SUBSTITUENT EFFECT OF 2ND ROW ELEMENTS ON SILYL CENTERS

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.