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.

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) 3 [Rh III (NCS)(SCN) 5 ]·H 2 O ( 2 ) is coordinated by five S atoms and one N atom of the SCN - ligands with a C 4 v 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.