Systems Using Outer d Orbitals

Some known monocyclic conjugated hetero jr systems containing heteroatoms like phosphorous, sulphur or selenium, which may take partin the ir system by means of either pv or outer dn orbitals, are shown in Fig. 1. 

Among the typically organic heterocycles of Fig. 1, the 1-methylphos-phole 12) 1 does not necessarily contribute dn orbitals to the rr-system besides its p lone pair13). The same holds for the phosphabenzene derivatives 2a 14 and 2b 1ST But in the phosphorine derivatives 3a 16 , 3b l7 , 3c 18 and 3d 19 , as well as in 1,1,3,3-tetraphenyl-l,3-diphosphabenzene 2°) 4 and 1-phenyl-1 H-phosphepine-l-oxide 21 5 which contain tetrahedral phosphorous atoms, cyclic conjugation can only be achieved by the use of outer d orbitals. The resultant bonding situation has been treated theoretically by Mason 22 using HMO theory. 

The extent of sulphur s d orbital contribution to the jr system in addition to that of one p lone electron pair was greatly overemphasized in the early HMO treatment 23 of thiophene 6. But more advanced calculations show that 

The same conclusion holds true in principle for the thiadiazoles 7 281 and 9 29) or the selenadiazoles 8 3°) and 10 31), whose structures are very accurately known experimentally, but which have been studied theoretically only by means of HMO 32 33) or PPP 7r-electron theories 34 without any specific exam- 

In the positively charged thiapyrylium ion 36 37) 11 or selenapyrylium ion 38 12 the contribution of d orbitals to the rr system may be more important 27 39 , as in the other sulphur compounds listed here. 

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