Stability constants of complexes

In coordination chemistry, a stability constant (also called formation constant or binding constant) is an equilibrium constant for the formation of a complex in solution. It is a measure of the strength of the interaction between the reagents that come together to form the complex. There are two main kinds of complex: compounds formed by the interaction of a metal ion with a ligand and supramolecular complexes, such as host–guest complexes and complexes of anions. The stability constant(s) provide(s) the information required to calculate the concentration(s) of the complex(es) in solution. There are many areas of application in chemistry, biology and medicine. History Jannik Bjerrum (son of Niels Bjerrum) developed the first general method for the determination of stability constants of metal-ammine complexes in 1941. The reasons why this occurred at such a late date, nearly 50 years after Alfred Werner had proposed the correct structures for coordination complexes, have be

Organometallics versus P4 Complexes of Group 11 Cations: Periodic Trends and Relativistic Effects in the Involvement of (n-1)d, ns, and np Orbitals in Metal-Ligand Interactions

Hui-Chung Tai

The nature of the bonding in ethylene and h2-P4 complexes, M(C2H4)2+ and M(h2-P4)2+, of group 11 metal cations (M = Cu, Ag, Au) has been explored by d. functional calcns. On the basis of the evaluation of symmetry orbitals, the contributions from the interactions of ligand orbitals with metal ns, np, and (n-1)d orbitals have been investigated. The anal. shows that the metal-ligand bonds in the organometallics and phosphorus complexes fit to a unified scheme, whereas traditional concepts such as the isolobality principle would hardly predict such a bonding analogy between C2H4 and P4 complexes. Bond energies increasing in the order Ag < Cu < Au have been predicted. The stronger metal-ligand bonds in the gold(I) compds. compared to those in the silver(I) compds. can be elucidated by the relativistic stabilization of the orbital interactions, particularly of those involving 6s and 5d orbitals. The stronger metal-ligand bonds in Cu(h2-P4)2+ compared to those in the exptl. known Ag(h2-P4)2+ can be attributed partly to the strong back-donation from metal 3d orbitals to vacant ligand orbitals. This result stands in sharp contrast to the common belief that first-row transition metals form weaker bonds to ligands than do their second-row analogs because of a comparably small overlap between ligand orbitals and metal 3d orbitals. [on SciFinder (R)]

2004

Complexation of [Gd(DTTA–Me)(H2O)2]− by Fluoride and Its Consequences to Water Exchange

Lothar Helm, Gail Ruth Hunter, Shima Karimi, Loïck Moriggi

The displacement of water molecule(s) from the inner coordination sphere of [Gd(DTTA–Me)(H2O)2]− (DTTA = ethylenetriamine-N,N,N″,N″-tetraacetate) by fluoride has been studied by multinuclear NMR relaxation (1H, 17O, 19F) and DFT calculations. Fluoride anions can replace only one of the coordinated water molecules. The thermodynamic stability constant (KGdLF,2980 = 11.6 ± 0.3) and thermodynamic parameters characterizing the formation of [Gd(DTTA–Me)(H2O)F]2– were determined (ΔH0 = +6.3 ± 0.1 kJ mol–1; ΔS0 = +41.5 ± 3.4 J mol–1 K–1; ΔV0 = +4.5 ± 1.2 cm3 mol–1). Fluoride binding causes a marked acceleration of the water exchange, which is seven times faster for [Gd(DTTA–Me)(H2O)F]2– (kex,1298 = 177 × 106 s–1) than for [Gd(DTTA–Me)(H2O)2]− (kex,2298 = 24.6 × 106 s–1). Water exchange on both compounds is faster than formation of the fluoride complex. The analysis of the Gd–Owater distances, electron density, and electron localization function (ELF) at the bond critical points using DFT calculations reveals that F– binding weakens the Gd–Owater bonds, thereby facilitating the departure of the coordinated water molecule following a dissociative mechanism. The water exchange on both Gd(DTTA–Me) complexes follow dissociative reaction pathways as shown by the positive activation volumes ΔV⧧ = +8 ± 2 cm3 mol–1 and +15 ± 4 cm3 mol–1 for the bis-aqua complex and the monofluoro complex, respectively.

American Chemical Society2016

Physicochemical Properties of the High-Field MRI-Relevant [Gd(DTTA-Me)(H2O)2]− Complex

Lothar Helm, Loïck Moriggi, Cora Prestinari

To study the physicochemical properties of the DTTA chelating moiety (H4DTTA = diethylenetriaminetetraacetic acid = N,N′-[iminobis(ethane-2,1-diyl)]bis[N-(carboxymethyl)glycine]), used in several compounds proposed as magnetic resonance imaging (MRI) contrast agents, the methylated derivative H4DTTA-Me (N,N′-[(methylimino)bis(ethane-2,1-diyl)]bis[N-(carboxymethyl)glycine]) was synthesized. Protonation constants of the ligand were determined in an aqueous solution by potentimetry and 1H NMR pH titration and compared to various DTTA derivatives. Stability constants were measured for the chelates formed with Gd3+ (log KGdL = 18.60 0.10) and Zn2+ (log KZnL = 17.69 0.10). A novel approach of determining the relative conditional stability constant of two paramagnetic complexes in a direct way by 1H NMR relaxometry is presented and was used for the Gd3+ complexes [Gd(DTTA-Me)(H2O)2]− (L1) and [Gd(DTPA-BMA)(H2O)] (L2) [KL1/L2*(at pH 8.3, 25 °C) = 6.4 0.3]. The transmetalation reaction of the Gd3+ complex with Zn2+ in a phosphate buffer solution (pH 7.0) was measured to be twice as fast for [Gd(DTTA-Me)(H2O)2]− in comparison to that for [Gd(DTPA-BMA)(H2O)]. This can be rationalized by the higher affinity of Zn2+ toward DTTA-Me4− if compared to DTPA-BMA3−. The formation of a ternary complex with L-lactate, which is common for DO3A-based heptadentate complexes, has not been observed for [Gd(DTTA-Me)(H2O)2]− as monitored by 1H NMR relaxometric titrations. From the results, it was concluded that the heptadentate DTTA-Me4− behaves similarly to the commercial octadentate DTPA-BMA3− with respect to stability. The use of [Gd(DTTA-Me)(H2O)2]− as an MRI contrast agent in vitro and in animal studies is conceivable, mainly at high magnetic fields, where an increase of the inner-sphere-coordination water actually seems to be the most certain way to increase the relaxivity.

2008

Physicochemical Properties of the High-Field MRI-Relevant [Gd(DTTA-Me)(H2O)2]− Complex

Lothar Helm, Loïck Moriggi, Cora Prestinari

2008