A THEORETICAL INVESTIGATION OF CLASSICAL AND NONCLASSICAL COMPLEX MOLECULAR-STRUCTURES OF FE-2(CO)(N)(MU-PR2)(MU-PR'(2)) (N=5, 6) DIMERS

For the title compounds, three different complex molecular structures exist depending on the size of the bridging phosphido groups. A theore tical analysis based on complete geometry optimizations at the SCF lev el and various estimations of the influence of the correlation energy shows that the bonding situation in the bridging region is quite diffe rent in the three cases. In the ''classical'' system (CO)(3)Fe(mu-PPh2 )(2)Fe(CO)(3), with a puckered Fe2P2 moiety, the usual bent bond pictu re is confirmed. For the system (CO)(3)Fe(mu-PR2)(mu-PR'(2))Fe(CO)(3), with a planar bridging moiety, some direct Fe-Fe sigma-bonding as wel l as a-bonding interaction must be assumed. Finally, in the electron-d eficient system (CO)(3)Fe(mu-PR2)(mu-PR'(2))Fe(CO)(2), which also has a planar bridging unit, a strong direct Fe=Fe double bond acts. (C) 19 97 Elsevier Science B.V.