[1] Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
[2] Inst. of Math., Phys. and Mechanics, Jadranska 19, 1000 Ljubljana, Slovenia
[3] National Chemistry Institute, Hajdrihova 19, 1000 Ljubljana, Slovenia
[4] Instituto de Ciencia Molecular, Departamento Quemica Inorganica, Universidad de Valencia, 46100 Burjasot, Spain