Mononuclear metal complexes

The bonding in the mononuclear metal complexes is considered to involve the two nitrogen atoms of the ligand structure, giving the familiar... 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

Supported metal carbonyl clusters are alternatively formed from mononuclear metal complexes by surface-mediated synthesis [5,13] examples are [HIr4(CO)ii] formed from Ir(CO)2(acac) on MgO and Rh CCOlie formed from Rh(CO)2(acac) on y-Al203 [5,12,13]. These syntheses are carried out in the presence of gas-phase CO and in the absence of solvents. Synthesis of metal carbonyl clusters on oxide supports apparently often involves hydroxyl groups or water on the support surface analogous chemistry occurs in solution [ 14]. A synthesis from a mononuclear metal complex precursor is usually characterized by a yield less than that attained as a result of simple adsorption of a preformed metal cluster, and consequently the latter precursors are preferred when the goal is a high yield of the cluster on the support an exception is made when the clusters do not fit into the pores of the support (e.g., a zeolite), and a smaller precursor is needed. 

Syntheses in which a reaction of a mononuclear metal complex precursor gives a tethered metal cluster are rare an early example is the formation of a tetrairidium carbonyl on a phosphine-fimctionaUzed polymer [17]. 

An exhaustive treatment of the electrochemical behaviour of transition metal complexes is beyond the scope of this book, because the enormous number of ligands available, combined with the possibility to prepare mono- and/or polynuclear complexes using identical or mixed ligands, would render such a task almost impossible. Therefore, the discussion is limited to some aspects associated with the redox properties of (essentially) mononuclear metal complexes. In particular, we will concentrate representatively on the redox changes of first row transition metal complexes (excluding the metallocene complexes, as they have been already discussed in Chapter 4) that give stable, or relatively stable products. A systematic and useful examination of the redox activity of organometallic complexes of transition metals dated to 1984 has appeared.1... 

The reactions of titanocene derivatives TiX(C CsCR)Cp 2 (X = C1, C= CC=CR R = SiMe3, Et) or of cis-Pt(C=CC=CR)2(PR 3)2 with mononuclear metal complexes have given numerous products in which the diynyl ligand(s) are chelated to a low-valent metal via the internal C=C fragment(s). These cis-bis(diynyl) complexes are often referred to as molecular tweezers. 

Cationic mononuclear metal complexes, reaction with anionic carbonyl clusters, 30 155-158... 

Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4], But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, offer the potentiality as a good host for immobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about immobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 

A variety of tools address the stoichiometry and molecular weight of compounds. The necessary condition that at least two metals be present for multiple metal bond formation is a simple sorting method for initial studies the molecular unit as determined by any type of molecular weight study must correspond to that of at least two metals per molecule. Conductivity measurements supply similar data for ions, and mass spectral data can indicate the presence of at least two metals per molecule. Analytical data with nonintegral ligand-to-metal ratios require that some multiple number of metal centers be present in order to formulate a stoichiometric compound. This array of techniques only eliminates the possibility of metal—metal bonds for mononuclear metal complexes, and further studies are always necessary to confirm the presence of an attractive metal-metal interaction. 

Table 4 Photochemical Hydrogen Production from Mononuclear Metal Complexes and Water...

The current high level of interest in binuclear metal complexes arises from the expectation that the metal centers in these complexes will exhibit reactivity patterns that differ from the well-established modes of reactivity of mononuclear metal complexes. The diphosphine, bis(diphenylphosphino)methane (dpm), has proved to be a versatile ligand for linking two metals while allowing for considerable flexibility in the distance between the two metal ions involved (1). This chapter presents an overview of the reaction chemistry and structural parameters of some palladium complexes of dpm that display the unique properties found in some binuclear complexes. Palladium complexes of dpm are known for three different oxidation states. Palladium(O) is present in Pd2(dpm)3 (2). Although the structure of this molecule is unknown, it exhibits a single P-31 NMR reso-... 

This mechanism is notable for a possibility of molecular H202 to participate in two-electron oxidation implemented in one-stage ion, a mononuclear metal complex. 

The simplest supported catalysts are mononuclear metal complexes, exemplified by industrial supported metallocene catalysts, used (with promoters) for alkene polymerization these are the so-called single-site catalysts that are finding wide industrial applications (Kristen, 1999 Kaminsky, 1999 Roscoe et al., 1998). The most common supports are metal oxides and zeolites. The metals in these complexes range from oxophilic (e.g., Zr and Ta) to noble (e.g., Rh). Supported metal complexes are stabilized by ligands—in addition to those provided by the support—such as hydride (H), hydrocarbons, and carbonyl (CO). In a typical supported metal complex, the metal is present in a positive oxidation state. Although some such complexes are relatively stable, most are, befitting their roles as catalysts, highly reactive and air- and moisture-sensitive. 

When samples incorporate uniform mononuclear metal complexes, then EXAFS data may provide high-quality information about the interactions between the metal and oxygen atoms of the support. Evidence of the metal-support interface has also been determined by theory and, indirectly, by IR spectra indicating the symmetry and thus the number of atoms of the support that act as ligands bonded to the metal. 

A limitation of supported metal nanoclusters prepared from molecular metal carbonyl clusters is that, so far, clusters of only several metals (Ru, Rh, Ir, and Os) have been made in high yields (80 to 90%, with the likely impurity species being mononuclear metal complexes). However, this disadvantage is offset by the advantage of the characterizations, which show that some clusters are stable even during catalysis, at least under mild conditions. 

There are numerous precedents for such proposed intermediates for mononuclear metal complexes in solution (25), and such cyclic adsorbates on an extended metal surface should experience less ring strain than their mononuclear counterparts in solution. 

The examples in Fig. 1 symbolically show the transition from a mononuclear metal complex via small clusters with M3 and M4 units up to a relatively large cluster with 13 metal atoms. Numerous examples with M5, M7, Ms, M9 or M10, and M12 cluster units which are not shown in Fig. 1 have also been described. So, at least from Mj up to M13, a stepless transition is known. At this point several questions arise ... 

Additions of polar X-C bonds (e.g., X = Cl, Br, I, or OR) usually occur by nucleophilic attack by an electron-rich cluster at carbon this reaction is less common for clusters than it is for mononuclear metal complexes. One example is addition of iodobenzene to Ru3(CO)i2, forming Ru3(/x-I)(/x,) r] -C6H6)(C0)8. ... 

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