Chemical bonding, transition metal coordination

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

Divalent late transition metals like cobalt (d ), nickel (d ), and copper (d ) in the first row of the d-block can use five 3d orbitals, one 4s orbital, and three 4p orbitals to form 4-, 5-, or 6-coordinate complexes. As a general rule, if there are N ligands in the first-shell coordination sphere of a transition metal complex, then there should be N bonding molecular orbitals, N anti-bonding molecular orbitals, and 9-N nonbonding molecular orbitals. Exceptions to this rule occur in some square-planar complexes in which three orbitals with the same symmetry properties overlap and form chemical bonds. Usually, some coordination sites in the first-shell of the... 

Section 14 15 Coordination polymerization of ethylene and propene has the biggest eco nomic impact of any organic chemical process Ziegler-Natta polymer ization IS carried out using catalysts derived from transition metals such as titanium and zirconium tt Bonded and ct bonded organometallic com pounds are intermediates m coordination polymerization... 

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). 

Figure 1.13 Raman spectra for a number of transition metal oxides supported on y-AI203 [75,102], Three distinct regions can be differentiated in these spectra, namely, the peaks around 1000 cm-1 assigned to the stretching frequency of terminal metal-oxygen double bonds, the features about 900 cm 1 corresponding to metal-oxygen stretches in tetrahedral coordination sites, and the low-frequency (<400 cm-1) range associated with oxygen-metal-oxygen deformation modes. Raman spectroscopy can clearly complement IR data for the characterization of solid catalysts. (Reproduced with permission from The American Chemical Society.)...

A tremendous number of transformations of allenes have been reported owing to their high jt-coordination ability towards transition metals. Among them, intramolecular cycloaddition reactions of allenes, in particular, appear to be a practical means of carbon-carbon bond formation in a complicated system. The allenic moiety, however, should be precisely designed for the synthetic purpose of more complex frameworks. A formidable challenge is the synthesis of diversely functionalized allenes of high chemical and/or enantiomerical purity. 

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