Temperature Dependence of Chlorine NQR Frequencies and Pi Bonding

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Abstract
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.