The new tetrachelating diphosphanes 1,2-[(5H-dibenzo[a,d]cyclohepten-5-yl)phenylphosphano]ethane, R,S-11 [R,S-bis(tropp P h )], and the corresponding enantiomers R,R-bis(tropp P h ), R,R-12, and S,S-bis(tropp P h ), S,S-12, were synthesised from 1,2-bis(phenylphosphano)ethane (9), and 5-chloro-5H-dibenzo[a,d]cycloheptene (10). With the meso form R,S-11, the rhodium(I) complex [Rh{R,S-bis(tropp P h )}]PF 6 (15), and the iridium(I) complex [Ir(cod){R,S-bis(tropp P h )}]O 3 SCF 3 (16), were prepared. The cation of the 16-electron rhodium complex 15 has a square planar structure, which is markedly distorted towards a square pyramid with the rhodium centre in the apex. The structure of the cation of the 18-electron complex of 16 is trigonal bipyramidal and unprecedented a phosphorus and an olefin unit occupy the axial positions. Cyclic voltammetry data of 15 indicate, that considerable strain energy (ca. 30 kJ mol - 1 ) is build up when the [Rh{R,S-bis(tropp P h )}] + cation is reversibly reduced in THF by one electron to the paramagnetic neutral complex [Rh{R,S-bis(tropp P h )}] 0 .