CRYSTAL-STRUCTURE OF TRANS-FE(CO)3(PPH3)2, TRICARBONYLBIS(TRIPHENYLPHOSPHINE)IRON(0), AND AB-INITIO STUDY OF THE BONDING IN TRANS-FE(CO)3(PH3)2

The selective one-step preparation of trans-Fe(CO)3(Pph3)2, 1, in the absence of Fe(CO)4-(Pph3) has made it possible to obtain 1 pure and in crystalline form. trans-Fe(CO)3(Pph3)2 crystallizes in the orthorhomb ic space group Pbca with cell parameters a = 18.216(5) angstrom, b = 1 7.0380(10) angstrom, c = 21.804(3) angstrom, and Z = 8; R = 0.042 and R(w) = 0.056 for 3891 independent reflections with I > 2.0sigma(I). Th e crystal structure of 1 is compared to Ru(CO)3(Pph3)2, Os-(CO)3(Pph3) 2, Fe(CO)2(CS)(Pph3)2, and a selection of complexes of the types trans -Fe(CO)3-(PR3)2 (R = Ph, NMe2, Ome) and trans-Fe(CO)4PR3 (R = Ph, Nme2 , CMe3, SiMe3). Effects of equatorial CO substitution in 1 by the diaz onium ion and nitrile ligands are discussed. Ab initio calculations th at employ effective core potential basis sets at the Hartree-Fock leve l and which include perturbational corrections for electron correlatio n at the MP2 level are reported fortrans-Fe(CO)3(PH3)2. Good agreement between theory and experiment can only be obtained at correlated leve ls of theory in conjunction with double-zeta quality basis sets on iro n and all ligands. Population analyses are employed to examine the eff ects of electron correlation and to delineate the iron-ligand bonding. Electron correlation serves overall to reduce the molecular quadrupol e moment. Comparison between the crystal structure of 1 and the theore tical structure of the model system allow differentiation between intr insic structural preferences and packing effects. In particular, the c omparison points out that the Fe-P bond length differences and the non linearity of the P-Fe-P backbone are not intrinsic bonding features bu t are caused by the packing.