Silanylidene group

Silanylidene groups, or silene groups are saturated hydrosilicon functional groups, which means that they consist only of hydrogen and silicon atoms and all bonds are single bonds, except for the joining bond, which is a double bond. By definition, cycles are excluded, so that the silanylidene groups comprise a homologous series of inorganic functional groups with the general chemical formula Si
nH
2n.

Except for the joining atom, which has three bonds, each silicon atom has four bonds (either Si-H or Si-Si bonds), and each hydrogen atom is joined to a silicon atom (H-Si bonds). A series of linked silicon atoms is known as the silicon skeleton or silicon backbone.

The simplest possible silanylidene group (the parent functional group) is silylidene, SiH
2. There is no limit to the number of silicon atoms that can be linked together, the only limitation being that the functional group is acyclic, is saturated, and is a hydrosilicon.

Structure classification

Saturated hydrosilicon functional groups with a double joining bond, can be:

• linear (general formula Si
  nH
  2n) wherein the silicon atoms are joined in a snake-like structure
• branched (general formula Si
  nH
  2n, n > 3) wherein the silicon backbone splits off in one or more directions
• cyclic (general formula Si
  nH
  2n-2, n > 2) wherein the silicon backbone is linked so as to form a loop.

According to the definition by IUPAC, the former two are silanylidene groups, whereas the third group is called cyclosilanylidene groups. Saturated hydrosilicon functional groups with a double joining bond can also combine any of the linear, cyclic (e.g., polycyclic) and branching structures, and they are still silanylidene groups (no general formula) as long as they are acyclic (i.e., having no loops). They also have single covalent bonds between their carbons.

Isomerism

Silanylidene groups with more than two silicon atoms can be arranged in various ways, forming structural isomers. The simplest isomer of a silanylidene group is the one in which the silicon atoms are arranged in a single chain with no branches, and a terminal joinining silicon atom. This isomer is sometimes called the n-isomer (n for "normal", although it is not necessarily the most common). However the chain of silicon atoms may also be branched at one or more points, and the joining atom may also be non-terminal. The number of possible isomers increases rapidly with the number of silicon atoms. For example:

• Si₁: no isomers: silylidene group
• Si₂: no isomers: disilanylidene group
• Si₃: two isomers: n-trisilanylidene & isotrisilanylidene groups
• Si₄: three isomers: n-tetrasilanylidene, isotetrasilanylidene & sec-tetrasilanylidene groups
• Si₅: seven isomers: n-pentasilanylidene, isopentasilanylidene, 2-silyl-n-tetrasilanylidene, neopentasilanylidene, sec-pentasilanylidene, 3-silyl-sec-tetrasilanylidene & pentasilan-3-ylidene groups

Branched silanylidene groups can be chiral. For example the 2-silyl-n-tetrasilanylidene group and its higher homologues are chiral due to their stereogenic center at silicon atom number two. In addition to these isomers, the chain of silicon atoms may form one or more loops. Such functional groups are called cyclosilanylidene groups.

References