PHOSPHORUS-PHOSPHORUS COUPLING-CONSTANTS IN MIXED-PHOSPHINE TRICARBONYL IRON COMPLEXES, FE(CO)(3)LL' - CRYSTAL-STRUCTURE OF TRANS-FE(CO)(3)(PET3)(PPH3)

Mixed-ligand complexes, trans-Fe(CO)(3)LL' (L = PPh3, L' = PPh2Me, PPh Me2, PMe3, PPh2Et, PEt3, PPh2CH=CH2, PPh2H, AsPh3, P(OPh)(3); L = PMe3 , L' = PEt3, PPh2Et, PCy3, PPh2Me, PPhMe; L = PEt3, L' = PPh2Me; L = P Ph2H, L' = PPh2CH=CH2, PPh2Et; L = AsPh3, L' = PPhMe2, P(OPh)(3), P(OM e)(3), P(OEt)(3)), have been obtained from the stepwise reaction of ph osphines with Fe(CO)(3)(BDA) (BDA = benzylideneacetone) or Fe(CO)(3)(A sPh3)(2) and from the reaction of phosphine with Fe(CO)(4)PPh3 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 strongl y with both chi and E-ar, but not with theta. Although a near perfect fit is obtained from the three-parameter equation, a statistical analy sis suggests that for this small data set there are no predictive adva ntages over the one-parameter pK(a) model. It is possible to calculate reliable (2)J(PP) values for trans-Fe(CO)(3)-L-2 complexes with eithe r 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 s trength equalization may occur when two rather different phosphines oc cupy trans coordination sites.