Coordination-addition

H R = Br) [85JCS(D)973] containing a rhodium-rhodium brmd. Each rhodium atom is octahedrally coordinated. Addition of iodine to 176 (E =... 

A careful distinction must be drawn between transition states and intermediates. As noted in Chapter 4, an intermediate occupies a potential energy minimum along the reaction coordinate. Additional activation, whether by an intramolecular process (distortion, rearrangement, dissociation) or by a bimolecular reaction with another component, is needed to enable the intermediate to react further it may then return to the starting materials or advance to product. One can divert an intermediate from its normal course by the addition of another reagent. This substance, referred to as a trap or scavenger, can be added prior to the start of the reaction or (if the lifetime allows) once the first-formed intermediate has built up. Such experiments are the trapping experiments referred to in Chapters 4 and 5. 

To illustrate further aspects of the interplay between covalent and coordinate bonding, let us consider the successive coordinative additions of ammonia molecules (ammine ligands, NH3) to tungsten hydrides in the series H6 2n W(NH3) , n = 1-3,... 

Complex 2 efficiently catalyzes the cycloaddition reaction of methacrolein with the nitrones I-V. Table 1 lists some results obtained. The reactions were performed in CH2CI2 in the presence of 4 A molecular sieves, with 5 mol% of catalyst loading and a 1/140/20 catalyst/methacrolein/nitrone molar ratio. Typically, quantitative yields are obtained after a few hours at —25°C. The acyclic nitrone 11 generates the less active system but, even so, 78% conversion was achieved after 24 h at —10°C (entry 2). Enantiomeric excesses greater than 90% were achieved in most cases. A greater excess of methacrolein improves both rate and enantioseletivity (compare entry 6 with a catalyst/methacrolein/nitrone molar ratio 1/28/20 with entry 3 with a 1/140/20 molar ratio). To avoid undesired nitrone coordination, addition of the cyclic nitrones III-V was accomplished over 10 h. 

Structural diversity is achieved through the use of nonbonded pairs of electrons on the ligand of both type II complexes to coordinate additional metal atoms. The S—S distances of known complexes range from 1.98 to 2.15 A. Most S—S distances are intermediate between the distance of 1.89 A for Sj ( Zg ) (104) and 2.13 A for ( Zg+) in Na2S2 (50). The main S—S distances show no clear systematic trend with structural type (cf. Table II). 

Dilithiated diamine 2 was synthesized by Karsch by a two-fold metalation of N,N,N, N tetramethylmethylenediamine (TMMDA) (1). The reaction was effected in n-pentane at low temperatures, yielding the poorly soluble Af,Af -bis(fithiomethyl)-Af,Af -dimethyl-methylenediamine (2) (Scheme 1). Due to its low solubility in toluene or THE, the highly pyrophoric compound was characterized by derivatization with several electrophiles, mainly chlorosilanes. Obviously, the addition of coordinating additives, such as TMEDA, DME (dimethoxyethane) or THE, does not enhance the solubility of the dilithium compound. Interestingly, as the author comments, TMEDA is only monolithiated in modest yields by alkyllithium bases. 

The geminal dihalogenated cyclopropane derivatives 83a and 83b were lithiated by Vlaar and Klumpp . 7,7-Dichloro- (83a) and 7,7-dibromonorcarane (83b) were reacted with four equivalents of LiDBB in diethyl ether and several reaction conditions were examined by the authors such as reaction temperatures, the influence of different coordinating additives as well as various methods (Scheme 31). The achieved maximum yield for the geminal dilithium compound 84 was 55% (from 83b). Side-products, like the 1,2-dilithioethane derivative 85, the dilithiated dicyclohexylacetylene 86 or 1,3-dilithium compound 87, were observed in different quantities, sensitively depending on the reaction conditions. Also, carbenoid intermediates were formed as verified by trapping reactions (deuteriolysis). 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

A related zinc-alkyne interaction was suggested on the basis of the 13C and Raman spectra of di(4-hexynyl)zinc27. Interestingly, addition of coordinating additives such as pyridine prevented the formation of the latter interaction whereas weaker Lewis bases such as diethyl ether did not. 

Of the steps listed in Table 1. some are encountered more frequently, while others are less common. Transition metal catalyzed processes usually begin with oxidative addition or coordination-addition as an Entry, which is commonly followed by transmetalation or insertion in the Attachment phase. The final Detachment step is either reductive elimination, or p-hydride elimination, depending on the nature of the intermediate. 

The entries into transition metal catalysis discussed so far, required the presence of a specific bond (a polar carbon-heteroatom bond for oxidative addition or a carbon-carbon multiple bond for coordination-addition processes) that was sacrificed during the process. If we were able to use selected carbon-hydrogen bonds as sacrificial bonds, then we could not only save a lot of trouble in the preparation of starting materials but we would also provide environmentally benign alternatives to several existing processes. In spite of the progress made in this field the number of such transformations is still scarce compared to the aforementioned reactions. 

