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 c
omplexes: [Re(Br)(2)(NO)(eta(2)-H-2)(PR3)(2)] (R = Pr-i, 2a; Cy, 2b) and [R
e(H)(BH4)(NO)(PR3)(2)] (R = Pr-i, 3a; Cy, 3b). The coordinated BH3 of the d
erivatives 3 can be replaced by the H-2 or the NO ligand, thus leading to t
he tetrahydride and dinitrosyl species [Re(H)(4)(NO)L-2] (R = Pr-i, 4a; Cy,
4b) or [Re(H)(NO)(2)(PR3)(2)] (R = Pr-i, 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 rea
ctions of 3a,b with NOBF4 have revealed that the compounds initially formed
are the isolable BF3 adducts [Re(H)(NO)(NOBF3)(PR3)(2)] (R = Pr-i, 6a; Cy,
6b). The source of BF3 is the nitrosonium salt. Dissociation of BF3 from 6
a,b takes place in donor solvents such as THF, affording the BF3-free compo
unds 5a,b, whereas in noncoordinating solvents such as toluene, benzene, or
CH2Cl2 only the species 6a,b are observable. Apparently due to an unfavora
ble position of the dissociation equilibrium, the existence of the complex
[Re(H)(NO)(NOBF3)(PPh3)(2)] could only be made plausible from dynamic NMR s
pectroscopic 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)(eta(2)-H-2)((PPr3)-Pr-i)(2)] (2a), [Re(H)(BH4)(NO)(PR3)(2)]
(R = Pr-i, 3a; Ph, 3c), [Re(H)(NO)(2)((PPr3)-Pr-i)(2)] (5a), and [Re(H)(NO
)(NOBF3)((PPr3)-Pr-i)(2)] (6a). The hydrogen atoms of the eta(2)-H-2 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 ba
sed on DFT calculations.