Metal atoms coordinatively unsaturated

Class II dependence for the activation of a chemical bond as a function of surface metal atom coordinative unsaturation is typically found for chemical bonds of a character, such as the CH or C-C bond in an alkane. Activation of such bonds usually occurs atop of a metal atom. The transition-state configuration for methane on a Ru surface illustrates this (Figure 1.13). 

A clear trend is observed as long as the coordination number of the C atom does not change, its interaction energy will increase when the coordinative unsaturation of the metal surface atom increases. This trend predicted in the previous section is confirmed by the more rigorous quantum-chemical DFT results of Table 10.1. The adsorption energy of the C atom increases with increasing surface metal atom coordination number N. This relates to the C atom valency, as is discussed in detail in Section 10.3.5. 

In case when an 0x0 bridge is formed between two metal atoms, the condensation process is called as oxolation. When the metal is coordinatively unsaturated, oxolation proceeds by nucleophilic addition with rapid kinetics [114,115], leading to edge- or face-shared polyhedra (Equation (24.8) and Equation (24.9)). 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

Catalysis by Metals. Metals are among the most important and widely used industrial catalysts (69,70). They offer activities for a wide variety of reactions (Table 1). Atoms at the surfaces of bulk metals have reactivities and catalytic properties different from those of metals in metal complexes because they have different ligand surroundings. The surrounding bulk stabilizes surface metal atoms in a coordinatively unsaturated state that allows bonding of reactants. Thus metal surfaces offer an advantage over metal complexes, in which there is only restricted stabilization of coordinative... 

The structure of the diamagnetic, cherry-red vitamin B12 is shown in Fig. 26.6 and it can be seen that the coordination sphere of the cobalt has many similarities with that of iron in haem (see Fig. 25.7). In both cases the metal is coordinated to 4 nitrogen atoms of an unsaturated macrocycle (in this case part of a corrin ring which is less symmetrical and not so unsaturated as the porphyrin in haem) with an imidazole nitrogen in the fifth position. A major... 

As we discussed in the previous section, the primary parameter that determines the interaction strength between an adsorbate and a (transition) metal surface is the coordinative unsaturation of the surface metal atoms. The lower the coordination number of a surface atom, the larger the interaction with interacting adsorbates. 

The coordinatively unsaturated [Ni(PPh3)3] (1034) has trigonal planar Ni,2502 the structure showing some close approach of three o-phenyl H atoms to the central Ni (2.74-3.09 A) with implications for ortho-metallation. 

Potentially coordinatively unsaturated dithiolene-metal complexes are rare,298-306 and 1 1 dithiolene-transition-metal complexes with no other ligands are, to our knowledge, unprecedented.307 The neutral complex [PdS2C2(COOMe)2]6,308 is homoleptic containing one dithiolene unit for each palladium atom and no other ligands. Electrochemical reduction of the compound depicted in Figue 21 proceeds in four reversible steps. 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

Some of the carbonyls with a high degree of coordinative unsaturation were produced by co-condensation of metal atoms and CO rather than photolytically (13). 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Coordinative unsaturation arises from the fact that because of steric and electronic reasons, only a limited number of ligands or nearest neighbors can be within bonding distance of a metal atom or ion. In most transition metal oxides, the oxygen anions in the bulk form closed-packed layers and the metal cations occupy holes among the anions as schematically depicted in Fig. 2.1. In this picture, the oxide ion ligands appear to have saturated the coordination sphere of the bulk cation. 

In fact, it is just the unsaturated central double bond of the pyracylene unit which commonly becomes the reactive site for coordination to metal fragments. This is the case not only for [( 2-C6O) (K20,0)-Os(O4)(4-terf-butylpyridine) ], but for example also for C60[Pt(PPh3)2]25 and [Ir(CO)( 2-C6o)Cl(PPh3)2].26 In both compounds the C1-C2 bond length ( 1.50 A) is considerably longer than the value (1.38 A) that the 6 6 C = C bond should have in the absence of coordination (in the strictly related [Pd( 2-C60)(PPh3)2] the C1-C2 distance is 1.45 A27). At variance with [O72-C6o) (K20,0)-Os(O4)(4-terr-butylpyridine) ], in the latter cases the metal atom is directly linked to the fullerene unit. 

The difference between the close-packed crystal faces, such as (110) in bcc or (111) in fee crystals, and the open faces that prevail in the surface of nano particles, is of relevance to heterogeneous catalysis, because most adsorbates are more strongly bonded to the coordinatively unsaturated metal atoms in the open faces. 

SH-Ag clusters, 87 6-FeOOH, Co304,257 colloidal catalysts, 281 coordinatively unsaturated metal atoms, 141 ensembles, 150 methanol cross-over, 289 platforming, 140 volcano shaped curves, 150 chemical anchoring, 143 volcano-shaped curves, 141... 

Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the a-carbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [129 -135]. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [136-141]. Vinylidene complexes can further undergo [2 -i- 2] cycloadditions with imines, forming azetidin-2-ylidene complexes [142,143]. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [143] (Table 2.7). 

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