Five-coordinate intermediate

A detailed examination of the kinetics of dimethylaminolysis of N3P3C16 by Krishnamurthy and co-workers has revealed that there is a gradual and subtle mechanistic change that occurs as the degree of replacement of chlorines increases (92). While the first chlorine replacement follows an Sn2 pathway involving the formation of a neutral five-coordinate intermediate [Fig. 8(A)], at the second stage the mechanism can be induced to follow a concerted path [Fig. 8(B)] by using acetonitrile as the solvent. The polar transition state of the concerted path reaction pathway is stabilized in acetonitrile. This postulate has sup-... 

An n.m.r. study of the permutational isomerisations within a series of amidium fluorophosphates (133) has revealed an irregular mechanism with dissociation of the P-N and also the P-0 bonds of the bidentate ligands and the formation of five-coordinate intermediates (134)61. Reversible intramolecular migrations of pentaco-ordinate phosphorus in the N-C-N triad of amidines (e.g. 135, 136,... 

The formation of the complexes 6s CHCHC(6)R1 Cl(CO)(P Pr3)2 (R1 =Ph, H) from the corresponding Os CH=CHC(OH)R1R2 species could initially in-volve the elimination of HC1 to give five-coordinate intermediates of the type 6s CH=CHC(6)R1 R2 (CO)(P Pr3)2, which by a subsequent hydrogen 13-elimination (R2 = H) should give OsjCIICIICtOiR1 JIKCOXP1 Pibh- Then the... 

Casey et al. have studied the decarbonylation reactions of [cis-(OC)4M(MeCO)(PhCO)], in which M is Mn or Re (16,17). These complexes lose a carbonyl ligand to form five-coordinate intermediates of the type [(OC)3M(MeCO)(PhCO)]. Reversible methyl migration proceeds much more rapidly than does phenyl migration. In the course of these studies, a phosphine substituted rhena-/3-diketonate complex, [fac-(OC)3(Et3P)Re(MeCO) (PhCO)], was prepared. 

Mechanistic details are very similar for DMSO and water exchange. The reaction proceeds through a distorted trigonal bipyramidal reactive intermediate [Li(DMSO)5]+ that is reached via a late transition state. The enthalpy profile (see Fig. 13) is in line with the experimentally observed very fast exchange process. The five-coordinate intermediate is computed to be 7.9kcalmol 1 less stable than [Li(DMS0)4]+ and free DMSO, while an overall activation barrier of only 8.4kcalmol 1 is computed. Obviously,... 

Irradiation of [Pt(hfac)2] with an excess of ethene at 350 nm produced an X-ray-characterized five-coordinated intermediate containing both hfac ligands still bidentate (i.e. limiting A mechanism) (208). Quenching of photoexcited [Pt(terpy)Cl]+, and 4 -substituted terpy and NCS- analogues variously, by a range of Lewis bases, such as MeCN, DMSO, py, acetone, has been documented (209). 

Intramolecular coordination site exchange reactions in square-planar platinum(II) complexes [Pt(tbte)Cl]+ of the potentially four-donor-atom tripodal ligand tris[2-(t-butyl-thio)ethyl]amine (tbte) N,S,S-bonded occur through a trigonal bipyramidal five-coordinate intermediate... 

Flash photolysis of the dianion of Roussin s Red Salt, [Fe2S2(NO)4]2, in particular the initial photoinitiated loss of NO (382) and the reverse recombination reaction, en route to the eventual product, the anion of Roussin s Black Salt, [Fe4S3(NO)7] , has been documented (383). A 4-RC6H4S group (R = H, Me, OMe, Cl, or CF3) replaces one of the chloride ligands in [Fe4S4Cl4]2 via a five-coordinated intermediate, with the detailed sequence of steps acid-dependent (384). Loss of chloride is... 

