Mononuclear entities

Amino acid is one of the most important biological ligands. Researches on the coordination of metal-amino acid complexes will help us better understand the complicated behavior of the active site in a metal enzyme. Up to now many Ln-amino acid complexes [50] and 1 1 or 1 2 transition metal-amino acid complexes [51] with the structural motifs of mononuclear entity or chain have been synthesized. Recently, a series of polynuclear lanthanide clusters with amino acid as a ligand were reported (most of them display a Ln404-cubane structural motif) [52]. It is also well known that amino acids are useful ligands for the construction of polynuclear copper clusters [53-56], Several studies on polynuclear transition metal clusters with amino acids as ligands, such as [C03] [57,58], [Co2Pt2] [59], [Zn6] [60], and [Fe ] [61] were also reported. 

Hydrolysis of polynuclear hydroxo-bridged chromium (III) complexes in concentrated solutions of strong acid yields the corresponding mononuclear species. Such cleavage reactions are fast in comparison with the hydrolysis in dilute acid and proceed with retention of configuration of the mononuclear entities. A few representative examples are shown in Eqs. (46)-(49) (40, 42,161, 252). 

IR-7.1.2 Choosing a central atom or atoms, or a chain or ring structure IR-7.1.3 Representing ligands in additive names IR-7.1.4 Ions and radicals IR-7.2 Mononuclear entities IR-7.3 Polynuclear entities... 

A few examples are given now in order to illustrate some of the general remarks above. In these examples, and in the remainder of this chapter, alternative formulae are sometimes provided for clarity in connection with the discussion of additive names. These are based on a perception of the structures in question as generalized coordination entities. For mononuclear entities, this means that the central atom symbol is listed first and then the ligand symbols in alphabetical order, as prescribed in Section IR-4.4.3.2. 

As a result, during the whole process the active site alternates between the primary [M]Z and the secondary M—0 Z centers, which drive both the N2 and 02 production cycles. While alternative scenarios involving dinuclear active sites also exist [5,40,45], we focus on the better defined model involving the M Z/ M—OJZ couple, and by assuming these entities to be the active species, we develop a consistent mechanistic description of DeNO, process catalyzed by mononuclear TMIs. 

The benzeneselenolates of mercury, [Hg(SePh)2] and (Bu4N)[Hg(SePh)3], both adopt simple structures, with mononuclear complex entities which posses near-linear Se—Hg—Se and near trigonal-planar HgSe3 cores, respectively.335... 

Thus, in the next examples, we comment on supramolecular entities, including linear chains, two-dimensional sheets, and even three-dimensional networks. From them, perhaps the most common structural arrangement is that in which the metals form extended linear systems, usually built from mononuclear units, dinuclear, using polydentate donor ligands and polymetallic units. It is worth mentioning that in this part and also in the following we consider only structures built by gold-heterometal interactions, not bonds thus formal clusters will not be considered. 

Molecules or ions that can formally be regarded as mononuclear coordination entities may be named additively, applying the rules described in Chapter IR-7. 

Reactivity of [M (por)] Species. The mononuclear square-planar d [M (por)] (M = Rh, Ir) complexes (either as stable entities for the TMP, TTEPP, or TTiPP complexes, or as transient intermediates for the other complexes) behave very much like organic radicals. For example, they tend to dimerize, either directly via metal-metal bonds (only for TXP, TTP, TPP, or OEP) or via reducible ligands like CO or olefins (with net 2e ligand reduction). The [(por)M ] complexes also show a remarkable reactivity toward a variety of otherwise rather inert substrates. Activation under mild conditions of H2, benzylic, and aUylic C—H bonds, and even methane has been reported. 

Fig. 6. Simulation of the thermal variation of the Debye-Waller factor in the case of a mononuclear complex surrounded by 4 identical entities (see text)...

Despite the rich catalytic alkyne cyclization chemistry of Fe , there are few mononuclear ferracyclopentadienes, perhaps because of their high reactivity e.g., reduction of FeCl2[P(CH3)3]2 with Na-Hg in 2-butyne gives the blue, highly volatile and thermally unstable [(CH3)3P]4Fe[C4(CH3)4] and the cyclotrimer hexamethylbenzene . A similar blue entity IV arises when the 4-coordinate complex (dad)2Fe is exposed to dimethyl butynedioate [dad = ethanedialbis(cyclohexylimine)] . 

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