Anomalous temperature variation of NQR frequencies and bonding in metal complexes

Publisher Summary This chapter discusses pure quadrupole resonance or nuclear quadrupole resonance (NQR) that has been applied to a number of organic as well as inorganic compounds and has yielded valuable information on the electronic structure of chemical bonds. Inorganic compounds having a simple chemical formula are not necessarily simple from the standpoint of nuclear quadrupole spectroscopy. On the other hand, some metal complex compounds are amenable to this method of attack, because to the first approximation one can discuss the electronic state of a single complex ion. The first experimental observation of the NQR absorption of coordination compounds was reported on potassium tetraiodomercurate. The chapter discusses the nature of metal-ligand bonds in metal complexes, use is often made of rather vague terms such as covalent character, ionic character, and the hybrid of these to explain various properties of complex compounds such as, for instance, spectroscopic data, magnetic behavior. However, in order to establish these concepts in a quantitative manner and to express the transition from an ideal covalent bond to a pure ionic bond, the extent of covalent character is defined by resorting to some experimental parameters that are characteristic of metal-ligand bonds and are measurable accurately.

Nuclear Quadrupole Resonance and Its Application in Inorganic Chemistry

The chlorine resonance in titanium tetrachloride has been re‐examined, confirming the existence of four resonances. A chlorine‐35 resonance at about 6 Mc has been discovered in thorium tetrachloride. The frequency suggests highly ionic bonding, which contradicts earlier conclusions from x‐ray studies. Some double‐bond character may be present in the Th–Cl bond.Niobium and chlorine resonances have been discovered in niobium pentachloride. The niobium resonances are described by a single set of parameters, the coupling constant eqQ being approximately 78 Mc, and the asymmetry parameter, η=0.32. Because of the large asymmetry parameter, it was possible to observe two ΔM=2 and one ΔM=3 transitions. Only a single Cl35 resonance was found at about 13 Mc, although two more chlorine resonances would be expected on the basis of crystal structure. The frequency of the chlorine resonance suggests appreciable covalency in the niobium‐chlorine bond in niobium pentachloride.A single resonance has been found in tantalum...

Nuclear Quadrupole Resonance in TiCl4, ThCl4, NbCl5, and TaCl5

A theory is presented for the anomalous temperature dependence of 35Cl NQR frequencies observed for certain transition‐metal—chloride compounds. The basic feature of the theory is a break down of π bonding due to bending vibrations, and a consequent increase in the electron population in chlorine 2pπ orbitals, causing in turn an increase in the observed NQR frequency. An approximate separation of lattice‐motion effects and intramolecular effects is made for the series of compounds K2MCl6, M = W, Re, Os, Ir, Pt, and it is shown that the predicted magnitude and behavior of the intramolecular effects agree well with the observed values.

Temperature Dependence of Chlorine NQR Frequencies and Pi Bonding

Nuclear Quadrupole Coupling in the Alums

The nuclear quadrupole resonance spectrum of bromine was detected and measured in TiBr/sub 4/. The temperature dependence of the resonance frequencies was determined from data points obtained between 77 deg K and room temperature. In addition, the pressure dependence at room temperature to 10,000 psi was determined. The resonance frequency depends on pressure through direct volume dependence of the field gradients, and through the volume dependence of the vibration amplitudes which in turn average the field gradients. (J.R.D.)

R. D. Engardt

Papers

Unusual Temperature Dependence of Bromine Quadrupole Resonances in TiBr4