A theoretical study including full geometry optimizations is carried out at the IMOMM(MP2:MM3) (IMOMM = integrated molecular orbital molecular mechanics) computational level on the [ReH5(PPh(i)Pr(2))(2)(SiHPh(2))(2)] and [ReH5(PCyp(3))(2)(SiH(2)Ph)(2)] systems, the results being compared with available experimental diffraction data, as well as with MP2 results on the model system [ReH5(PH3)(2)(SiH3)(2)]. A simple scheme for the analysis of the relative weight of different contributions to the "steric" distortion is also proposed and applied to the same [ReH5(PPh(i)Pr(2))(2)(SiHPh(2))(2)] and [ReH5(PCyp(3))(2)(SiH(2)Ph)(2)] species.