Multi-ligand complexes

Using the concept of group overlap integrals, equation (15) can be rewritten for a multi-ligand complex, and for a specific symmetry, in the form... 

Similar examples are afforded by chromium(m)-halide-ammine complexes. Kinetic parameters have been determined for the aquation of [CrBr2(NH3)(OH2)en]+ to [Cr(OH2>4en] + as the kinetics of aquation of the latter are already known, the complete kinetic picture of aquation of the former is now established. Kinetic results have been reported for several other multi-ligand complexes of this type, including [CrBr2(NH3)2(OH2)2l+, cis- and /mnj-[Cra2(NH3)(OH2)3]+ [CrCl(NH3)(OH2)4] +, and [CrCl2(NH3)3(OH2)]+, which last is an intermediate in the aquation of [Cr(Oa)2(NH3)3] in hydrochloric acid. ... 

Fig. 15-3 Mixed ligand and multi-dentate complexes (a) a hypothetical Me(OH)Cl° complex (b) nitrilotriacetate chelate of a divalent metal ion in a tetrahedral configuration.

It is important to note that, even when the coordination geometry prescribed by the macrocyclic cavity is ideal for the metal ion involved, unusual kinetic and thermodynamic properties may also be observed (relative to the corresponding open-chain ligand complex). For example, very often the macrocyclic complex will exhibit both enhanced thermodynamic and kinetic stabilities (kinetic stability occurs when there is a reluctance for the ligand to dissociate from its metal ion). These increased stabilities are a manifestation of what has been termed the macrocyclic effect - the multi-faceted origins of which will be discussed in detail in subsequent chapters. 

The dissociation rate constant measures directly the value of k 2 in (4.52). The strain resident in multi-ring complexes is clearly demonstrated by some hydrolysis rate studies of nickel(II) complexes. The AFT values for the first bond rupture for Ni(II)-polyamine complexes fall neatly into groups. They are highest for en, containing the most strain-free ring (84 kJ mol ), about 75 kJ mol for complexes with terdentate ligands and only —63 kJ mol for complexes of quadridentate and quinquedentate amines and with NH3 itself. See also Ref. 109. 

Analyses in environmental samples, involve reactions with several ligands. The stability- and rate constants thus obtained do not reflect the interaction with a single ligand but rather with the complex multi-ligand system. 

Lakowicz JR, Gryczynski I, Tolosa L, Dattelbaum JD, Castellano FN, Li L, Rao G. Advances in fluorescence spectroscopy multi-photon excitation, engineered proteins, modulation sensing and microsecond rhenium metal-ligand complexes. Acta Physica Polonica A 1999, 95, 179-196. 

A pronounced chirality effect on the chemical reactivity of ylid complexes is to be expected from propeller-type ligands with a C2 rotation axis. In a first attempt to synthesize pertinent examples, a spirobicyclic ylide 1 was designed (11) and converted into a novel lithio-derivative 2, which on reaction with metal halides yields multi-spiro complexes of unusual geometries ... 

Fig. 6. Schematic representation of the biosynthetic pathway associated with the assembly of the NiFe-hydrogenase catalytic metal center. The precursor to the large subunit is represented by pre-HycE-Fe, and the CO (and/or CN) ligands are provided by the multi-protein complex formed between HypE, HypC and HypD. The source of the ligands to HypE is carbamoyl phosphate, and the insertion reaction is catalyzed by HypF. The Ni atom is inserted into the pre-HycE-Fe-CO-CN-HypC complex in a reaction catalyzed by HypA and HypB. Finally, the Hycl endopeptidase processes the C-terminus of the pre-HycE-Fe-CO-CN-HypC complex, yielding the mature NiFe-hydrogenase large subunit.

They may be readily transformed to r] -aUcynyl and vinylidene complexes. The coupling of two ) -alkynyl ligands results in platinacyclopentadiene species." A platinum(II) disilyl( -alkyne) undergoes insertion (see Insertion) and reductive elimination (see Reductive Elimination) to disilacyclohexene (Scheme 31)." Metal-diaUcynyls can also play jr-donors to form the coordination complexes of various configurations, as shown in (10-13) and so on." Such a,tt-bridging modes may have interconversion. Multi- aUcynyl complexes can even constitute clusters or higher-order stractures. ... 

The exchange of water molecules in the hydration sphere for other dissolved species can be extended to include "mixed ligand complexes", i.e. those in which water molecules have been replaced by two or more different types of ligands, and "multi-dentate"... 

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