End-On Cyanogen Complexes of Iridium, Palladium, and Platinum

The cyanogen complexes of iridium, palladium, and platinum were prepared via the reactions of noble metal atoms with cyanogen in argon matrixes, and the product structures were determined by infrared spectroscopy and density functional theory calculations. These complexes were predicted to possess linear geometries with the metal center coordinated by the nitrogen atom of cyanogen in end-on fashions. On the basis of the B3LYP calculations, doublet, singlet, and singlet spin states are most stable for [Ir(NCCN)], [Pd(NCCN)], and [Pt(NCCN)]. Bonding analysis revealed the presence of electron donation from the polarized σ orbital of cyanogen into the empty metal d² orbital and back-donation from the metal d orbitals to the 2π_u orbitals of cyanogen, the latter of which destabilizes the C-N bond and stabilizes the C-C bond. Such an effect causes an increase in C-N bond length by 0.01-0.015 Å and a decrease in C-C bond length by 0.013-0.02 Å for noble-metal-cyanogen complexes in comparison to the same bond in neutral cyanogen, and this is also consistent with the changes in vibrational frequencies. Although [Ir(NCCN)], [Pd(NCCN)], and [Pt(NCCN)] were formed spontaneously during sample annealing, neither cyanide nor isocyanide product was observed in the experiments, which is different from the cases of early transition, lanthanide, and actinide metals.