碱金属-汞和碱金属-铅原子簇的成键本质

Abstract

Relativistic density-Functional calculations on Free Hg2, Hg4, Hg8, Pb2 and Pb4 clusters, and those clusters embedded in an alkali (A) atom surrounding are reported.Heavy metal-metal (M-M) distances, binding energies and eFFective charges have been cal culated.The Hg-Hg distance in the weakly bound Free clusters is over 330 pm.Upon embedding in an alkali surrounding, the bonding is strengthened; the bond lengths are re duced For square planar Hg4 to below 300 pm.The bonding in the hypothetical tetrahedral alkali-mercury cluster appear to be quite diFFerent, where Hg4 win undergo a transition to Four Hg1.This explains the strict preFerence of square over the tetrahedral geomety of Hg4 clusters in arnalgams.Pb4 is predicted to be stable in a pure cluster and does not show signiFicanly diFFerent bonding Feature in the lead-alkali crystal.The eFFective charges on Hg and Pb are calculated to be -0.4 and -0.6, respectively.The bonding characteristics are also examined by calculation of the M4-An interaction energies.The amalgamation energies lie in the range of 50 to 70 kJ/molHg.The Formation energy of alkall-Pb is up to 150 kJ/molPb, much stronger than that of alkali-Hg.