Selective Crystallization of Linkage Isomers, [Rh^III(NCS)(SCN)₅]^3– and [Rh^III(SCN)₆]^3–, to Investigate Structural Trans Influence and Thermal Stability

Abstract

Linkage isomers of homoleptic complexes, [Rh III (SCN) 6 ] 3− and [Rh III (NCS)(SCN) 5 ] 3− , formed in aqueous solution were successfully separated by employing methyltriphenylphosphonium (MePPh 3 + ) and 1-ethylquinolinium (EtQu + ) ions as countercations, respectively. The single-crystal X-ray analysis of (MePPh 3 ) 3 [Rh III (SCN) 6 ] (1) indicated that all of the SCN − ligands coordinate to the Rh III ion by S atoms with an octahedral symmetry, where the average bond length of Rh−S is 2.374(7) Å. On the other hand, the Rh III ion of (EtQu 2) is coordinated by five S atoms and one N atom of the SCN − ligands with a C 4v symmetry. Structural trans influence was observed in the shorter bond length of Rh−S at the trans position of Rh−N. The Rh−S bond length is 2.3398(13) Å significantly shorter than those of 1 by ca. 0.04 Å, although DFT calculations based on the crystal structures indicated that the effective bond order of Rh−N is higher than those of Rh−S. Thermal stability examination by thermogravimetric and differential thermal analyses (TG/DTA) and IR spectroscopy indicated that the linkage isomerization of [Rh III (SCN) 6 ] 3− to [Rh III (NCS)(SCN) 5 ] 3− proceeded after melting around 174 °C. These results clearly indicate that [Rh III (NCS)(SCN) 5 ] 3− is thermodynamically more stable than [Rh III (SCN) 6 ] 3− in solid states, although further linkage isomerization hardly occurs.