Bonding of Alkali-mercury and Alkali-lead Clusters
Relativistic density-functional calculations on free Hg-2, Hg(4)m Hg-6 Pb-2 and Pb-4 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 calculated. The Hg-Hg distance in the weakly bound free clusters is over 330 pm. Upon embedding in an alkali surrounding, the bonding is strenghthened; the bond lengths are reduced for square planar Hg-4 to below 300 pm. The bonding in the hypothetical tetrahedrat alkali mecury cluster appear to be quite different. Where Hg-4 will undergo a transition to four Hg-2. This explains the strict preference of suare over the terrahedral geometry of Hg-4 significantly 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 M-4-A(n) interaction energies. The amalgamation energies lie in the range of 50 to 70 kJ/molHg. the formation energy of alkali-Pb is up to 150 kJ/molPb, much stronger than that of alkali-Hg.