F. Five-coordinate Intermediates and a-Complexes Computational Approaches... 

A. Five-Coordinate Intermediates Involved in Carbon-Heteroatom Coupling Reactions from Pt(TV)... 

Chelating ligands have a much lower propensity for dissociation. Yet detailed kinetic studies on similar systems with bidentate phosphine ligands, /ac-(L2)PtMe3X (L2 = dppe (bis(diphenylphosphino)ethane), dppbz (bis(diphenylphosphinobenzene) X = I, OAc, OPh) also showed that ligand dissociation was required prior to any C-C coupling (48-51). In this case, however, the X- group rather than the phosphine was lost to form a five-coordinate intermediate, as shown in Scheme 11. A competitive C-X reductive elimination also occurs from these complexes and involves the same five-coordinate cation (Section V. A). 

Thus to date, virtually all studies of C-C reductive elimination to form alkanes from Pt(IV) have found that these reactions proceed via five-coordinate intermediates. Only very recently have stable examples of Pt(IV) alkyl hydrides been synthesized (53-69). Detailed studies of C-H reductive elimination to form alkanes from these related complexes have identified similar five-coordinate intermediates on the reaction pathway (see following section). 

What specific properties of these complexes have allowed isolation of five-coordinate Pt(IV), in the form of the trimethyl complex and the dihy-dridosilyl complexes These two types of complexes are significantly different, and their stability is apparently due to different factors. Comparing the trimethyl complex in Scheme 21(A) with the related but six-coordinate complexes of a similarly bulky oc-diimine ligand (98), shown in Scheme 23, is instructive. In Scheme 23A, triflate is clearly coordinated, exhibiting an O-Pt distance of 2.276(3) A (98), which is typical for Pt-coordinated triflate (108). This triflate complex A in Scheme 23 was obtained from dry tetrahydrofuran. The aqua complex cation B, also structurally characterized, was obtained from acetone containing trace water. An equilibrium between coordinated triflate and coordinated water, very likely via a common five-coordinate intermediate, was indicated by NMR spectroscopy (98). 

In the case of ethylene hydrogenation, the mechanism proposed by Parshall [61] involves the coordination of an alkene molecule through a five-coordinate intermediate (Eq. (13)) the subsequent alkene insertion into the Pt-H bond (Eq. (14)) and intervention of a second molecule of H2 (Eq. (15)) leads to the elimination of ethane and restoration of the catalytic active species [PtH(SnCl3)]2. However, in 1976 Yasumori and coworkers reported a kinetic analysis conducted on the hydrogenation of ethylene catalyzed by the Pt-Sn complex [(Me)4N]3[Pt(SnCl3)5] [70], under much milder conditions than those... 

All the kinetic features expected for a D mechanism and rate law (4.9) i. e. marked effects of L and Lj on the rate constants, are shown in the comprehensive studies in nonaqueous solution of substitution in low-spin Fe(II) complexes of the type FeN4XY where N4 are planar porphyrins, phthalocyanins and macrocycles and X and Y are neutral ligands, CO, R3P, pyridines etc. Small discrimination factors (Ar, /kj) suggest that the five-coordinated intermediate in these systems is very reactive.There have been problems in the confirmation of curvature in the plots of A o,j/[L ] for classical reactions of a number of aquapent-ammine complexes. ... 

An intermediate of the type Co(NH3)4NH + is postulated in (4.45). The subsequent reactions of this intermediate should be independent of the nature of the X group in the starting material. The results of early experiments to verify this point have represented some of the most powerful support for the mechanism. Base hydrolysis of Co(NH3)jX + in an H2 0/H2 0 mixture was found, as required, to give a constant proportion of Co(NH3)5 OH + and Co(NH3)5> OH +, independent of X being Cl, Br or NO3-. The competition experiments described in Sec. 2.2.1 (b) support a five-coordinate intermediate which is so short lived that it retains the original ion-atmosphere of Co(NH3)jX "1+ but has lost memory of the X group. ... 

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