Cluster compounds bonding scheme

SCHEME 5. Hypothetical reaction scheme for the insertion reaction of MX into MX bonds leading to a cluster compound. 

The higher catalytic activity of the cluster compound [Pd4(dppm)4(H2)](BPh4)2 [21] (20 in Scheme 4.12) in DMF with respect to less coordinating solvents (e.g., THF, acetone, acetonitrile), combined with a kinetic analysis, led to the mechanism depicted in Scheme 4.12. Initially, 20 dissociates into the less sterically demanding d9-d9 solvento-dimer 21, which is the active catalyst An alkyne molecule then inserts into the Pd-Pd bond to yield 22 and, after migratory insertion into the Pd-H bond, the d9-d9 intermediate 23 forms. Now, H2 can oxidatively add to 23 giving rise to 24 which, upon reductive elimination, results in the formation of the alkene and regenerates 21. 

Cluster compounds and remarks on the 2e, 2c bonding scheme . A formal metal-metal bond designation may be introduced having as reference the simple bonding schemes based on the inert gas rules ( octet rule). 

Scheme 2. Schematic relationships between C2 hydrocarbon adsorbates and the surface species that could be derived from them. Possible relationships are indicated between surface species that involve not more than one addition/subtraction of H and/or M atoms. Strong bonding of these species to metal atoms could give rise to structures approximating to metallocyclopropanes or metallocyclopropenes, respectively. These structures with C=C or C = C groupings frequently occur on surfaces with additional n bonds to further metal atoms, M in other cases, two additional CM bonds may replace a CC double bond. JOn surfaces the metal atoms are usually bonded to each other so that the analogous molecular cluster compounds would be metallocyclopropanes or metallocyclobutanes, etc. 1 See footnote to Scheme I. The dashed rectangles indicate surface species that involve no CH bond breaking on adsorption of the parent hydrocarbon. These are most likely to be present under low-temperature adsorption conditions.

The oxidation of the reactive M R compounds through cycloaddition with organic multiple-bonded reagents, such as alkynes, dienes, and heterodynes, produces heterocychc compounds that contain M (Scheme 8). In the case of A1 C1, the initial cycloaddition products often ohgomerize to form larger heterocycles and clusters compounds. 

The search for new reactivity and new reactions is an important target in homogeneous catalysis. A declared goal is the selective activation of C-H bonds under mild conditions. Although there are numerous examples of stoichiometric C-H bond oxidative additions to transition metal centers, successful examples regarding catalytic functionalization of C-H bonds have been made only during the last five years. Notable advances have been achieved by Moore and coworkers who described in 1992 the ortAo-acylation of pyridine with olefins and carbon monoxide. The cluster compound triruthenium dodecacarbonyl has been used as catalyst (Scheme 10). 

Fig. 12.21 The structures of some representative aluminium-nitrogen cluster compounds. Localized bonding schemes are appropriate for each cage (see problem 12.17 at the end of the chapter).

Seven contributions covering aspects of transition metal cluster chemistry and one concerning group-14 element clusters constitute this chapter on solid-state cluster chemistry. The general topics discussed are the structures of the cluster compounds and the electronic bonding schemes used to describe them. The importance of the bridges between the molecular units which form the solid-state cluster structures should be emphasized because of the general treatments of the electronic structures by use of modified molecular orbital calculations that are based on discrete cluster units. 

Formation of cluster opened methano- and imino[60]fullerenes (fulleroids and azafulleroids) thermal [3+ 2]-cycloadditions of diazo compounds or azides lead to the formation of fulleropyrazolines or fullerotriazolines. The thermolysis of such adducts after extrusion of N2 affords as kinetic products the corresponding [5,6] -bridged methano and iminofullerenes with an intact 60 7T-electron system and an open transannular bond (Scheme 6) [113-128]. The corresponding [6,6] -bridged structures with 58 rr-electrons and a closed transannular bond are formed only in traces. 

Wesemann et al. reported the synthesis of two cluster compounds 65a and 65b containing a Cd-Sn and Hg-Sn bond, which were generated by reacting the stanna-c/oso-dodecaborate salts [Et3NH ]2[SnBiiHii"] or [Me3NH+]2[SnBjjHjj ] withCdBrj or Hg2Cl2, respectively (Scheme 12.21) [48]. 

Gold-mercury cluster compounds have been obtained by different groups. Fackler et al. first prepared two Au-Hg-bonded complexes 143 and 145 in 1988 (Scheme 12.42) [97]. The two complexes are isomeric and the Au-Hg bond lengths are 3.088(1) and 2.989(1) A, respectively. Compound 144 could also be converted into 146 by treating with Au(THT)Cl (THT = tetrahydrothiophene) without T1 salt. The solid structure of 146 showed the Hg-Au distances of 3.310(1) and 3.361(1) A. 

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