Radical CpNi(dithiolene) and CpNi(diselenolene) complexes: Synthetic routes and molecular properties

The formally Ni(III) d7 radical organometallic complexes formulated as [CpNi(dithiolene)] can be prepared by different routes involving different CpNi sources such as the Ni(I) [CpNi(CO)]2, the Ni(II) [Cp2Ni] or [CpNi(cod)]+ or the Ni(III) [Cp2Ni]+ complexes. As dithiolene precursors, the naked dithiolate, the mono- as well as bis-(dithiolene) metal complexes were investigated. The highest yields are generally associated with an appropriate redox match, that is a CpNi(II) precursor with a formally Ni(IV) [Ni(dithiolene)2]0 complex, or a CpNi(III) precursor with a formally Ni(III) [Ni(dithiolene)2]− complex. The structural, electrochemical and spectroscopic (UV–vis–NIR, EPR) properties of more than twenty complexes are described and compared, with the help of DFT calculations. They all exhibit a small optical gap with a low-energy absorption band in the Near Infra-Red region, between 700 and 1000 nm. The smaller electrochemical and optical gap found in the [CpNi(dmit)] and [CpNi(dddt)] complexes is correlated with an extensive delocalisation of the spin density in these complexes, while the other members of the series are characterized with a larger and sizeable spin density on the cyclopentadienyl ring.