Structure and Bonding in the Gaseous Monomers

It was long taken for granted that all monomeric dihalides of the Group 2 and 12 metals have linear equilibrium stmctures in the gas phase, similar to the stractures of HgCl2 and HgBr2 in the solid phase. The first evidence that this might not be so was provided by the molecular beam studies published by Klemperer and coworkers in 1963 and 1964 [27, 28]. Some of the Group 2 dihalides. 

and all the Ba dihalides, were deflected by inhomogeneous electric fields. These observations suggested that the molecules have permanent electric dipole moments, i.e., that their molecular structures are angular. 

The experimentally determined MX bond distances of 27 dihalides of the Group 2 and 12 metals are listed in Table 2. In most cases they are accurate to better than 2 pm. As in the case of the alkali metal monohalides, the bond distances of the Group 2 metal dihalides increase monotonically with increasing atomic numbers of the metal or halogen atoms. The bond distances in the Group 12 metal dihalides increase with the atomic number of the halogen, but the variation with the atomic number of the metal breaks the pattern Hg forms shorter bonds than Cd. The shortening is probably due to a combination of relativistic effects and the lanthanide contraction [17]. 

Somewhat to our surprise, we have been unable to find reference to any published attempt to calculate these bond energies of the gaseous dihalides on the basis of the Bom-Lande lattice energy model. We have therefore carried out such calculations at a level that reproduced the bond energies of the alkali metal halide monomers with an average deviation from the experimental values of 8%. All calculations were carried out on linear XMX structures. The ratio between these calculated values and their experimental counterparts are listed in the last column of Table 2. 

Experimental and calculated bond distances (R) and valence angles (Z) calculated APT net atomic charges on the metal atoms (i m) mean bond energies at 0 K calculated from thermodynamic data, MBEexpi and the ratio between mean bond energies calculated from ionic models, MSEcaic, and the experimental values. Experimental data from [29-32]. The experimental bond distances are in most cases accurate to better than 2 pm. The experimental MBEs. are probably accurate to about 10 kJ mol . For information about calculated parameters, see text 

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