R. K. Pierens

Rayleigh scattering depolarization ratio and molecular polarizability anisotropy for gases

The depolarization ratio for Rayleigh scattering by 39 molecular species are reported for λ= 488.0, 514.5 and 632.8 nm. The literature has been critically surveyed for refractivities which are used in conjunction with the observed depolarization ratios to yield the anisotropy of the molecular polarizability of the molecules. Results are given for CO2, OCS, CS2, C2H6, C3H8, isobutane, cyclopropane, C2H4, allene, C2H2, methylacetylene, dimethylacetylene, spiropentane, cyclohexane, C6H6, C6H5F, 1,3,5-C6H3F3, C6F6, CH3CN, (CH3)2CO, CH3F, CH2F2, CHF3, CH3Cl, CH2Cl2, CHCl3, CF2Cl2, CH3Br, CH3I, t-butylchloride, t-butylbromide, ethylene oxide, (CH3)2O, (CH3)2S, H2S, SO2, N2, N2O and CO.

Metal—ligand bonding in LM(CO)5 complexes (L = NMe3, PMe3, PCl3, PBr3 or AsMe3; M = Cr, Mo or W) from dielectric, electro-optic and spectroscopic evidence

Abstract A comparative study has been made of the electric dipole moments, optical polarisability anisotropies (from electric birefringence measurements), infrared carbonyl stretching frequencies and 13C NMR spectral shifts of LM(CO)5 complexes where L = NMe3, PMe3, PCl3, PBr3 or AsMe3 and M = Cr, Mo or W. Analysis of the polarisability anisotropies of the complexes provides direct, experimentally based evidence indicating a substantial π-component in the phosphorus—metal and arsenic—metal bonding in P(As)Me3M(CO)5 within a highly polarisable P(As)M(trans)CO delocalised π-electron system. The LM π-bonding appears not to be significantly affected by change of L from PMe3 to AsMe3 or by change of M within the chromium triad. It is shown that π-bonding between the phosphorus and the metal and electron delocalisation along the PM(trans)CO linkage are enhanced by electronegative halogen substituents in the phosphine ligand. The conclusions reached are in general accord with results from interpretation of the infrared and 13C NMR spectral and dipole moment data.

Electric dipole moments and anisotropic optical polarisabilities of methyl- and t-butyl-substituted (η6-benzene)tricarbonylchromium(0) complexes: Their relevance to the nature of the chromium-arene bond

Determinations of the electric dipole moments, carbonyl stretching frequencies and anisotropic optical polarisabilities are reported for [Cr(CO)3(η6-C6H6)] and for nine of its methyl- and t-butyl-substituted derivatives in cyclohexane solution. The effects of progressive substitution are approximately additive. For each complex, the molecular optical anisotropy from experiment is opposite in sign to that predicted from additivity of the anisotropies of the component arene and Cr(CO)3 fragments. Coordination between the fragments leads to a very large enhancement of polarisability along the Cr-arene axis while the polarisabilities perpendicular to that axis (parallel to the benzenoid ring) are lowered. The electron system constituting the Cr-arene bond is highly polarisable along the bond and provides a channel of relatively free electron movement between the arene and Cr(CO)3. The results are consistent with a postulated large magnetic anisotropy for the Cr-arene bond. The aromatic character of the complexed arene is diminished relative to that of the free ligand.

Polarities and anisotropic electron polarisabilities of neutral ligands

Dipole moments and molar Kerr constants are reported for the molecules Me3MX (M = P or As, and X = lone pair, O, or S) as solutes in dioxan and in benzene. The results are analysed to provide quantitative estimates of the anisotropic electron polarisabilities for each of these neutral ligands. The parameters so obtained are potentially of value in studies of metal–ligand interactions. Evidence is presented also of stereospecific Me3MX,C6H6 complex formation in the aromatic solvent.

A study of polarities, polarisabilities and infrared spectra of LM(CO)5 complexes (M=Cr, Mo or W; L=trimethylamine, quinuclidine, pyridine, p-methyl- or p-t-butylpyridine) and of the X-ray crystal structure of quinuclidine-Cr(CO)5; evidence for π-acceptor behaviour of coordinated pyridine

Carbonyl infrared frequencies, electric dipole moments and optical polarisability anisotropies of pyridine and of p-methyl and p-t-butyl-pyridine-M(CO)5 complexes (M  Cr, Mo or W) are compared with the corresponding parameters for the analogous trimethylamine and quinuclidine-M(CO)5 complexes. For the same ligand L in LM(CO)5, the sequence of v(CO)A1 is Cr < W ≤ Mo. The carbonyl bands are only slightly affected by variation in L or M in the complexes studied. Trends in the molecular dipole moments of the LM(CO)5 complexes suggest a degree of π-acceptor capability for the bonded pyridine system but the evidence is not conclusive. Anisotropic molecular polarisabilities derived from experiment for the pyridine and p-substituted pyridine-M(CO)5 complexes, when compared with parameters calculated for a purely σ-bonded NM model for such systems, show a strong enhancement of polarisability specifically along the pyridine-M-trans(CO) axis. This is related to conjugative electron delocalisation between the pyridine and M(CO)5 moieties and indicates a degree of π-bond character in the connecting NM bond. The crystal structure of quinuclidine-Cr(CO)5 is reported. Comparison of bond dimensions of this complex with those of pyridine-Cr(CO)5 yields positive though qualified support for a π-component in the NCr bond of pyridine-Cr(CO)5.

