Hexacoordinate lead complexes

For the hexacoordinate starting complex (1) derived from the crystal structure of the binary complex, the most stable conformer (la, Fig. 6) was found to feature the imidazole ring in His 290 rotated 180° compared to that of the crystal structure, and this conformation of His290 was found to be preferred for all stationary points leading up to the FeIV=0 intermediate. Unless otherwise noted, all energies in this study are given relative to la. 

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. 

A convenient method for the preparation of neutral bis(N->Si) hexacoordinate silicon complexes has been developed and reported recently, consisting of ligand exchange between a polychlorosilane (1) and 0-trimethylsilyl derivatives of hydrazides (2, Eq. 1) [2]. An attempt to utilize this synthetic route for the preparation of isomeric 0->Si coordinated chelates did not lead to the expected hexacoordinate complexes, but to ionic siliconium chloride salts stabilized by two (O—>Si) dative bonds (5, Eq. 2) [3]. 

Higher temperatures, increased K2TaF7 concentrations and other factors mentioned above shift the equilibrium in Equation (174) to the left and lead to the formation of coarser tantalum particles. Form this point of view, it can be concluded that smaller hexacoordinated complexes, TaF6 lead to the formation of coarser tantalum powder, whereas the predominant presence of larger heptacoordinated complexes ions initiates the formation of finer particles. 

When the bis(isopropylamino)iodocyclopropenylium iodide is reacted with platinum black in acetonitrile, the reaction takes a different course, affording mainly the trans-bis[bis(diisopropylamino)cyclopropenylidene] diiodoplatinum complex (equation 277)351. A plausible pathway for this reaction involves two consecutive oxidative additions to platinum leading to the hexacoordinated intermediate Ptlv-complex [ -Pr2N)2C3]Ptf4, followed by reductive elimination of I2 to form the product (cf Section VI. A. 1. a). 

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

In addition to photosubstitution and photoelimination reactions, in the cases of some Ni(II) complexes, photoexcitation of square-planar complexes Ni(TP) and formation of the photoassociative ligand-field (LF) excited state 3Blg can lead to photoaddition reactions yielding hexacoordinate complexes Ni(TP)L2 [65, 66, 75-77], Such processes differ from the second step of photosubstitutions since an excited complex participates in them and the addition is conditioned by the electronic structure of the complex in its excited state (see Table 3). 

Complexation of the well-known dithiocarbazate chelate ligand (DTCA ) with Al3+, Sn2+, and Sn4+ ions leads to the formation of hexacoordinated complexes with 1 1 (M + DTCA) ratio, which were investigated for antimicrobial activity <2001SRI115>. These compounds, however, were found to be thermally unstable. 

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) ... 

We had already seen that oxidation of [Cr I(CO)10]2- with I2 leads to yellow [Cr°(CO)uI]- (57). With excess iodine, however, the oxidation goes further, to the deep-blue, hexacoordinated, thermally very unstable Cr+I(CO)sI, by which reaction the dinuclear, deep red, paramagnetic, neutral complex of Crj(CO)10I may be isolated as the intermediate. The actual compound formed is solely dependent upon the mole ratio of [Cr2(CO)1o],- I, (57, 58, 68) ... 

Pathways B and C describe the process that could occur to form the symmetrical silyl ketal. Formation of silyl ketal-metal complex VII could occur in one step via an associated pathway B, or in two steps via a dissociative pathway C. Pathway B could occur by attack on the pentacoordinated silyl ketal-metal complex IV by alcohol ROH, forcing the dissociation R OH leading to complex VII. The associative pathway would precede through a hexacoordinated silicon species. Hexacoordinated silicon compounds have been shown to be very reactive.36,23d The choice of the alcohol that dissociates would be determined by electronic and/or steric factors. 

An interesting case of a Jahn-Teller distorted hexacoordinate copper(II) complex is shown in Fig. 12.3. The elongation along the O-Cu-O axis leads to a loss of delocalization within the hfacac ligand skeleton. Clearly, the simple... 

A similar reaction to that described above (Eq. 7) occurs also with chloromethyl(methyl)dichlorosilane (12) (Eq. 10).41 The reaction leads to a tautomeric equilibrium which interconverts a pentacoordinate with a hexacoordinate complex. This topic is discussed separately in Section III.A.6. 

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