Nitrogen-14 nuclear quadrupole resonance study of coordinated thiocyanate

Article abstract of DOI:10.1021/ic50220a024

A series of twenty palladium(II) thiocyanate complexes have been synthesized and their ¹⁴N nuclear quadrupole resonance (NQR) spectra recorded by using the recently developed technique of adiabatic demagnetization in the laboratory frame-double resonance level crossing (ADLF-drlc). A clearly definitive trend for determining the mode of coordination of the thiocyanate moiety, via its ¹⁴N nuclear quadrupole coupling constant, has been established. The field gradients at nitrogen are found to be largest in sulfur-bound thiocyanates and smallest in nitrogen-bound thiocyanates, while ionic thiocyanates have field gradients of intermediate values. The asymmetry parameter does not vary consistently with the mode of attachment but, on the average, increases in the order NCS^- < M-NCS < M-SCN. These results indicate that, in general, no single nitrogen 2p orbital is occupied largely by a lone electron pair. For coordinated thiocyanate, the occupation of a single 2p orbital is greatest for the sulfur-bound complexes. These results support the resonance forms generally accepted, based on bond angle measurements from other techniques, e.g., X-ray crystal structures. Nitrogen-14 NQR via the ADLF-drlc experiment is the most unambiguous technique to date, short of a single-crystal X-ray crystal structure analysis, for the determination of the bonding mode of diamagnetic, hydrogen-containing complexes of thiocyanate in the solid state.