It has been previously demonstrated that stable singlet electrophilic carbenes can behave as metal surrogates in the activation of strong E–H bonds (E = H, B, N, Si, P), but it was believed that these activations only proceed through an irreversible activation barrier. Herein we show that, as is the case with transition metals, the steric environment can be used to promote reductive elimination at carbon centers.

The preparation and reactivity with H 2 of two Ru complexes of the novel ZnPhos ligand (ZnPhos = Zn(o-C 6 H 4 PPh 2 ) 2 ) are described. Ru(ZnPhos)(CO) 3 (2) and Ru(ZnPhos)(IMe 4 ) 2 (4; IMe 4 = 1,3,4,5tetramethylimidazol-2-ylidene) are formed directly from the reaction of Ru(PPh 3 )(C 6 H 4 PPh 2 ) 2 (ZnMe) 2 (1) or Ru(PPh 3 ) 3 HCl/LiCH 2 TMS/ ZnMe 2 with CO and IMe 4 , respectively. Structural and electronic structure analyses characterize both 2 and 4 as Ru(0) species in which Ru donates to the Z-type Zn center of the ZnPhos ligand; in 2, Ru adopts an octahedral coordination, while 4 displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, 2 loses CO to give Ru(ZnPhos)(CO) 2 that then adds H 2 over the Ru−Zn bond to form Ru(ZnPhos)(CO) 2 (μ-H) 2 (3). In contrast, 4 reacts directly with H 2 to set up an equilibrium with Ru(ZnPhos)(IMe 4 ) 2 H 2 (5), the product of oxidative addition at the Ru center. DFT calculations rationalize these different outcomes in terms of the energies of the square-pyramidal Ru(ZnPhos)L 2 intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H 2 to add across the Ru−Zn bond, but for L = IMe 4 , this species is kinetically inaccessible and reaction can only occur at the Ru center. This difference is related to the strong π-acceptor ability of CO compared to IMe 4 . Steric effects associated with the larger IMe 4 ligands are not significant. Species 4 can be considered as a Ru(0)L 4 species that is stabilized by the Ru→Zn interaction. As such, it is a rare example of a stable Ru(0)L 4 species devoid of strong π-acceptor ligands.

Studies on alkyne hydrophosphination employing nickel‐NHC catalysts (NHC=N‐heterocyclic carbene) revealed that the free N‐alkyl substituted NHCs themselves were catalytically active. DFT calculations showed the mechanism involves the NHC acting as a Brønsted base to form an imidazolium phosphide species which then undergoes rate‐limiting nucleophilic attack at the terminal alkyne carbon. This mechanism explains the preference seen experimentally for reactions with aryl substituted phosphines and alkynes, while the rearrangements of the alkenyl anion formed upon P−C bond formation account for the observation of both Z‐ and E‐regioisomers of the products.

The addition of PPh 2 H, PPhMeH, PPhH 2 , P(para-Tol)H 2 , PMesH 2 and PH 3 to the two-coordinate Ni 0 Nheterocyclic carbene species [Ni(NHC) 2 ] (NHC = IiPr 2 , IMe 4 , IEt 2 Me 2 ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [HÀ NiÀ PR 2 ] or novel trans [R 2 PÀ NiÀ PR 2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe 4 > IEt 2 Me 2 > IiPr 2 ) and phosphines are employed. PÀ P activation of the diphosphines R 2 PÀ PR 2 (R 2 = Ph 2 , PhMe) provides an alternative route to some of the [Ni(NHC) 2 (PR 2 ) 2 ] complexes.DFT calculations capture these trends with PÀ H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the NiÀ P bond. PÀ P bond activation from [Ni(NHC) 2 (Ph 2 PÀ PPh 2 )] adducts proceeds with computed barriers below 10 kcal mol À 1 . The ability of the [Ni(NHC) 2 ] moiety to afford isolable terminal phosphido products reflects the stability of the NiÀ NHC bond that prevents ligand dissociation and onward reaction.

Treatment of [Ru(PPh 3 )(C 6 H 4 PPh 2 ) 2 H][Li(THF) 2 ] with AlMe 2 Cl and SnMe 3 Cl leads to elimination of LiCl and CH 4 and formation of the heterobimetallic complexes [Ru(C 6 H 4 PPh 2 ) 2 {PPh 2 C 6 H 4 AlMe(THF)}H] 5 and [Ru(PPh 3 )(C 6 H 4 PPh 2 )-(PPh 2 C 6 H 4 SnMe 2 )] 6, respectively. The pathways to 5 and 6 have been probed by variable temperature NMR studies, together with input from DFT calculations. Complete reaction of H 2 occurs with 5 at 60 °C and with 6 at room temperature to yield the spectroscopically characterized trihydride complexes [Ru(PPh 2 ) 2 {PPh 2 C 6 H 4 AlMe}H 3 ] 7 and [Ru(PPh 2 ) 2 {PPh 2 C 6 H 4 SnMe 2 }-H 3 ] 8. In the presence of CO, 6 forms the acylated phosphine complex, [Ru(CO) 2 (C(O)C 6 H 4 PPh 2 )(PPh 2 C 6 H 4 SnMe 2 )] 9, through a series of intermediates that were identified by NMR spectroscopy in conjunction with 13 CO labeling. Complex 6 undergoes addition and substitution reactions with the N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene (IMe 4 ) to give [Ru(IMe 4 ) 2 (PPh 2 C 6 H 4 )(PPh 2 C 6 H 4 SnMe 2 )] 10, which converted via rare N-Me group C−H activation to [Ru(IMe 4 )(PPh 3 )-(IMe 4 )′(PPh 2 C 6 H 4 SnMe 2 )] 11 upon heating at 60 °C and to a mixture of [Ru(IMe 4 ) 2 (IMe 4 )′(PPh 2 C 6 H 4 SnMe 2 )] 12 and [Ru(PPh 3 )(PPh 2 C 6 H 4 )(IMe 4 -SnMe 2 )′] 13 at 120 °C.