely studied, especially from a molecular structure point of view. This work is focused on the understanding of how different phosphines react with Co(acac)₂(acac = acetylacetonate). To do so, a variety of analytical tools, including NMR and IR spectroscopy, X‐ray crystallography, mass spectrometry, and elemental analysis, have been used to study the reactions and characterize the isolated products. These results have shown that the monodentate ligand, HPPh₂, binds to Co(acac)₂weakly and reversibly to produce Co₂(acac)₄(HPPh₂), whereas the bidentate ligand, 1,2‐bis(diphenylphosphino)ethane (dppe), interacts with Co(acac)₂more strongly to yield a 1D coordination polymer of Co(acac)₂(dppe). 2‐(Dicyclohexylphosphino)methyl‐1 H‐pyrrole (^CyPN^H), which is a pyrrole‐tethered phosphine, forms an unusual 5‐coordinate cobalt complex, Co(acac)₂(^CyPN^H), in which the pyrrole moiety participates in a bifurcated hydrogen–bonding interaction with the [acac]^–ligands. In contrast, another bidentate ligand, 4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene (xantphos), fails to react with Co(acac)₂, presumably due to its wide bite angle and difficulty in bridging two metals. 

A dodecanuclear copper hydride L₄Cu₁₂H₁₂is shown to be a transient species after the cluster expansion from L₂Cu₄H₄to L₃Cu₆H₆and before the degradation to Cu(0), L, and H₂. 

This work centers around the nickel complexes derived from two tetrahydrosalen-type proligands: N , N ′-bis(2-hydroxybenzyl)- o -phenylenediamine (H 2 salophan) and N , N ′-bis(2-hydroxy-3-methylbenzyl)- o -phenylenediamine (H 2 salophan_Me). The reaction of H 2 salophan with Ni(OAc) 2 ·4H 2 O generates a dinuclear complex Ni 2 (Hsalophan) 2 (OAc) 2 or Na[Ni 2 (salophan) 2 (OAc)] when NaOH is added to assist ligand deprotonation. The reaction of H 2 salophan_Me with Ni(OAc) 2 ·4H 2 O, however, yields a mononuclear complex Ni(Hsalophan_Me) 2 . Unlike the corresponding salen-type nickel complexes, these tetrahydrosalen-type complexes are paramagentic and air sensitive (in solution). Oxidation by O 2 or peroxides results in dehydrogenation of the ligand backbone to form the salen-type complexes.