Reorientation of the alkyne moiety in the heterometallic cluster [FeCo2(CO)9(EtC2Et)], induced by phosphine, phosphite, or isonitrile substitution for CO. Crystal structure of [FeCo2(CO)8(PPh3)(EtC2Et)]

Abstract

The [FeCo₂(CO)₉(EtC₂Et)] cluster represents the prototype of heterotrimetallic complexes having a nido-octahedral structure or in other words a µ₃-(η²-∥) alkyne co-ordination mode. The cluster exhibits a symmetrical (MM∥CC) geometry in the solid state; since fluxionality of the alkyne is observed in solution and therefore the rotamers are characterized by subtile energy differences, we have attempted to understand the factors affecting the alkyne orientation over the metallic triangle. The spectroscopic and structural data of the parent cluster have been compared with those of its mono derivatives [FeCo₂(CO)₈L(EtC₂Et)][L = PEt₃, PPh₃, P(OMe)₃, P(OPh)₃, or CNCH₂Ph]. The structure of [FeCo₂(CO)₈(PPh₃)(EtC₂Et)] has been determined by X-ray methods. It crystallizes in the triclinic space group P, with Z= 2 in a unit cell of dimensions a= 10.690(4), b= 16.267(8), c= 9.457(5)Å, α= 95.01(3), β= 107.55(3), and γ= 89.23(2)°. The structure has been solved from diffractometer data by direct and Fourier methods and refined by full-matrix least squares to R= 0.0563 and 4 165 observed reflections. The structure consists of an isosceles triangle of one Fe and two Co atoms. Of the eight carbonyls, seven are terminally bound to the metal atoms [three to Fe, three to the unsubstituted Co(2), one to the Co(1) atom having the phosphine ligand] and one asymmetrically bridges the Fe–Co(1) edge. The EtC₂Et ligand interacts with all three metals, being σ bonded to Fe and Co(2) and π bonded to Co(1) through the triple bond which is disposed nearly parallel to the Fe–Co(2) edge of the cluster. Thus the substitution of a CO for PPh₃ has caused the reorientation of the alkyne moiety in the ground state. Finally, dimers of formula [{FeCo₂(CO)₈(-EtC₂Et)}₂{Ph₂P(CH₂)_nPPh₂}](n= 2–4) have been characterized by means of multinuclear n.m.r. spectroscopy.