Trimeric species

In very acid media, Zr, (OH), becomes dominant (see Appendix D). The species forms according to Reaction (V.IO)  

The following equilibrium constant is selected in this review consistent with the explicit consideration of water activity  

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As discussed in preceding sections, FI and have nuclear spin 5, which may have drastic consequences on the vibrational spectra of the corresponding trimeric species. In fact, the nuclear spin functions can only have A, (quartet state) and E (doublet) symmetries. Since the total wave function must be antisymmetric, Ai rovibronic states are therefore not allowed. Thus, for 7 = 0, only resonance states of A2 and E symmetries exist, with calculated states of Ai symmetry being purely mathematical states. Similarly, only -symmetric pseudobound states are allowed for 7 = 0. Indeed, even when vibronic coupling is taken into account, only A and E vibronic states have physical significance. Table XVII-XIX summarize the symmetry properties of the wave functions for H3 and its isotopomers. 

Polymeric isocyanates or PMDI ate cmde products that vary in exact composition. The main constituents are 40—60% 4,4 -MDI the remainder is the other isomers of MDI, trimeric species, and higher molecular weight oligomers. Important product variables are functionaHty and acidity. Rigid polyurethane foams are mainly manufactured from PMDI. The so-called pure MDI is a low melting soHd that is used for high performance polyurethane elastomers and spandex fibers. Liquid MDI products are used in RIM polyurethane elastomers. 

Silicagel. Silicagel is usually prepared from sodium silicates, which are produced as glasses with a stoichiometry of Si02/Na20 = 0.6-0.25. At the right pH they arc soluble in water and the solution contains mixtures of silicate anions monomeric, dimeric, trimeric species, etc. (Fig. 3.13). 

The chemistry of silver(I) with crown thioethers has developed in the last years,1094,1095 but now numerous examples with a great variety of ligands have been reported. The first homoleptic silver compounds have been described with 1,4,7-trithiacyclononane ([9]aneS3), [Ag([9]aneS3)2]OTf (168), which is octahedral and shows a reversible oxidation assigned to the Ag1/Ag11 couple,1096,1097 and the trimeric species [Ag3([9]aneS3)3]3+ (169).1097 The complexes... 

Dimeric zinc complexes of tertiary phosphines, [Zn(PR3)I2]2, are also formed from zinc powder and R3PI2, (R = Me, Et, n-Pr, ra-Bu). The crystal structure of the ethyl derivative demonstrates the dimeric nature of the complexes.295 Metallation of diphenylphosphine with ZnEt2 results in a trimeric species with a Zn3P3 core and bridging diphenyl phosphide ligands. Two protic (HPPh2)... 

Dialkyl zinc compounds form phosphine complexes of formula RZnP(SiMe3)2 on addition of one equivalent of bis(trimethylsilyl)phosphine. Solution and solid-state studies showed that the complexes are dimeric or trimeric in solution or the solid state. Bis(trimethylsilyl)phosphido-methylzinc crystallizes as a trimeric molecule with a Zn3P3 core in the twist-boat conformation. Bis(trimethylsilyl)phosphido- -butylzinc, shows a similar central Zn3P3 fragment. The sterically more demanding trimethylsilylmethyl substituent forms a dimeric species of bis(trimethylsilyl) phosphido-trimethylsilylmethylzinc. Solution studies of bis(trimethylsilyl)phosphido- .vo-propyl-zinc demonstrate a temperature-dependent equilibrium of the dimeric and trimeric species and the crystalline state contains a 1 1 mixture of these two oligomers. A monomeric bis(trimethyl-silyl)phosphido-tris(trimethylsilyl)methylzinc has also been synthesized.313... 

A trimeric species was formed with one equivalent of a bis(benzimidazole) ligand in the presence of benzylthiolates (59) (Figure 10). Zinc-zinc distances were in the range 3.069-3.958(2) A.126 Other zinc-thiolate trinuclear compounds include Zn3(CH2SiMe3)3(SC6H2iPr3)3 and Zn3(o-phen)2(toluene-3,4-dithiolate)3 but have little structural similarity.81,525... 

