Monomeric mononuclear

The possibility of formation of dimeric structures should be taken into account in the syntheses of metal-halide complexes of other ligands. In this respect, the transformation (3.72) is interesting and representative, in which not only monomeric mononuclear 637, but also dinuclear 638 complexes were isolated and their structures determined by x-ray diffraction [89] ... 

Routes to monomeric , mononuclear , monolanthanide alkoxides, enolates, siloxides and aryloxides - an expanded title which will put the scope of the article in a more concrete form. The synthesis of mononuclear alkoxides, in particularly homoleptic derivatives [1], was decisively stimulated by the discovery of high temperature superconducting ceramics based on YBa2Cu307<, where yttrium represents the lanthanide elements [2]. The support of volatile and highly soluble molecular precursors is a prerequisite for synthesizing thin films of these materials by means of MOCVD [3] and sol gel processes [4], respectively. More recently, lanthanide alkoxide reagents became established in... 

Metalloporphyrin catalysts are characterised by their isolated metal atoms surrounded by a large, planar, rigid ring and, therefore, form monomeric mononuclear species (Mt-X). The polymerisation of epoxides with these well-defined catalysts appeared to exhibit a living character the polyethers obtained are characterised by a small molecular weight and very narrow molecular weight distribution [30,36]. 

There is much discussion on the nature of the aluminum species present in slightly acidic and basic solutions. There is general agreement that in solutions below pH 4, the mononuclear Al " exists coordinated by six water molecules, ie, [ ( 20) ". The strong positive charge of the Al " ion polarizes each water molecule and as the pH is increased, a proton is eventually released, forming the monomeric complex ion [A1(0H)(H20) ]. At about pH 5, this complex ion and the hexahydrated Al " are in equal abundance. The pentahydrate complex ion may dimerize by losing two water molecules... 

Because they possess an odd number of valence electrons the elements of this group can only satisfy the 18-electron rule in their carbonyls if M-M bonds are present. In accord with this, mononuclear carbonyls are not formed. Instead [M2(CO)s], [M4(CO)i2] and [M6(CO)i6] are the principal binary carbonyls of these elements. But reduction of [Co2(CO)g] with, for instance, sodium amalgam in benzene yields the monomeric and tetrahedral, 18-electron ion, [Co(CO)4] , acidification of which gives the pale yellow hydride, [HCo(CO)4]. Reductions employing Na metal in liquid NH3 yield the super-reduced [M(CO)3] (M = Co, Rh, Ir) containing these elements in their lowest formal oxidation state. 

Cobalt(II) alkoxides are known and monomeric forms are part of a wider review.413 The interest in these compounds pertains to a potential role in catalysis. For example, a discrete cobalt(II) alkoxide is believed to form in situ from a chloro precursor during reaction and performs the catalytic role in the decomposition of dialkyl pyrocarbonates to dialkyl carbonates and carbon dioxide.414 A number of mononuclear alkoxide complexes of cobalt(II) have been characterized by crystal structures, as exemplified by [CoCl(OC(t-Bu)3)2 Li(THF)].415 The Co ion in this structure and close relatives has a rare distorted trigonal-planar coordination geometry due to the extreme steric crowding around the metal. 

Monomeric, dialkylcopper complexes have been prepared and structurally characterized. The reaction of CpCuPPh3 with lithium fluorenyl18 provided the complex [(fluorenyl)2CuPPh3] [Li(THF)4]+ 11. The use of the anionic ligand CH(Me)PEt2NSiMe319 afforded the mononuclear complex 12 in which the ligand acts as a bridge... 

An alternative mechanism has been proposed by Schneider (1988) who considers that ferritin could be also filled via a transient, mononuclear Fe " species. This species is similar to Fe in size, but is more versatile in its interaction with the protein shell. Experiments have shown that as the pH of a system containing diferric-trans-ferrin and ferritin was lowered very slowly from 7.5 to 5.0, monomeric Fe was released from the transferrin and redeposited in the ferritin (Glaus, 1989). Calculations of the iron flux across the cell membrane and estimates of the rates of interaction of the mononuclear species with ferritin and with the cell mitochondria indicated that the steady state concentration of the mononuclear Fe species would be sufficiently low for this species alone to enter the protein shell and be deposited as the iron core. Uptake of this species by the protein shell is about fiftyfold faster than the rate of hydrolytic polymerization or even of the dimerization of Fe (tiy2 1 vs. 50 ms). This hypothesis suggests an interesting direction for further research. 

A number of iridium-porphyrin systems, including mononuclear and cofacial diporphyrins, have been adsorbed on pyrolytic edge-plane graphite electrodes and tested for their ability to reduce dioxygen to water (82). The original system investigated, [Ir(oep)H], where oep = 2,3,7,8,12,13,17,18-octaethylprophyrinato, was unique in that, while monomeric, the complex was still active in acidic solutions at potentials of +0.72 V vs NHE at pH 1 (82a). The [Ir(oep)H] did become inactive at potentials less than +0.2 V vs NHE, unlike the cofacial dicobalt diporphyrin systems. In the more recent report of these systems (82b),... 

Most monomeric calcium amides involve the use of neutral donor molecules to prevent association. The only example of a three-coordinate mononuclear Ca amide, [Ca N(SiMe3)-(SiBu Ph2) 2(thf)], was prepared from CaF and KN(SiMe3)(SiBu Ph2) similar reaction... 

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