Pentacoordinate species

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

The crystal structures of [Ag(l,4-dioxane)]AsF69 9 and [Ag2(l,4-oxathiane)](N03)295° have been reported. Two adducts of the 4-nitropyridine A-oxide with silver nitrate have been characterized, one is a mononuclear tetracoordinated compound [Ag(N03)(0NC5H4N02)2] and the other is a dinuclear pentacoordinated species [Ag2(N03)2(/u-0NC5H4N02)2(0NC5H4N02)2] (130).951,952 In the mixed compound /nmv-[RhCl2(py)4]N03-AgN03 exists the dinitroargentate(I) ion in a distorted tetrahedral environment 953... 

Chart 1. Feni-TAML activators mentioned in this account are shown as pentacoordinated species with an axial aqua ligand as obtained in the solid state. The aqua complexes are synthesized as such or as the corresponding chloro species with Cl- instead of H20 (with two M+ = Li+, Na +, NR4 counter ions). The subscript Cl as in lei is used throughout to indicate the corresponding chloro species. 

Common "parent" phosphorus-halogen species include particularly the P(III) system PC13 and the P(V) species P(0)C13. Of course, the pentacoordinated species PC15 must also be considered for its availability as reagent precursor. These are species that often serve not only in a direct manner in carbon-phosphorus bond formation but also in the construction of partially or fully esterified species prior to carbon-phosphorus bond incorporation. 

Heme proteins in their various forms contain mainly the ferrous or ferric porphyrin moieties [6—77] (R some organic side chain of the protein A a small molecule act-ingas a-donor-TT-acceptor ligand, e.g., CO, 02, NO, CH3CN, CH3NC) (7, 20-34). In fact the binding of dioxygen to the pentacoordinate species [6] and [7] — an essential... 

Since the first reaction undoubtedly proceeds with inversion of configuration at sulfur, and since additional experiments demonstrated that the formation of chlorosulfurane 177 from sulfoxide 180 takes place with retention at sulfur, (5>chirality was assigned to (+>177. As for the designation of absolute configuration, Martin and Balthazor (195) proposed a system of nomenclature for optically active pentacoordinate species. 

Figure 14.5 DR-UV spectra of (a) Ti-silsesquioxanes complexes, with the assigned major species for each maximum, and (b) the Ti(OPr )4 grafted on MCM-41 and deconvolution. The fifth ligand in the pentacoordinated species is represented as an adjacent surface siloxane bridge, but water or surface silanols are also possible (see text). (From Reference [64].)...

The coupling of organosilicon compounds with organic electrophiles was not disclosed until 1988 by Hatanaka and Hiyama, when they demonstrated that through the addition of an appropriate silicophilic nucleophile, those desired pentacoordinate species can be generated in situ and transfer an unsaturated group. Nucleophilic fluoride sources were found to be the additive of choice, typically TASK, TBAF, and, in some cases, KF and GsF. These are the fundamental concepts of what is nowadays called the Hiyama reaction. The use of fluoride... 

It has been known for some time, as a result of the work of Prof. Adamson and others, that cobalt (I I) forms a stable pentacyano complex in aqueous solution. However there have been some questions raised about the structure of this species particularly as to whether it is a truly pentacoordinated species or whether there is a loosely held water molecule. 

Both four- and five-coordinate platinum(II) complexes can be prepared with phosphole ligands L (119).1328 The complexes /rans-PtX2L2 (X = Cl, Br, I R = Me, Bun, Bu, Ph, CH2Ph R = H, Me) are non-electrolytes in methanol solution. The formation of the five-coordinate adduct (equation 411) can be analyzed in terms of intra- and inter-molecular equilibria of the pentacoordinate species PtX2L3. The formation of PtX2L3 is enthalpy favored and entropy disfavored. 

In these reactions the pentacoordinate species is a high-energy transition state, not an intermediate. 

Corriu and coworkers have proposed50 an alternative mechanism involving extension of the coordination of the silicon atom. The first step involves rapid and reversible attack by a molecule of the nucleophilic catalyst, Nu, to give the pentacoordinate species 23 (equation 11). 

Numerous experimental observations of the increased reactivity of pentacoordinate silicon over tetracoordinate silicon have been reported. Corriu and coworkers have found that the relative reactivity of PbMeSiI 3 and PhMeSiF2 towards t-BuMgBr was >1000 155. The pentacoordinate species 29 reacts with alcohols and acids to give mono- or disubstitution products whereas the corresponding four-coordinate species 30 does not react at all56. 

Substitution with inversion and retention of the pentacoordinate species 33 has been interpreted in terms of two competing dissociative mechanisms Si—O cleavage and Si—NMI cleavage65. A hexacoordinate intermediate is not implicated. 

This reaction, which initially was assumed to be a simple deprotonation8a,b, was shown later by Corriu and coworkers9 to proceed via a two-step mechanism. The initial step is the reversible formation of a pentacoordinated species, which decomposes irreversibly under formation of molecular hydrogen and the metalated silane. 

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