Pentacoordinated radicals

Tri- and pentacoordinate phosphoms compounds often react by electron-pair mechanisms as demonstrated by the nucleophilic reactivity of the lone pair electrons in trivalent compounds, and the electrophilicity of the phosphoms atom in the pentavalent compounds. Some compounds also react by free-radical mechanisms. The theoretical and synthetic aspects of the chemistry of phosphoms compounds have been described (6—9). 

Bimolecular substitution and oxidation reactions of 17-electron pentacoordinate metal carbonyl radicals. A. Poe, Transition Met. Chem. (Weinheim, Ger.), 1982,7, 65-69 (41). 

Three- and pentacoordinate organic phosphorus compounds can be oxidized through a free radical Arbuzov reaction, i.e., formation and p-scission of a phosphoranyl radical (Scheme 24). The P-scission is regioselective homolysis occurs on a ligand located in an equatorial site. Both a- and P-scissions are strongly dependent on the strength (bond dissociation energy) of the cleaved... 

Some characteristics of free-radical terpolymerization of tri-butylstannyl methacrylate, styrene and maleic anhydride governed by the pentacoordination state of the tin atom are reported in Refs. 95),96). It is shown that a coordination-bound monomer has a considerable effect on chain initiation and propagation. Copolymerization mainly involves the participation of complex-bound monomers. 

This would explain the fact that groups which form stable radicals, e.g., benzyl, allyl, or phenacyl, are easiest to remove electrolytically, and that tertiary alkyl groups are removed more readily than secondary or primary. This mechanism is more likely than one previously suggested, which involves a pentacoordinate phosphorus hydride ( 70) 61 > ... 

The rearrangement 12 13 is not straightforward and two alternatives routes have been suggested. One of them involves a 1,3 transfer from silicon to oxygen to give the silyl radical 15 followed by Sui reaction on the peroxide moiety [15]. The other indicated by PM3 calculations is a dioxirane-like three-membered intermediate 16 having a pentacoordinated central silicon followed by a 1,2 silyl migration to afford radical 13 [19]. 

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

The proposed intermediate is the pentacoordinate silyl radical, which eliminates the most stable alkyl radical to give the corresponding fluorosilane (equation 1 and Table 2). 

The methane dication (CH42+) with a lifetime of only 3 jxs803 has been detected experimentally in the gas phase.804-807 A square-planar D4h symmetrical structure was predicted by early calculations.808,809 However, the planar but not square C2v symmetrical form (443) was shown to be the preferred structure by Wong and Radom.810 The cation contains an, vp2-hybridized carbon, an empty p-orbital perpendicular to the plane of the molecule, and a 2e-3c interaction. It can be visualized as a complex of a hydrogen molecule weakly bounded by a 2e-3c bond to the CH22+ core as shown by ab initio calculations [MP2 and B3LYP levels with 6-311++G(2df,2pd) basis set].811 The pentacoordinate CH52+ radical dication was studied by Stahl... 

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. 

A detailed discussion on the properties of Mils is given in Section V.D.l and its calculated structure is shown in Figure 9. In this section we will discuss the role of pentacoordinated MRs in free-radical homolytic substitution reactions. 

Immediately upon contact with air, the high-spin complex (g 6.1) attributable to Fe(III) TPP py (pentacoordinated complex) is observed together with the free radicals whose g values are 1.99-2.04 and the low-spin Fe(III) complex (see Figure 4). The formation of a pair of low-spin complexes attributable to the hexacoordinated Fe(III) TPP complexes (g = 2.66, 2.19, 1.80 and g = 2.31, 1.93) occurs with time. Of those, one should be the Fe(III) TPP py2 and another the product-coordinated Fe(III)-TPP complex. The appearance of ESR absorptions attributable to Fe(III) complexes (high-and low-spin) indicates that a part of the Fe(II)-TPP complex is converted to Fe(III) TPP by reaction with molecular oxygen. 

Evidence supporting Co-C bond homolysis has been obtained from the X-ray crystal structures of several Bi2-binding proteins [36-39], where cobalt is found in a form consistent with Co (i.e., pentacoordinated). An abundance of electron spin resonance (ESR) studies have dso provided support for the homolysis of the cobalt-carbon bond [31, 40-43]. ESR spectra of the holoenzymes can be analyzed in terms of two dissimilar, strongly interacting, spins [41]. One spin is almost certainly cob(II)alamin [42], while the other is most likely a carbon-centered radical [41]. 

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