Molecules with a Genuine Si-Si Triple Bond and a Stable Derivative of [SiH]~+

摘要

The multiple-bond rule, often simply named as the double-bond rule was considered to be valid for the molecular chemistry of the main-group elements. According to this rule, the elements of the first eight-element period, but not those of the following periods, are capable of forming multiple bonds. Although the origins of this are obscure, it probably arose from the work of Pitzer[1] and Mulliken[2] who, more than 50 years ago, posed the question as to why nitrogen, for example, has such a high bond-dissociation energy whereas the homologous P_2 molecule only exists as a species at high temperature and on cooling reverts to the P_4 molecule with P-P single bonds. The repulsive forces between the fully occupied inner electron shells were postulated as a reason for the highly different stabilities of the triple bonds in N_2 and P_2. In the case of N_2 there are only repulsive forces between the first shells, each occupied by merely two electrons, whereas in the P_2 molecule there are the additional eight electrons of the second shells, making the P-P triple bond unstable at room temperature.