The 31P NMR spectra of 28-30 showed signals characteristic of their structural features. Dimeric species 28 with l,l,l-tri(diphenylphosphinomethyl)ethane affords two singlets at 23.0 and —28.6 ppm due to the coordinated and nonco-ordinated P donors, respectively. For trimeric species 29, two singlets occur at 24.3 and 14.4 ppm, ascribed to two classes of P donors in bis[2-(diphenylphos-phino)ethyl]phenylphosphine. Because of two types of P donors in tri[2-(diphenylphosphino)ethyl]phosphine, tetrameric species 30 exhibits two singlets at 16.4 and 24.0 ppm. 

The 2,2 -bipyrazine (bpz)-substituted RU3 cluster monomer 42, dimer 43, and trimer 44 could be accessed by reaction of triruthenium precursor 2 with different amounts of 2,2/-bipyrazine [30]. The trimeric species 44 containing two parent Ru30(0Ac)6(py)2 n m and one derivate Ru30(0Ac)5(py)2II m m units could be directly prepared by reaction of 3.8 equivalents of 2 with 2,2/-bipyrizine. It is also accessible by reaction of dimeric species 43 with 1.8 equivalents of 2. The bpz adopts ri1 (N),(x-r 1 (N),r(1 (N) and p,4-r 1(N),r 1(N),r 1(C),r 2(N,N) bonding modes in 42, 43, and 44, respectively. Reduction of 3+ trimer 44 by addition of aqueous hydrazine allowed isolation of neutral intracluster mixed-valence species 44b containing three Ru 11,111,11 units. Oxidation of 44b with two... 

Fig. 8. Perspective view of the trimeric species [Be3(0H)3(H20)6]3+. Reproduced with permission from Ref. (89). Copyright 1998, American Chemical Society.

Ru(CO)4X2, the dimeric Ru2(CO)4X4, the trimeric species Ru3(CO)12X6, and the polymeric [Ru(CO)2X2] . The relative proportions differ with variation in the reaction conditions. However, the initial product in all these reactions is cis-Ru(CO)4X2, which subsequently polymerizes to yield the other derivatives 142). 

The second order in catalyst points to the involvement of two chromium salen molecules in the transition state complex. Therefore several dimeric species were synthesised with suitable linkers the dimeric catalysts gave reaction rates that were one or two orders of magnitude higher than that of the monomeric catalyst. Trimeric species gave still higher reaction rates The... 

Hyperfine coupling constant, 22 267, 269 Hyperfine interaction, ESR data for, 22 274 Hyperfine parameters for O, 32 128-130 Hyperfine splitting, 31 81 Hyperfine structure, trimer species, 31 98-99 Hyperfine tensor, 22 267, 273-279, 336, 340 constants, 32 20-21 dioxygen species, 32 18-25 equivalent oxygen nuclei, 32 18-21 ionic oxides, 32 40... 

For modulation of the size and shape of these metallacalixarene cavities, combinations of various pyrimidine and phen ligands have been used to form heterotopic metallacalixarenes. The multicomponent reactions of 3 1 2 mixture of [(en)Pd l, pyrimidine, and phen preferably formed heterotopic trimeric species [(en)3Pd3(L-Nl N3)(phen-N4 N7)2] (127) (L, a = 2-pymo, b = 2-mpymo,... 

Accordingly, the plot of hi([A p]) versus ln([I]o) is linear with a slope of /n. The aggregation state can thus be directly deduced from this plot. For instance, if the polymerization of sCL is initiated by A3, the three-arm growing species is an unimer. Conversely, when the same polymerization is initiated by Et2A10R, trimeric species are observed [21]. 

A further point of interest is that in both the dimeric and trimeric species shown, the beryllium atom still has a vacant orbital available which may be used in adduct formation without disruption of the electron-deficient bond. This type of behavior leads to the formation of dimers with four-coordinate beryllium atoms, e.g., structure XX (86). This structure has been determined in the solid state and shows that the phenylethynyl-bridging group is tipped to the side, but to a much smaller extent than observed in the aluminum derivative (112). One cannot be certain whether the distortion in this case is associated with a it - metal interaction or is simply a result of steric crowding, crystal packing, or the formation of the coordination complexes. Certainly some differences must have occurred since both the Be—Be distance and Be—C—Be angle are substantially increased in this compound relative to those observed in the polymer chain. 

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