Phosphorus−Phosphorus Coupling Constants in Mixed-Phosphine Tricarbonyl Iron Complexes, Fe(CO)3LL‘. Crystal Structure of trans-Fe(CO)3(PEt3)(PPh3)

Mixed-ligand complexes, trans-Fe(CO)3LL‘ (L = PPh3, L‘ = PPh2Me, PPhMe2, PMe3, PPh2Et, PEt3, PPh2CHCH2, PPh2H, AsPh3, P(OPh)3; L = PMe3, L‘ = PEt3, PPh2Et, PCy3, PPh2Me, PPhMe; L = PEt3, L‘ = PPh2Me; L = PPh2H, L‘ = PPh2CHCH2, PPh2Et; L = AsPh3, L‘ = PPhMe2, P(OPh)3, P(OMe)3, P(OEt)3), have been obtained from the stepwise reaction of phosphines with Fe(CO)3(BDA) (BDA = benzylideneacetone) or Fe(CO)3(AsPh3)2 and from the reaction of phosphine with Fe(CO)4PPh3 in the presence of base. A strong negative correlation exists between 2 J PP coupling constant values and the sum of the phosphine pK a values. By application of quantitative analysis of ligand effects, it has been shown that 2 J PP for the mixed-ligand complexes correlates strongly with both χ and E ar, but not with θ. Although a near perfect fit is obtained from the three-parameter equation, a statistical analysis suggests that for this small data set there are no predictive advantages over the one-parameter pK a model. It is possible to calculate reliable 2 J PP values for trans-Fe(CO)3L2 complexes with either model. An X-ray structure of solid-state trans-Fe(CO)3(PEt3)(PPh3) shows equal Fe−PEt3 and Fe−PPh3 bond distances, implying that bond strength equalization may occur when two rather different phosphines occupy trans coordination sites.