Protonation of Cp*Fe(dppe)H (1; Cp* = η 5 -C 5 Me 5 , dppe = Ph 2 PCH 2 CH 2 PPh 2 ) by HBF 4 Et 2 O at -80 °C in diethylether affords the dihydrogen complex [Cp*Fe(dppe)(η 2 -H 2 )] + BF 4 -(2 + BF 4 -) in 90% yield. Its PF 6 -salt analogue (2 + PF 6 -) is obtained in 94% yield by reaction between the 16-electron derivative [Cp*Fe(dppe)] + PF 6 -(3 + PF 6 -
) with H 2 gas at -80 °C. The presence of a bound dihydrogen ligand in 2 + is indicated by a short T 1 minimum values consistent with a H-H distances of 0.98(1) Å. For the partially deuterated derivative 2 + -d 1 , the observed J HD value of 27.0 Hz confirms the presence of the coordinated dihydrogen ligand, which displays an H-H separation of 0.97(1) Å, in complete agreement with the distance calculated using the T 1 static rotation model. Variable temperature NMR study shows the gradual, complete and irreversible transformation of the dihydrogen complex into its classical dihydride isomer trans-[Cp*Fe(dppe)(H) 2 ] + (4 + ). Thermal solid state reaction (-20 °C, 48 h) of 2 + BF 4 -gives quantitatively 4 + BF 4 -, whereas 4 + PF 6 -is obtained by simple contact of H 2 with a solution of 3 + PF 6
-in THF at room temperature. The crystal structure of 4 + BF 4 -has been determined and shows a transoid arrangement of hydride ligands, consistent with the formulation of 4 + as an iron(IV) dihydride. DFT calculations on both dihydride and dihydrogen isomers of [Cp*Fe(dppe)H 2 ] + indicate that 4 + is more stable than 2 + by 0.19 eV, while this energy difference is reversed in the case of [CpFe(dpe)H 2 ] + (dpe = H 2 PCH 2 CH 2 PH 2 ). The preference for the dihydride form in the case of [Cp*Fe(dppe)H 2 ] + and of the dihydrogen one in the case of [CpFe(dpe)H 2 ] + is due to the larger π-donor and σ-acceptor abilities of the [Cp*Fe(dppe)] + fragment, as compared to the [CpFe(dpe)] + unit.