Conformational studies of EPh3M(CO)5 complexes (E P, As or Sb; M Cr, Mo or W)

Abstract Studies are reported of the electro-optical Kerr effect, electric dipole moments and IR carbonyl stretching frequencies of the nine EPh 3 (CO) 5 complexes (E  P, As or Sb and M  Cr, Mo or W). The experimental results are applied in conjunction with molecular mechanics calculations and data from X-ray crystal structure analysis, to explore the stereostructural preferences of the EPh 3 M(CO) 5 molecules with regard to orientations about the ME bond and phenyl group rotations about EC ar . Evidence is drawn together and compared for EPh 3 M(CO) 5 in the solution and solid states and as isolated molecules. It is found that for all states, phenyl ring dispositions of like helicity are favoured for the coordinated EPh 3 ligands in these complexes.

Molecular polarisability. The dipole moments and molar Kerr constants of some para-substituted anisoles

Dipole moments and molar Kerr constants are reported for the molecules p-XC6H4·OMe (X = H, Me, Cl, Br, l, NO2, or CN) examined as solutes in carbon tetrachloride and benzene. The experimental data are analysed to indicate that (a) the Car–O–C group is virtually coplanar with the aromatic ring if X = NO2 or CN; for X = H, Me, or halogen, the apparent dihedral angle between the planes is ca. 20°(b) stereospecific benzene–solute complex formation occurs in the aromatic solvent.

Molecular polarisability. The conformation of NN′-dinitropiperazine and other cyclic nitroamines in dioxan solution

Dipole moments and Kerr constants are reported for dimethylnitroamine, N-nitropiperidine, N-nitromorpho-line, NN′-dinitropiperazine, 3,7-dinitro-1,3,5,7-tetra-azabicyclo[3.3.1]nonane, 1,3,5-trinitro-1,3,5-triazacyclohexane, and 1,3,5,7-tetranitro-1,3,5,7-tetra-azacyclo-octane at 25 °C. The effective molecular polarisability anisotropies are derived for dimethylnitroamine and the values obtained used to investigate the possible solution conformations of the monocyclic secondary nitroamines.

Anisotropic optical polarisabilities of the complexes LCr(CO)5 (L=PMe3, NMe3 or NH3): a study of π-back-bonding

Abstract Dipole moments and electric birefringences are reported for the complexes LCr(CO)5 where L is PMe3, NMe3 or NH3. Comparison of the molecular optical polarisability anisotropies of PMe3Cr(CO)5 and NMe3Cr(CO)5 shows a large enhancement of polarisability specifically in the L-Cr-(trans)CO direction for the phosphorus compound; the polarisabilities perpendicular to the symmetry axis are closely similar for the two complexes. The results are interpreted as direct evidence for a highly deformable π-component in PCr bonding. This work presents a new experimental approach to the study of π-back-bonding.

Dielectric, electro-optic and infrared and 1H NMR spectral properties of methyl- and t-butyl-substituted (η6-benzene)tricarbonylchromium(0) complexes: a study of solvent effects

Abstract Dipole moment, microwave dielectric absorption, electric birefringence and infrared and 1H NMR spectral data are presented for [Cr(CO)3(η6-C6H6)] and for some of its methyl- and t-butyl-substituted derivatives in the non-dipolar solvents, cyclohexane, benzene and dioxane. Atomic polarisations for these complexes are shown to be negligibly small. Specific interactions are shown to occur between dioxane or benzene solvent molecules and the coordinated arene which acts as an electrophile. The effects on such interactions of progressive methyl or t-butyl group insertion on the arene nucleus are studied. In 1,3,5-tri-t-butylbenzenetricarbonylchromium(0), “face-to-face” associations between the arene ligand and solvating benzene are sterically inhibited. Direct complexation with chromium is shown to be improbable with these solvents. The solute polarisability tensor of [Cr(CO)3(η6-C6H6)], unlike the dipole moment vector, is not changed by interaction with dioxane. With methyl or t-butyl substitution, the solute polarisability anisotropy in dioxane tends to decrease, relative to that in cyclohexane. Electric birefringences and 1H NMR chemical shifts are interpreted in terms of transient stereospecific solute-benzene associations additional to “face-to-face” charge-transfer interactions. Benzene-induced upfield shifts or aryl protons (ASIS) decrease in magnitude with progressive methyl or t-butyl substitution.