Mononuclear complexes, size

As already mentioned, larger ring size triazamacrocyclic ligand complexes have also been characterized. The zinc bromide complex of 1,5,9-triazacyclododecane has been characterized as a mononuclear complex with a tetrahedral zinc center.672 Kimura and co-workers have extensively studied zinc complex formation with 1,5,9-triazacyclododecane-based ligands.445,673-676 1,6,11-triazacyclopentadecane forms a zinc complex of the composition Zn3L2Cl6. NMR studies... 

The above complexes have been shown to mimic the second step of RNA hydrolysis as well, i.e. the-efficient cleavage of ribonucleoside 2, 3 -cyclic monophosphates [55] with bell-shaped pH-rate profile. With these substrates 37 showed much higher bimetallic cooperativity the dime/2 m0nomer ratios range between 64 and 457 for the different 2, 3 -NMPs used, while for 38 this ratio varies between 1 and 26. Since the mononuclear complexes have nearly the same activity toward the different 2, 3 -NMPs, these kinetic data indicate a notable base-selectivity of the dimer complexes. Since no correlation was observed with the size,... 

Hetero site reactivity The simplest difference between a cluster and a mononuclear complex is that the cluster can do two or more things where a mononuclear complex can do one. Simple as it is, this difference has hardly ever been verified other than in multiple ligand substitutions. One verification is hetero site reactivity, i.e., different modes of reaction at different sites on one and the same cluster. Two examples of this appear to exist. Different phosphine ligands substitute CO on different metal atoms in H2FeRu3(CO),3 depending on their size and basicity (210), and Ru2Co2 (CO), 3 reacts with H2 at the ruthenium atoms (cf. Section IV, A) and... 

Prussian blue, Fe4[Fe(CN)6]3xH20, consists of a cubic lattice of alternating Fe2+ and Fe3+ ions connected with cyanide bridges (Figure 8.3). The [Fe(CN)6]4 sites are 75 % occupied and the exposed Fe3+ ions are coordinated with H2O. Dehydration and hydration by moisture is reversible hence, Prussian blue is a porous solid albeit one with very small pores. To increase pore size, the [Fe(CN)6]4 complexes were replaced with octahedral [Rer TeACNAJ4 clusters (Figure 8.4). This cluster displays an octahedral array of N donors on a sphere about 4 A larger in diameter than the mononuclear complex. Reaction of Fe3+ with the cluster in... 

The effectiveness of the binuclear complex 11 (Fig. 13), with two mononuclear cyclen-cobalt(III) units linked together by an anthra-cenyl spacer (cyclen = 1,4,7,10-tetraazacyclododecane), was compared with the monomer in the hydrolysis of phosphate monoesters (354). The reaction assisted by this rigid binuclear complex, having a phosphate-sized pocket, was 10 times faster than that promoted in the presence of two equivalents of the single cyclen-Co complex. In these experiments the substrate concentration was 25 pM and the total cobalt concentration was 2 mM at 25°C and neutral pH (354). No such cooperativity could be noted using a diester substrate because the pseudo-first-order rate constants were similar for both 11 and the mononuclear complex. With 11 as catalyst, an overall rate enhancement of 10 was achieved over the uncatalyzed hydrolysis of paranitrophenyl phosphate monoester as substrate. 

Factors Affecting the Size of Dipole Moments in Mononuclear Complexes... 

It should be noted that in a graft layer with crazing polymers, at least when copper nanoparticles are being generated, the reaction is observed to be reversible owing to the formation of a galvanic pair Cu°/Cu, nanoparticles with sizes less than 3 nm are oxidized in time (a few hours) and converted into mononuclear complexes with migration of copper ions the specific metallic luster of the polymeric particles disappears. 

If a piece of metal is reduced to a size of a few thousand atoms we enter the world of metal colloids, unique particles which were already handled by Michael Faraday in the last century. Smaller units of a few hundred or dozen atoms are usually called clusters . This term is also well tried for small molecular species consisting of only a few metal atoms. Metals, especially transition metals, offer an exceptional opportunity to study the pathway which leads from the bulk to the molecular state and finally to mononuclear complexes (Figure 1-2). 

Although 50 and 51 were shown to be poorly controlled initiators for MMA polymerization, it was reasoned that an alkyl group intermediate in size between a methyl and a ferf-butyl might produce a mononuclear complex, yet still be sufficiently small enough to allow facile nucleophilic attack on a molecule ofMMA (and therefore initiate polymerization more efficiently). In previous studies, ... 

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