Synthesis of diphosphine-substituted selenido carbonyl iron clusters: Progressive deformation of the Fe3Se2 core in the nido clusters [Fe3Se2(CO)(7){mu-(Ph2P)(2)R}] by widening the bite of the bridging ligand

The reactions of [Fe-3(CO)(12)] with three diphosphine diselenides, dppmSe(2), dppeSe(2) and dppfcSe(2), produce the disubstituted clusters [Fe-3(mu(3)-Se)(2)(CO)(7){(Ph2P)(2)R}] (R = CH2 (dppm) 3; R = CH2CH2 (dppe) 7; R = (C5H4)(2)Fe (dppfc) 8) as the main products. Other products are [Fe-3(mu(3)Se)(2)(CO)(9)] 1, [Fe(CO)(4)(dppm)] 2, [Fe-2(mu-Se-2)(CO)(4)(dppm)] 4 in the case of dppm and [{Fe-3(mu(3)-Se)(2)(CO)(8)}(2)(dppe)] 5 in the case of dppe. Clusters 1, 3, 5, 7 and 8 have a square-pyramidal structure with two iron and two selenium atoms alternating in the basal plane and the third iron atom (Fe-ap) at the apex of the pyramid, and should be regarded as nido-clusters with seven skeletal electron pairs. The phosphine substitution is regioselective, occurring only on the two basal iron atoms. H-1 and P-31 NMR data in solution suggest a fluxional behaviour for 3 and 7 in solution due to the migration of a metal-metal bond to link the two iron atoms bound to the bidentate ligand. This is probably related to the deformation of the Fe3Se2 core induced by the steric demand of the diphosphines in such a way that the Fe ... Fe non-bonding distance decreases as the bite of the ligand shortens. The crystal structures of 3, 4, 5, 7 and 8 are described. (C) 1997 Elsevier Science S.A.