Transition metal rings 5-type molecular orbitals

The discovery in 1951 of the transition metal rr-complex, ferrocene or bis-cyclopentadienyl-iron, Fe(Cp)2, (1, 2) led to enormous interest being shown in the possible structures of such compounds and in the nature of the metal-ring bonding. Within a year the sandwich type structure (Fig. 1) had been proposed (2), and an outline treatment of the metal 3d-ligand 7r-orbital interaction by Jaffe (4) was soon followed by a more detailed molecular... 

Aromatic rings composed of transition metal elements only also show signs of aromaticity, and one of the most salient features of these compounds is the existence of the 5-aromaticity arising from the molecular orbitals made of linear combinations of (7-type orbitals [22-24]. 

Let us focus on four-member transitiou metal rings. Eor simplicity, we will assnme that each class of the five atomic d-type orbitals of each of the four transition metals of the ring will combine linearly to produce five sets of molecular orbitals. The... 

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