Broadening the scope of ancillary phosphane-type ligands:: A systematic comparison of PR3, PR2BH3-, and SiR3 and their chalcogen derivatives

This work describes a systematic experimental and theoretical study of the properties of two series of isoelectronic and largely isosteric ligands, namely PPh2Me, PPh2BH3-, and SiPh2Me- and SPtBu3, SPtBu2BH3-, and SSitBu(3)(-). In addition, we have also investigated the oxo derivatives OPPh2BH3 and OSiPh2Me-. Based on X-ray crystal structure determinations (Fe-CO and C-O bond lengths) as well as NMR [e.g. delta((CO)-C-13)] and IR [v(CO)] spectroscopic investigations of the corresponding [CpFe(CO)(2)](+) complexes, we can conclude that, with respect to electron donor strength, phosphanyl borohydrides occupy an intermediate position between phosphanes (weakest donors) and silyl ligands (strongest donors). The same is true for the thio derivatives, although the differences are smaller. In the reaction with [CpFe(CO)(2)](+), the oxo derivative OPPh2BH3- transfers a hydride ion rather than forming a stable [CpFe(CO)(2)(OPPh2BH3)] complex. The tendency to undergo hydride-transfer reactions was studied by density functional calculations for the series PtBu2BH3-, OPtBu2BH3-, and SPtBu2BH3-. The results reveal that OPtBu2BH3- is the strongest and SPtBu2BH3- the weakest hydride donor, in accordance with the experimental observations. Theoretical analysis indicates that the three derivatives PPh2Me, PPh2BH3-, and SiPh2Me- are truly isolobal species despite variations in their charge distributions. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).