NIDO-CLUSTERS AND CLOSO-CLUSTERS AS HALF-SANDWICH AND TRIPLE-DECKER COMPLEXES - SIMPLE MO MODEL FOR INTERPRETATION OF REDOX BEHAVIOR OF CLUSTERS

The halfsandwich-type compounds eta-4-[RPactivated...Fe(CO)3...X...Fe(CO)3]Fe(CO)3, (X = Se, Te) 1a, 1b (type A), may be stacked to the tripeldecker-type compounds mu-2-eta-4-[RPactivated...Fe4-(CO)3...X...Fe(CO)3]Fe2(CO)5, 2a (X = Se), 2b (X = Te) (X-ray analysis) (type B). The relation between the compounds 1 and 2 is the same as the one between the nido pentagonal pyramidal clusters eta-5-[RPactivated...Fe(CO)3...R'C...R''C...Fe(CO)3]Fe(CO)3, 3 (type C) and their closo derivatives mu-2-eta-5-[RPactivated...Fe-(CO)3...R'C...R''C...Fe(CO)3]Fe2(CO)5, 4 (type D). The analysis of a specific dynamic behaviour of compounds 3 is reported. In addition a model is proposed, which explains the observed pairwise relation in the redox potentials of structurally related nido and closo compounds in a simple manner.