A tetra-decker sandwich complex with normal geometry can be visualized as a common-vertices conjuncto-heteroborane and the no. of its valence bonding orbitals, 21, has been obtained by means of the structural rule of conjucto-heteroboranes. By using EHMO quantum chem. calcns., the variations of MO energies with respect to the extent of stretch of the skeleton have been discussed to explain that the no. of valence bonding orbitals of the mol. changes from 21 to 23. Anal. of the results about the relationships between the electron configurations and geometries for actual tetra-decker sandwich complexes shows that the above discussion is correct. The comparison of tetra-decker with triple- and double-decker sandwich complexes indicates that the relationships are qual. the same, while varying rates increase with increasing no. of transition metal atoms.

Key words: QUANTUM CHEMISTRY, ELECTRONIC STRUCTURE, TRANSITION METAL COMPOUND, BORANE, BORON HETEROCYCLIC COMPOUNDS, SANDWICH COMPOUNDS, HUCKEL MOLECULAR ORBITAL