text: X-ray analysis indicates certain important bond lengths and angles of the first diphosphene, bis(2,4,6-tri-tert-butylphenyl)diphosphene: P-P = 2.034 (2) Å; P-C = 1.826 (2) Å; P-P-C = 102.8 (1); C-P-P-C = 172.2 (1). Compared with the bond length of a P-P single bond in HPPH (2.238 Å), the P-P bond distance is much shorter, which reveals double bond character. The orientation of the substituents about the P-P double bond in bis(2,4,6-tri-tert-butylphenyl)diphosphene exhibits an E (trans) configuration. But by visible irradiation of the trans isomer, an interconversion between cis isomer and trans isomers would occur. In 1984, M. Koening et al. noticed a different splitting mode and chemical shift in H NMR and P NMR under a direct irradiation of a toluene solution of E-bis(2,4,6-tri-tert-butylphenyl)diphosphene, which suggesting a cis-trans isomerization.