Cage Silaphosphanes with a P→Si Dative Bond

The design of the cage silaphosphanes XSi(-L-)(3)P (X = Me (a), F (b); L = YCH(2)Z (4-19), YC6H4 (20-23), C(6)H(4)Z (24-27), C10H6 (28), YCH(2)CH(2)Z (29-37), YC(6)H(4)Z (38-46); Y, Z = O, NH, CH2, S) has been carried out by MP2 and B3LYP methods using 6-31G(d) and 6-311+G(d) basis sets. These species can exist in two forms, namely, with the phosphorus lone electron pair (LEP) directed inside the cage (endo), enabling P -> Si coordination, or outside the cage (exo), excluding it. The relative stabilities of the two isomers depend oil the properties of the Si and P bridgehead atom surroundings as well as on the size and nature of the side chains (L). Among the 50 studied cage species XSi(-L-)(3)P Possessing the bridgehead Si and P atoms in the 1- and 5-positions, only 3 structures (L = SC6H4 (23a,b), C10H6 (28b)) exist exclusively in the endo form with the pentacoordinate silicon atom. On going from the systems XSi(-L-)(3)P With the 1,5-bridgehead Si and P atoms to the similar 1,6-bridgehead systems, the number of stable endo isomers increases to 16. Remarkable among the latter is the molecule FSi(NHC6H4NH)(3)P (42b), with a record short P -> Si dative bond of covalent character, as follows from the AIM and ELF analyses. The configuration of the bonds around the Si-v atom in 42b corresponds to practically an ideal trigonal bipyramid (eta(e) = 100%).