Versatility of Coordination Modes in Complexes of Monohydroxamic Acids with Half‐Sandwich Type Ruthenium, Rhodium, Osmium and Iridium Cations

Complexation of the secondary N‐methyl‐acetohydroxamic acid (HMeaha) with [(η6‐p‐cym)Ru(H2O)3]2+, [(η5‐Cp*)Rh(H2O)3]2+, [(η6‐p‐cym)Os(H2O)3]2+ or [(η5‐Cp*)Ir(H2O)3]2+ cations and the primary acetohydroxamic acid (HAha) with the former two cations (first time in the literature) has been studied in aqueous solution using pH‐potentiometry, NMR, ESI‐MS and DFT calculations. Because the arene or arenyl ligands take three coordination sites of the metal ion, only three remain free for interaction with additional ligand(s). Meaha– forms five‐membered hydroxamate type chelated [ML]+ and [M(OH)L], being the third coordination site occupied by a water molecule in the former complex and by a hydroxide ion in the latter one. While [ML]+ remains predominant even under basic conditions in the Ru‐ and Rh‐containing systems, it is measurable only in slightly acidic region with the two 5d metal ions. Also the tendency for hydrolysis of the aqua ligand in [ML(H2O)]+ complexes follows the trend of the cations for hydrolysis, Os > Ir > Ru > Rh. The X‐ray result did not show any role of the H substituent of Aha– in [{(η5‐Cp*)Rh}2(μ2‐Aha)2]2+, in solution, however, its significant role was supported especially with Ru. All the results support the formation of a stable trinuclear [M3H–2L3(H2O)]+ species in which doubly deprotonated (hydroxymate type) ligands bridge two metal centres by coordinating via the two oxygens to the one cation and via the N‐donor to another one. Unlike secondary hydroxamates, the primary derivative, acetohydroxamic acid (HL), was shown to form a trinuclear [M3H–2L3(H2O)]+ type species (M=[(η6‐p‐cym)Ru]2+) containing two doubly deprotonated hydroxymate and one mono deprotonated hydroxamate type ligands in which the formers bridge two metal centres by coordinating via the two oxygens to the one cation and via the N‐donor to another one.