The trans effect: methodological musings

The trans effect (TE) in the present context refers to the electronic compensation which in collinear homoligand L—Z—L* trans bond pairs lengthens the Z—L* bond when the Z—L bond is shortened. The existence of a functional relation d* = f(d) between the conjugated bond lengths d(Z—L) and d*(Z—L*) (d and d* not equivalent by symmetry; population A) has been demonstrated for a variety of Z-L combinations, with Z mostly from Groups VI and V and L mostly a halogen. The two model functions investigated in detail are the empirical DPF (difference power fit), d* – d0 = K(d − d0)−c, and the semiempirical CSBO (constant sum of bond orders) based on a modified 3-centre 4-electron bond concept, d* − d0 = −B ln {1 − expt[−(d − d0)/B]}, where B = b0 + b1(d − d0). Fitting DPF and CSBO to experimental d,d* data sets involves 3-parameter nonlinear optimization; in this CSBO differs from the 2-parameter treatment of Sheldrick etal., in which the limiting bond length d0 was supplied externally. Modified versions of DPF and CSBO have been devised to accommodate, along with A, d,d* pairs in which d = d* by symmetry (population S).The relative merits of DPF and CSBO and the various aspects of TE quantification are discussed at length, among these the effect of the oxidation state of Z and of the presence of heteroligands on Z. The meaning of the parameters of optimization and the existence of "chemical" trends between them are examined as well as the importance of the symmetrically balanced bond length de = d = d* and of the total d range Δ = de−d0 resulting from the d,d* regressions. Attempts to extend TE quantification to collinear heteroligand L1—Z—L2trans bond pairs have provided insight into the nature of the bond-length variation in such systems. The very good DPF and CSBO fits to d,d* sets obtained from 6-31G* optimizations of the equilibrium geometries of the OBOX, XOCN, and OCNY (X, Y = H, F, Cl, Li, Na, or no ligand) molecules and ions support the validity of the modified 3c4e model in accounting for the TE bond-length relationships.

Geometric and positional fatty acid isomers interact differently with desaturation and elongation of linoleic and linolenic acids in cultured glioma cells

A range of geometrical and positional isomers of 18-carbon acyl chains are potential components of diets containing fats from processed vegetable oils or ruminant animals. We have examined seventeen 18-carbon fatty acids, with cis or trans double bonds in positions 8–15, for their effects on desaturation and chain elongation of [1-14C]18:2(n − 6) or [1-14C]18:3(n − 3), primarily to 20:4(n − 6) or 20:5(n − 3) respectively, in cultured glioma cells. All trans monoenoic positional isomers inhibited Δ5 desaturation by approximately 60% when 18:2(n − 6) was the substrate, with no positional discrimination evident; with 18:3(n − 3) as substrate, only the 11- and 12-trans isomers were effective inhibitors of formation of 20:5(n − 3). All cis positional monoene isomers, except for 12-cis 18:1, produced 25–30% inhibition of conversion of 18:2(n − 6) to 20:4(n − 6), but had little effect on the conversion of 18:3(n − 3) to 20:5(n − 3). For dienoic isomers, the presence of a 12-trans bond inhibited formation of both 20:4(n − 6) and 20:5(n − 3) and this was enhanced markedly when the other bond in the dienoic acid was 9-trans. Presence of a 9-trans or 15-trans bond with 12-cis gave little effect except for a slight inhibition of 20:4(n − 6) formation by the 12-cis,15-trans 18:2 isomer. All-cis 20:3(n − 9) blocked Δ5 desaturation, increasing 20:3(n − 6) accumulation from 18:2(n − 6) and suggesting that formation of 20:3(n − 9) from 18:1(n − 9) during essential fatty acid deficiency may further exacerbate the already compromised formation of 20:4(n − 6). Further, the differential effects of various cis and trans isomers on the metabolism of 18:2(n − 6) and 18:3(n − 3) suggest that either Δ5 desaturation is not common in both pathways of conversion of 18:2(n − 6) or 18:3(n − 3) to their primary products or that selective interactions of trans and cis isomers occur when the essential fatty acid substrates are different.Key words: trans isomers, essential fatty acids, desaturation, glioma cells, fatty acid metabolism.

Steric Modification of M-P Bond Lengths: Crystal and Molecular-Structures of the Platinum(II) Complexes trans-(PPrI3)2ClnH2-nPtII (N = 0-2)

Crystal structure analyses of the square-planar platinum complexes trans-(PPri3)2ClnH2-nPtII(n = 0-2) (C), (B), (A) are described. Diffraction data were recorded at room temperature with a Picker FACS-1 diffractometer. Convergence R values, for reflection numbers given in parentheses, are 0.020 (2373), (A), 0.021 (5771), (B) and 0.018 (1982), (C). The ligand conformation about the Pt-P bonds is perpendicular for (A) and (B) and eclipsed for (C). The Pt-P distances decrease systematically with increasing hydride content [2.339(1)Ǻ, (A); 2.286(1)Ǻ, (B); 2.252(1)Ǻ, (C)]. The decrease reflects the low steric requirement of hydride vis a vis chloride ligands and concomitant changes in anionic ligand/phosphine substituent non-bonding interactions. The trans-bond lengthening of the Pt- Cl bond in (B)(due to trans hydride) is 0.092(1)Ǻ.

Alkoxohalogenotellurate(IV): Darstellung und Struktur von Tetraphenylphosphonium-trichloro(dioxo-ethylen-O.O')tellurat(rV) / Alkoxohalogenotellurates(IV): Preparation and Structure of Tetraphenylphosphonium Trichloro(dioxo-ethylene-O,O')tellurate(IV)

The deprotonated glycolate dianion (OCH2CH2O)2- is shown to react as a bidentate chelating ligand towards chalcogen(IV) halides. With TeCl4, the anion TeCl3(O2C2H4)- is formed. According to a single crystal X-ray analysis of the tetraphenylphosphonium salt (P21/c, a = 7.501, b = 26.328, c = 13.582 A, β= 104.55°, Z = 4) Te is in a tetragonal pyramidal (ψ octahedral) coordination with the alcoholate ligands in the apical (Te-O 1.919 Å) and in one of the equatorial positions (Te O 1.973 Å), the Te-Cl bond lengths being between 2.495 and 2.590 Å. The trans bond lengthening effect of the equatorial alcoholate ligand (i.e. the polarizing effect towards the lone pair) is unusually low. Vibrational spectra are reported.