Broadening the Scope of Ancillary Phosphane‐Type Ligands: A Systematic Comparison of PR 3 , PR 2 BH 3 – , and SiR 3 – 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 PPh 2 Me, PPh 2 BH 3 – , and SiPh 2 Me – and SP t Bu 3 , SP t Bu 2 BH 3 – , and SSi t Bu 3 – . In addition, we have also investigated the oxo derivatives OPPh 2 BH 3 – and OSiPh 2 Me – . Based on X‐ray crystal structure determinations (Fe–CO and C–O bond lengths) as well as NMR [e.g. δ( 13 CO)] and IR [ν(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 OPPh 2 BH 3 – transfers a hydride ion rather than forming a stable [CpFe(CO) 2 (OPPh 2 BH 3 )] complex. The tendency to undergo hydride‐transfer reactions was studied by density functional calculations for the series P t Bu 2 BH 3 – , OP t Bu 2 BH 3 – , and SP t Bu 2 BH 3 – . The results reveal that OP t Bu 2 BH 3 – is the strongest and SP t Bu 2 BH 3 – the weakest hydride donor, in accordance with the experimental observations. Theoretical analysis indicates that the three derivatives PPh 2 Me, PPh 2 BH 3 – , and SiPh 2 Me – are truly isolobal species despite variations in their charge distributions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)