The entire set of single- and double-bonded second row element-substit
uted silicon compounds, silyl radicals and silylenes, along with their
parent silicon hydrides, SiHn (n = 2-4), have been investigated by us
ing ab initio methods. All structural parameters were optimized by use
of the 6-31G basis sets at the HF and MP2 levels of theory. The main
structural features of the low-valent silicon species do not differ s
ignificantly from those of the first row element-substituted counterpa
rts. Shortening of the Si-R (R = substituent) bond (relative to the un
saturated derivatives) is observable with substituents from the left s
ide of the periodic table if there is an odd electron on the silyl cen
ter (silyl radical or triplet silylene), while substituents from the r
ight side of the periodic table cause shortening of the otherwise long
er (cf. SiH2 and SiH4) Si-R bonds in the case of singlet silylenes. St
abilization energies (relative to the saturated silanes) due to the su
bstitution were evaluated by using isodesmic reactions. It has been sh
own that, while silyl radicals and triplet silylenes are stabilized by
substituents having empty p orbitals (being capable of delocalizing t
he odd electron on the silicon center), singlet silylenes are stabiliz
ed by substituents having unshared electron pairs (being capable of de
localizing to the empty p orbital of the silylene unit). Rotational ba
rriers about the Si-R bonds give similar values to the isodesmic stabi
lization energies. Stabilization caused by the second row elements is
not less than that of first row substituents, but depends instead on t
he electron donor/acceptor properties of the substituting group. In ce
rtain cases (H2AlSiH2, HSiPH2) the delocalization stabilization energy
is significantly reduced (compared to the expectations based on bond
shortening and electronegativity of the substituting group), as the in
version barriers (at the silyl and phosphine centers respectively) sho
uld be surmounted by the delocalization stabilization. The strong elec
tron-acceptor character of singlet silylene can be rationalized by the
reduction of the phosphine inversion barrier (about 35 kcal mol(-1) i
n PH3) to 0.2 kcal mol(-1) at the MP2/6-31G//MP2/6-31G* level of theo
ry (0.35 and 0.4 kcal mol(-1) at the MP4/6-31G//MP2/6-31G* and MP2/6-
31G*//MP2/6-311G** levels of theory respectively). This reduction of
the inversion barrier is even larger than that reported for BH2 substi
tuent.