The history of the subject of this chapter starts in 1877, when Kohler and
Michaelis (82) concluded that the product of the reaction of PhPH2 and PhPClz
was “phosphobenzene,” PhP=PPh, the phosphorus analogue of azobenzene.
Although double bonding between the phosphorus atoms was implied, subsequent
physical studies revealed that “phosphobenzene” is in fact a a-bonded cyclopolyphosphine,
(PhP), (n = 5, 6) (5). Likewise, compounds that were thought
to involve arsenic-arsenic (50) or silicon-silicon (8 1) double bonds have since
been shown to possess cyclic oligomeric structures