A series of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized bis-N-heterocyclic carbene ligand of formula [Cp*MinCl{(Me1m)2CHCOO}1 and [MI(diene){(MeIm)(2)CHCOO}] (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; M = Rh, Ir; MeIm = 3-methyldiene = 1,5-cyclooctadiene (cod), norbornadiene (nbd)) were prepared from the salt [(MehnH)(2)CHCOO]}r and suitable metal precursor. The solid-state structure of both types of complexes shows a boat shaped six-membered metallacycle derived of the c2C,C' coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the bowsprit position in the Cp*M111 complexes, whereas in the M1(diene) complexes it is at the flagpole position of the metallacycle. The complexes [Rhi(diene){(MeIm)(2)CHCOO}] (diene = cod, nbd) exist as two conformational isomers in dichloromethane, bowsprit and flagpole, that interconvert through the boat-to-boat inversion of the metallacycle. An inversion barrier of 17 kcal-I-nor' was determined by two-dimensional exchange spectroscopy NMR measurements for [Rhi(cod){(Me1m)(2)CHCOO}]. Reaction of zwitterionic Cp*M111 complexes with methyl triflate or tetrafluoroboric acid affords the cationic complexes [Cp*MIIIC1{(MeIm)2CHCOOMe}1+ or [Cp*M-III C1{(MeIm)(2)CHCOOH}]+ (M = Rh, Ir) featuring carboxy and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands, respectively. Similarly, complexes [Mi(diene){(Melin)(2)CHCOOMe}]+ (M = Rh, Ir) were prepared by alkylation of the corresponding zwitterionic M1(diene) complexes with methyl triflate. In contrast, reaction of [IrI(cod){(MeIm)(2)CHCOO}1 with HBF4-Et20 (Et = ethyl), CH3OTf, CH3I, or 12 gives cationic iridium(III) octahedral complexes [IrifiX(cod){(MeIm)(2)CHCOO}r (X = H, Me, or I) featuring a tripodal coordination mode of the carboxylate bridge-functionalized bis-NHC ligand. The switch from K2C,C' to OC,C',0 coordination of the bis-NHC ligand accompanying the oxidative addition prevents the coordination of the anions eventually formed in the process that remain as counterions.