A detailed comparative study of the structural and spectroscopic features and of the reactivity of ruthenium phosphinidene complexes (η 6 -Ar)(PCy 3 )Ru(PMes*) (2a, Ar ) p-cymene; 2b, Ar ) benzene) has been undertaken. The structures of complexes 2a and 2b have been determined by single-crystal X-ray diffraction and display similar features. Both compounds possess identical chemical behavior toward Bro ¨nsted acids such as HBF 4 : protonation of the phosphinidene ligand yields the new cationic complexes [(η 6 -Ar)(PCy 3 )Ru(PHMes*)]BF 4 (3aBF 4 , Ar ) p-cymene; 3bBF 4 , Ar ) benzene), which exhibit an unprecedented phosphenium-bearing hydrogen substituent. 3aBF 4 has been characterized using X-ray diffraction techniques. The lone pair of the phosphorus atom of the phosphinidene ligand remains also accessible to the Lewis acid BH 3 : the reactions of 2a and 2b with borane give the adducts (η 6 -Ar)-(PCy 3 )Ru[P(BH 3 )Mes*] (4a, Ar ) p-cymene; 4b, Ar ) benzene). In the presence of the larger borane BPh 3 , no reaction occurs until water is introduced in the reaction vessel. This results in the generation of [(η 6 -Ar)(PCy 3 )Ru(PHMes*)]BPh 3 OH (3aBPh 3 OH, Ar ) p-cymene; 3bBPh 3 OH, Ar ) benzene) presumably through protonation of 2a and 2b by the previously unknown adduct H 2 O‚BPh 3 . Phosphinidene complexes react also with electrophilic alkylating reagents such as organic iodides provided the alkyl substituent is small. Treatment of 2a and 2b with 1 equiv of methyliodide leads to the alkylation at the phosphinidene center and yields the phosphenium complexes [(η 6 -Ar)(PCy 3 )Ru(PMeMes*)]I (5a, Ar ) p-cymene; 5b, Ar ) benzene). Examination of the reactivity toward electron-rich reagents such as the alkynes RCCH (R ) Me 3 Si, Ph) yields unexpected results: 2a instantaneously reacts to generate phosphaindane complexes 6 and 7, whereas no reaction occurs when using 2b. A detailed kinetic study provides evidence for a dissociative mechanism involving the release of the phosphine ligand in 2a and explains its specificity. The p-cymene ligand in 2a acts as a reactivity switch due to the higher steric hindrance of this arene.

The metaphosphate anion has attracted steady interest since 1955 [1] in connection with wide applications in phosphorylation reactions.[2] It has relevance in biochemistry in particular with respect to ATP hydrolysis.[3] However, despite considerable efforts, monomeric metaphosphates and other dioxophosphoranes are only known as transient species in solution and their existence can be inferred from trapping experiments. [2][3][4] Their high reactivity arises from the powerful electrophilic character at the phosphorus center. Several approaches have been tested to stabilize these highly reactive species. Kinetic stabilization by steric protection [5] proved to be ineffective for dioxophosphorane and led to products that resulted from the insertion of the PO 2 moiety in a neighboring group.[6] The electrophilicity and the tendency to expand the valence shell were exploited for their stabilization as Lewis salts. This strategy allowed only spectroscopic characterization by 31 P NMR in a few cases. [7] Thus, the chemistry of dioxophosphorane suffers from a paucity of information and new approaches to stabilize and enable deeper investigation are highly desirable. This work describes the first isolation and full characterization, including X-ray structural analysis, of stabilized metaphosphonate.We have developed a novel and successful route to ruthenium-stabilized monomeric metaphosphonate by oxidation of a nucleophilic terminal phosphinidene complex of ruthenium. As recently described by Lammerstma and coworkers, the terminal phosphinidene ruthenium complexes [(h 6 -p-cymene)Ru(PR 3 )(PMes*)] (3 a: R = Cy, 3 b: R = Ph [8] ) are formed as dark green solids by the reaction of the primary phosphane complex [(h 6 -p-cymene)RuCl 2 (PH 2 Mes*)](1; pcymene = 4-methyl-iso-propylbenzene; Mes* = 2,4,6-tri-tertbutylphenyl) with two equivalents of 1,8-diazabicyclo[5-4-0]undec-7-ene (DBU) in the presence of PR 3 as a stabilizing ligand (R = Ph, Cy).[8] Alternatively, treatment of [(h 6 -pcymene)RuCl 2 (PR 3 )] (2 a: R = Cy, 2 b: R = Ph) and PH 2 Mes* with DBU affords 3 in high yields (Scheme 1).