Synthesis and characterisation of rhodium(0) and rhodium(-1) complexes stabilised by 1,1 '-bis(diphenylphosphino)ferrocene (dppf). Crystal structuresof [Rh(dppf)(2)] and [Na(THF)(5)][Rh(dppf)(2)]center dot THF

The rhodium(I) complex stabilised by the organometallic ligand 1,1'-bis(dip henylphosphino)ferrocene (dppf), [Rh(dppf)(2)](+)(1) can be electrochemical ly reduced to the Rh(0) and Rh(-1) species, [Rh(dppf)(2)] (2) and [Rh(dppf) (2)](-) (3), respectively, in two reversible single-electron processes. The se low-valent complexes have been obtained by the chemical reduction of 1 w ith sodium naphtalenide in tetrahydrofuran solution and their crystal and m olecular structures determined by single-crystal X-ray analyses. The anioni c complex [Rh(dppf)(2)](-) crystallises as sodium-solvated salt, [Na(THF)(5 )][Rh(dppf)(2)]. THF (3) in the monoclinic system. The coordination geometr y around the metal in the d(10) configuration is a slightly distorted tetra hedral in which the bite angles of the chelated diphosphines is 103 degrees (average) and the Rh-P bond distances is in the range of 2.230(5)-2.251(5) Angstrom. The sodium cation is surrounded by five molecules of THF in a sl ightly distorted trigonal-bipyramidal environment. Complex 2 is convenientl y prepared by reacting equimolar amount of I and 3 in THF and crystallises in the triclinic system. The dihedral angle defined by the planes containin g the atoms P(1)-Rh-P(2) and P(3)-Rh-P(4) is 75.2 degrees, indicating a geo metry of the rhodium in the d(9) configuration intermediately between the h ighly distorted square-planar (dihedral angle 49.7 degrees), found earlier in the cationic d(8) species 1, and in the anionic d(10) complex (dihedral angle 95.0 degrees). Reduction of the metal centre causes a decrease in the Rh-P bond length of 0.034 Angstrom (average) from Rh(I) to Rh(0) and 0.071 Angstrom from Rh(0) to Rh(-1), respectively, with a concomitant increase o f the bite angle of the diphosphine which ranges from 93.4 degrees (average ) in [Rh(dppf)(2)](+) to 102.6 degrees in [Rh(dppf)(2)](-). The EPR spectro scopic properties of 2 and the analogue iridium derivative, [Ir(dppf)2], ha ve been examined at 20 K in 2-methyltetrahydrofuran matrix. A linear depend ence between the hyperfine constants and the M-P (M = Rh, Ir) bond distance s has been evidenced. This fact suggests the possibility to use the phospho rus coupling as an indication of the metal-phosphorus bond length in simila r complexes. (C) 2001 Elsevier Science B.V. All rights reserved.