With terpy, compounds of the type Ag(terpy)X have been prepared and cannot be tetrahedral for steric reasons, in that terpy cannot span three corners of a tetrahedron. It has been argued that more likely only two of the N atoms are coordinated and that a distorted linear structure is adapted.115 However, if this was the case the central N group would necessarily be within bonding distance. The other alternative was that they were three-coordinate. Addition reactions with neutral ligands such as H20, pyridine or phosphines would then yield distorted square planar structures. 

Nickel(II) complexes display a variety of equilibria which involve spin state changes. Planar four-coordinate complexes are invariably diamagnetic. These can undergo an intramolecular isomerization to paramagnetic tetrahedral four-coordinate species. Alternatively, the planar complexes can coordinate additional ligands to form five- and six-coordinate paramagnetic complexes. The additional ligand molecules can be Lewis bases in solution, or solvent molecules, or, in par-... 

Tin forms dihahdes and tetrahahdes with all of the common halogens. These compounds may be prepared by direct combination of the elements, the tetrahalides being favored. Like the halides of tile lower main group 4 elements, all are essentially covalent. Their hydrolysis requires, therefore, an initial step consisting of the coordinative addition of Iwo molecules of water, followed by the loss of one molecule of HX. the process being repeated until the end product ft S111 Oil c is obtained. The most significant commercial tin halides are stannous chloride, stannic chloride, and stannous fluoride. 

Structural diversity is exhibited through the utilization of the nonbonded pairs of electrons on the S2 ligand of both type I and type II complexes to coordinate additional metal atoms. 

The ability of n-arene metal complexes to coordinate additional metal atoms may be significant in the early stages of metallization of some phenyl-rich polymers. At the lowest coverages of say chromium on polystyrene, (arene)2Cr may be formed. With continued accumulation of metal these compounds may rapidly convert to thermally unstable organometallic cluster species that eventually expel the metal core. 

Some other schemes of the process were suggested, but they were rejected for various reasons. At present, the problem under discussion is whether the reaction proceeds as coordinated addition or by two stages through the intermediate biradical state. Most often, it is assumed that the former mechanism prevails for solutions the latter one, for the gas phase [4-6, 10, 11]. However, at present, the mechanism of coordinated addition is considered prevailing for the gas phase too [12-14]. 

However, the calculations were performed in terms of the restrieted Hartree-Foek method (i.e., with a strictly-defined zero spin) in the one-determinant approximation. This approach may account for a loss of solutions for cases of eomplex systems where the spin squared deviates from zero although all reagents are in the singlet state. Henee, this solution may have the physical meaning [21], as is the ease with ozone. In the literature, these cases are considered as unstable Hartree-Foek s solutions. In [22], ealeulations were performed with allowanee for this condition in terms of the unrestrieted Hartree-Foek (UHF) method the ealeulations showed that the reaction may proceed by non-coordinated addition but with the induction of higher-spin states (S = 0.7-1.2). 

Here, pathway 1 (reaction 1) is the coordinated addition of ozone (1) to ethylene (2), which proceeds through the formation of a weakly-boimd complex that transforms into primary ethylene ozonide (PO) or 1.2.3-trioxolene upon passing through the symmetrical transient state (TSl). Pathway 2 (reaction 2, the DeMore mechanism [15]) involves the collision during spontaneous orientation of the reagents (3) and the rotational transition to the biradical transient state (TS2) (4) followed by the formation of the same PO. Proceeding from the above-said, we supplement this pathway with the reaction of detachment of molecular oxygen and the formation of intermediate biradical (5) the latter may either decompose with the formation of formaldehyde (6) and carbene (7) or transform into acetaldehyde (8) or epoxide (9). Finally, pathway 3 involves the transition of ozone into the triplet state (10). This pathway is similar to reaction 2. Here, the same biradical (5) is formed it transforms into the... 

Figure 4. Transient state (TS2) in the non-coordinated addition of ozone to ethylene.

Figure 5. Change in the energy of the ethylene+ozone system by the pathway of non-coordinated addition (1) in the singlet and (2) triplet states and (3) change in the y value in the former case UB3LYP calculations.

The mixed anhydrides of diphenylphosphinous acid and heavily substituted alkenoic acids displace one PPh3 ligand from [RhCl(PPh3)3] to form a complex where the anhydride is coordinated through phosphorus. If these products are treated with thaUium(I) or silver(I) salts, the anhydride coordinates additionally through the carbonyl oxygen and the chloro ligand is displaced. 

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