Strong magnetic interactions through weak bonding interactions in organometallic radicals:: Combined experimental and theoretical study

The magnetic properties of a series of three neutral radical organometallic complexes of general formula [CpNi(dithiolene)](.) have been investigated by a combination of X-ray crystal structure analysis and magnetic susceptibility measurements, while the assignment of the exchange coupling constants to the possible exchange pathways has been accomplished with the help of calculations based on density functional theory (DFT). The syntheses and X-ray structures of [CpNi(adt)] (adt = acrylonitrile-2,3-dithiolate) and [CpNi(tfd)] (tfd = 1,2-bis(trifluoromethyl)ethene-1,2-dithiolate) complexes are described, while [CpNi(mnt)] (mm = maleonitriledithiolate) was reported earlier. In the three complexes, we observed strong antiferromagnetic coupling that could not be explained solely by short S center dot center dot center dot S intermolecular contacts. Our calculations indicated that spin density in these complexes is strongly delocalized on the NiS2 moiety, with up to 20 % on the Cp ring. As a consequence, Cp center dot center dot center dot Cp and Cp center dot center dot center dot dithiolene overlap interactions have been identified as responsible for antiferromagnetic couplings. The [CpNi(adt)] complex thus has a value J= -369.5cm(-1) for an exchange interaction through a pi stacking due to the Cp center dot center dot center dot Cp overlap.