Out of the 14 lanthanide (Ln) ions, molecular complexes of Ln(IV) were known only for cerium and more recently terbium. Here we demonstrate that the +IV oxidation state is also accessible for the large praseodymium (Pr) cation. The oxidation of the tetrakis(triphenysiloxide) Pr(III) ate complex, [KPr(OSiPh 3 ) 4 (THF) 3 ], 1-Pr Ph , with [N(C 6 H 4 Br) 3 ][SbCl 6 ], affords the Pr(IV) complex [Pr(OSiPh 3 ) 4 (MeCN) 2 ], 2-Pr Ph , which is stable once isolated. The solid state structure, UV−visible spectroscopy, magnetometry, and cyclic voltammetry data along with the DFT computations of the 2-Pr Ph complex unambiguously confirm the presence of Pr(IV).

This study details syntheses of unsymmetrical magnesium(I)−adduct complexes, [( Ar Nacnac)-(D)Mg−Mg( Ar Nacnac)] ( Ar Nacnac = [(ArNC-Me) 2 CH] − ), Ar = xylyl (Xyl), mesityl (Mes), 2,6diethylphenyl (Dep), or 2,6-diisopropylphenyl (Dip); D = N-heterocyclic carbene or 4-dimethylaminopyridine, DMAP), which X-ray crystallographic studies show to have markedly elongated Mg−Mg bonds. Two of these highly reactive species are shown to reductively trimerize CO to yield rare crystallographically characterized examples of the planar, aromatic deltate dianion, incorporated in the complexes [{( Dip Nacnac)(D)Mg(μ-C 3 O 3 )Mg( Dip Nacnac)} 2 ] (D = DMAP or :C{N(Me)C-(Me)} 2 ). DFT calculations suggest that these complexes form via stepwise two-electron reductions of three CO molecules, resulting in the formation of three C−C bonds within the cyclic deltate unit. This work highlights the utility of activated magnesium(I) adduct complexes as soluble organometallic models for the study of reductive C−C bond-forming events in, for example, the heterogeneously catalyzed Fischer−Tropsch process.