Synthesis and protonation reactions of nitrosyl complexes [M2Cp(μ-κ:η5-C5H4)(μ-PR2)(CO)(NO)2] to give κ:η-NO-bridged derivatives (M = Mo, W; R = Cy, Bu)

• Me 3 NO promotes decarbonylation and deprotonation on [M 2 Cp 2 ( μ -PR 2 )(CO) 2 (NO) 2 ] + cations. • The title complexes are protonated at the C 5 H 4 group to give κ:η -NO-bridged cations. • [W 2 Cp( μ - κ:η 5 -C 5 H 4 )( μ -P t Bu 2 )(CO)(NO) 2 ] is methylated to give a methoxyimido derivative. • κ:η -NO-bridged derivatives are computed to be less stable than their μ - κ:η -CO isomers. The title compounds were prepared upon reaction of BAr’ 4 − salts of the known cations [M 2 Cp 2 ( μ -PR 2 )(CO) 2 (NO) 2 ] + (M = Mo, W; R = Cy, t Bu; Ar’ = 3,5-C 6 H 3 (CF 3 ) 2 ), with commercial Me 3 NO·2H 2 O in fluorobenzene solution at 273 K, this promoting decarbonylation and cyclopentadienyl deprotonation to yield complexes with cyclopentadienylidene ligands bridging the metal centres (W−W = 2.9917(5) Å when R = Cy). The use of dichloromethane as solvent, however, triggered a competitive chloride abstraction reaction by the Mo compounds that yielded the chloride complexes [Mo 2 Cp 2 Cl( μ -PR 2 )(CO)(NO) 2 ], specifically formed with an anti disposition of the Cp ligands relative to the Mo 2 P plane, and cis positioning of carbonyl and chloride ligands relative to the P atom (Mo−Cl = 2.336(4) Å when R = t Bu). Protonation of the cyclopentadienylidene-bridged complexes with [H(OEt 2 ) 2 ](BAr’ 4 ) yielded the BAr’ 4 − salts of the dicyclopentadienyl cations [M 2 Cp 2 ( μ - κ:η -NO)( μ -PR 2 )(CO)(NO)] + (Mo−Mo = 3.1027(2) Å when R = t Bu; W−W = 3.0446(7) Å when R = Cy), which display bridging nitrosyl ligands in the rare linear end-on:side-on coordination mode, and are kinetic rather than thermodynamic products, according to density functional theory calculations. When using HBF 4 ·OEt 2 as protonation reagent on the P t Bu 2 -bridged complexes, however, coordination of the external anion (in the κ 1 -mode) also takes place, to yield the tetrafluoroborate complexes [M 2 Cp 2 ( κ 1 -BF 4 )( μ -P t Bu 2 )(CO)(NO) 2 ] as unique (Mo) or major (W) products. Methylation of the same cyclopentadienylidene complexes with [Me 3 O](BF 4 ) was only observed for the tungsten cation, and took place specifically at the O atom of one nitrosyl ligand to give [W 2 Cp( μ - κ:η 5 -C 5 H 4 )(NOMe)( μ -P t Bu 2 )(CO)(NO)](BF 4 ), a complex displaying a linear methoxyimido ligand (W−N = 1.746(4) Å; W−N−O = 163.5(4) o ). Synopsis Decarbonylation of cations [M 2 Cp 2 ( μ -PR 2 )(CO) 2 (NO) 2 ] + with Me 3 NO·2H 2 O also resulted in deprotonation of a cyclopentadienyl ring to yield cyclopentadienylidene-bridged derivatives. Protonation of the latter restored the Cp ring and triggered nitrosyl rearrangement to yield rare linear κ:η -NO-bridged complexes, even if less stable than the corresponding κ:η -CO-bridged isomers, according to DFT calculations. Pictogram