Pentacoordinate lead complexes

A special case of reversible ionization of a hexacoordinate silicon complex has been described as a novel tautomeric equilibrium.41 It differs from the formation of siliconium-ion salts in that the positive charge resides on nitrogen, in a dimethylammonium cation, and not on silicon. The transsilylation of lg with 12 in equimolar concentrations leads to the pentacoordinate zwitterionic complex 13 (Eq. (10), Section II.B.5). However, when the molar ratio was 2 1, respectively, an equilibrium mixture of tautomers (58, 59) was obtained, as shown in Eq. (21). The same mixture was also obtained when a second mole-equivalent of lg was added to 13. 

The analogy developed above between pentacoordinated complexes and organic free radicals is capable of meaningful extension to coordination compounds of other electron configurations and coordination numbers see Table III). Thus, similar reasoning leads to the expectation of similarities between the reactivity patterns of tetracoordi-nated d complexes and carbenes, pentacoordinated d complexes and carbanions, and pentacoordinated d complexes and carbonium ions. In each case the stoichiometries of the reactions which restore the stable closed-shell configurations are the same for both species hence the similarity of reactivity patterns. 

The mechanism of asymmetric induction at a trivalent phosphorus atom was explained by formation of pentacoordinated transition state b, pseudo-rotation, and exchange of ligands at a pentacoordinated phosphorus atom, resulting in the thermodynamically more stable diastereoisomer. The effect of the reaction conditions on the diastereoisomeric ratio of products c shows thermodynamic control. For example, a decrease of the reaction temperature reduces the stereoselectivity, which is impossible with kinetic control, because the temperature lowering leads to a deceleration of the equilibrium establishment of pentacoordinated intermediate complex (Scheme 16) [33]. 

Scheme 24), the hydride migration from silicon to an adjacent unsaturated imino carbon atom leads to a pentacoordinated silicon complex 89 as final product [170]. For the intermediate 88 a dynamic equilibrium between two conformers 88a and 88b with pentacoordinated Si atom was observed by NMR spectroscopy. For related compounds with hexacoordinated Si atom within a (0,N>2SiMe(H) coordination sphere, the authors observed reversible neutral dissociation of the N-Si dative bond, i.e., an equilibrium between hexa- and pentacoordinated hydrido complexes of silicon [235]. 

Silicon-mediated nitrile activatirai has also been observed in trichloro [2-(dialkylphosphanyl)imidazol-l-yl]silanes (Scheme 29) [133]. These react with aceto- or propionitrUe to yield the dinuclear pentacoordinated silicon complexes 98. The reactions leading to 96,97, and 98 hint to new pathways of catalytic activation mediated by dinuclear sihcrai complexes. 

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... 

Orbital symmetry arguments and EHT calculations have also provided a way of discriminating between axial and apical substitution in the above mentioned case of pentacoordinate phosphorus. This analysis leads the way to more complex problems of coordination around transition metal atoms. 

Plumbene-NHC complex 36 is generated by the reaction of an NHC with a bis(aryl)-lead(II) compound. The NHC-silylene adduct 37 also features a long C-Si bond with significant Si polarity. Pentacoordinated silicon(lV) and... 

Pentacoordinated Rh3 + corrolates react, in mild conditions, with isocyanides leading to the formation of hexacoordinated complexes Rh(OMC)(CNR)2 [24]. 

A metal-carbon bond splitting is the first step in the sequence leading from methylcobalamin Im-Co(corrin)-CH3 to acetylcobalamin Im-Co(corrin)-COCH3 [117]. The radical CH3 formed in the primary photoredox step, associated with the reduction of Co(III) to Co(II), is trapped by a CO molecule and the redox addition of the radical CH3CO to the reduced pentacoordinated complex Co(II) results in the final Co(III) acetyl complex. 

At the same time, according to x-ray data for zinc chelate 909, the nitrogen atom is turned to the side of the metal. The distance Npy-Zn is 2.80 A, that allows us to consider the possible participation of the examined donor center in binding with the metal, leading to formation of a hexacoordinated structure (two-capped tetrahedron) [243]. In relation with this result, let s pay attention to the data reported in Refs. 244 and 248. The tetrahedral configuration without coordination of the nitrogen atom of pyridine is attributed to the cobalt complex 907 (X = NTs, M — Co), although this N atom is rotated to the side of the metal [244]. The pentacoordinated complex 910 is described in Ref. 248, in which only one pyridine substituent is coordinated (the distance Npy-Co is 2.45 A) ... 

DNA polymerases catalyze DNA synthesis in a template-directed manner (Box 16). For most known DNA polymerases a short DNA strand hybridized to the template strand is required to serve as a primer for initiation of DNA synthesis. Nascent DNA synthesis is promoted by DNA polymerases by catalysis of nucleophilic attack of the 3 -hydroxyl group of the 3 -terminal nucleotide of the primer strand on the a-phosphate of an incoming nucleoside triphosphate (dNTP), leading to substitution of pyrophosphate. This phosphoryl transfer step is promoted by two magnesium ions that stabilize a pentacoordinated transition state by complex-ation of the phosphate groups and essential carboxylate moieties in the active site (Figure 4.1.1) [2],... 

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