Indigo di- and monoimines can be protonated to form stable salts in which the central C=C bond has isomerized from a trans to cis configuration. Deprotonation of these salts regenerates the neutral trans species. The protonation chemistry of indigo is also explored.

A series of lanthanide complexes, {[CpCo(P═O(OR)2)3]2Ln(H2O)x}(+)Cl(-) (Ln = Nd, 3; Eu, 4; Tb, 5; Yb, 6; R = Et, a; R = Ph, b) bearing two cobalt metalloligands were prepared. Electrospray mass spectrometry and thermogravimetric analysis suggest that the cations are either solvent-free or contain very weakly bound water molecules. The related complex {[CpCo(P═O(OPh)2)3]2Yb}(+) [CoCl3(THF)](-), 7, was crystallographically characterized, and the cation in this case was confirmed to be 6-coordinate and solvent-free. Ligand exchange rates between the d0- and d60-isotopomers of 3a-6a and 5b were determined in acetonitrile by electrospray mass spectrometry. The ligand exchange rate was found to increase by almost 4 orders of magnitude from the smallest (Yb, 6a, k = 0.3 M(-1) s(-1)) to largest ion (Nd, 3a, >2500 M(-1) s(-1)) in acetonitrile. Additionally, the ligand exchange rate increased rapidly for 5a (Tb) with increasing water concentration from 30 M(-1) s(-1) in pure acetonitrile to 268 M(-1) s(-1) in 50:50 (v/v) acetonitrile/water. Changing the phosphite substituent had no significant impact on the rate of ligand exchange for 5b (R = Ph) relative to 5a (R = Et).