The new tetrachelating diphosphanes 1,2-[(5 H-dibenzo[a,d]cyclohepten-5-yl)phenylphosphano]ethane, R, S- 11 [ R, S-bis(tropp Ph)], and the corresponding enantiomers R, R-bis(tropp Ph), R, R- 12, and S, S-bis(tropp Ph), S, S- 12, were synthesised from 1,2-bis(phenylphosphano)ethane ( 9), and 5-chloro-5 H-dibenzo[a,d]cycloheptene ( 10). With the meso form R, S- 11, the rhodium(I) complex [Rh{ R, S-bis(tropp Ph)}]PF 6 ( 15), and the iridium(I) complex [Ir(cod){ R, S-bis(tropp Ph)}]O 3SCF 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 Ph)}] + cation is reversibly reduced in THF by one electron to the paramagnetic neutral complex [Rh{ R, S-bis(tropp Ph)}] 0.