Five-coordinated intermediate

The reactivity of the closely related system TpMe2PtMeH2 toward electrophiles in arene solvents has also been reported recently (68). The boron-based Lewis acid B(C6F5)3 induced elimination of methane and formation of an aryl(dihydrido) platinum(IV) complex via arene C-H activation (Scheme 17, A -> C). The active acid may be either B(C6F5)3 or alternatively a proton generated from B(C6F5)3 and trace water. It was proposed that the acid coordinates to a pyrazole nitrogen (shown in Scheme 17, B) forming an intermediate five-coordinate platinum(IV) complex, which readily eliminates methane. 

Fig. 4. An intermediate five-coordinate Class A compound before oxygen insertion...

Depends on the electron configuration of the central metal ion and also on the configuration of the intermediate five-coordinate transition complex. 

Five-coordinate complexes are far more common than was once supposed and are now known for all configurations from d to d. Two limiting stereochemistries may be distinguished (Fig. 19.4). One of the first authenticated examples of 5-coordination was [VO(acac)2] which has the square-pyramidal 4 structure with the =0 occupying the unique apical site. However, many of the complexes with this coordination number have structures intermediate between the... 

Note that the first step in Figure 21.6—reaction of the carboxylate with ATP to give an acyl adenylate—is itself a nucleophilic acyl substitution on phosphorus. The carboxylate first adds to a P=0 bond, giving a five-coordinate phosphorus intermediate that expels diphosphate ion as leaving group. 

The P450 reaction cycle (Scheme 10.4) starts with four stable intermediates that have been characterized by spectroscopic methods. The resting state of the enzyme is a six-coordinate, low-spin ferric state (complex I) with water (or hydroxide) coordinated trans to the cysteinate ligand. The spin state of the iron changes to high-spin upon substrate binding and results in a five-coordinate ferric ion (com-... 

One possibility is a limiting SN1 mechanism with a five-coordinate cobalt complex as an intermediate,... 

Yamamoto and Yamazaki 171) carried out reactions of Pt(PEt3)2CHjI and Pt(PPhMe2)(CH2CjH5)I with tert-butyl and cyclohexyl isocyanides. These reactions gave small yields of the ionic intermediate species, which readily reverted to the appropriate iminoacyl complexes. In reactions of analogous chloride complexes the intermediate species was not isolated. It is suggested on the basis of PMR data that these iminoacyl complexes are five-coordinate (see above). 

Another typical example for anisotropic covalency is found in five-coordinate ferric compounds with intermediate spin S = 3/2 (also discussed in Sect. 8.2). Crystal field theory predicts a vanishing valence contribution to the EFG, whereas large quadrupole splittings up to more than 4 mm s are experimentally found. 

Intermediate spin occurs mainly for square- and rhombic-pyramidal five-coordinate iron complexes and also for planar four-coordinate and for asymmetric six-coordinate... 

Coordination compounds of dianionic dithiolene (S2C2 R2) and benzene-1,2-dithiolene (bdt = (S2C6H4) and their derivatives have been studied since the 1960s by Mossbauer spectroscopy [87] and other techniques. Nevertheless, many aspects of their electronic structure remained uncertain for a long time. The five-coordinate ferric complexes with two equatorial dithiolene ligands exhibit intermediate spin and show the Mossbauer parameters = 0.25-0.38 mm s and A q = 1.6-3.2 mm s For example, [Fe° mnt)2/ y] with two mnt ligands (=S2C2(CN)2) and an... 

In a crystal-field picture, the electronic structure of iron in the five-coordinate compounds is usually best represented by a (d yf idyz, 4cz) ( zO configuration [66, 70], as convincingly borne out by spin-unrestricted DFT calculations on the Jager compound 20 [68]. The intermediate spin configuration with an empty d 2 yi orbital in the CF model, however, has a vanishing valence contribution to the... 

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,... 

There have been a number of hydrolysis studies. The mechanism of alkaline hydrolysis of phenyl dimethylthiophosphinate has been compared to that of phenyl acetate.27 Evidence for the formation of five coordinate oxyphosphorane intermediates in the alkaline hydrolysis of aryl diphenylthiophosphinates is based on the lack of development of negative charge on the leaving group in the transition state.279... 

Five-coordinate or 16-electron platinum(IV) complexes have been avidly sought for many years because of their relevance as intermediates in catalytic processes. However, the first two structurally... 

---