The paramagnetic rhenium complex [NEt4]2[Re(Br)5(NO)] (1) has been used to prepare a
series of novel mononitrosyl hydride and dihydrogen rhenium complexes: [Re(Br)2(NO)(η2-H2)(PR3)2] (R = iPr, 2a; Cy, 2b) and [Re(H)(BH4)(NO)(PR3)2] (R = iPr, 3a; Cy, 3b). The
coordinated BH3 of the derivatives 3 can be replaced by the H2 or the NO ligand, thus leading
to the tetrahydride and dinitrosyl species [Re(H)4(NO)L2] (R = iPr, 4a; Cy, 4b) or [Re(H)(NO)2(PR3)2] (R = iPr, 5a; Cy, 5b). While [Re(H)4(NO)(PPh3)2] does not seem to be stable,
[Re(H)(NO)2(PPh3)2] (5c) has been obtained in a fashion similar to the preparation of 3a,b
from the reaction of [Re(H)(BH4)(NO)(PPh3)2] and NOBF4. Detailed investigations of the
reactions of 3a,b with NOBF4 have revealed that the compounds initially formed are the
isolable BF3 adducts [Re(H)(NO)(NOBF3)(PR3)2] (R = iPr, 6a; Cy, 6b). The source of BF3 is
the nitrosonium salt. Dissociation of BF3 from 6a,b takes place in donor solvents such as
THF, affording the BF3-free compounds 5a,b, whereas in noncoordinating solvents such as
toluene, benzene, or CH2Cl2 only the species 6a,b are observable. Apparently due to an
unfavorable position of the dissociation equilibrium, the existence of the complex [Re(H)(NO)(NOBF3)(PPh3)2] could only be made plausible from dynamic NMR spectroscopic
observations. Attempts to isolate it failed even from nonpolar solvents. X-ray diffraction
studies have been carried out on the complexes [Re(Br)2(NO)(η2-H2)(PiPr3)2] (2a), [Re(H)(BH4)(NO)(PR3)2] (R = iPr, 3a; Ph, 3c), [Re(H)(NO)2(PiPr3)2] (5a), and [Re(H)(NO)(NOBF3)(PiPr3)2] (6a). The hydrogen atoms of the η2-H2 moiety of 2a could not be located in the
X-ray diffraction study, but their most probable position in the molecule has been traced by
an extensive search based on DFT calculations.
