The reaction of the cobalt(I) complex [(TIMMN mes )Co I ](BPh 4 )(2)(TIMMN mes = tris-[2-(3-mesitylimidazolin-2-ylidene)methyl]amine) with 1-adamantylazide yields the cobalt(III) imido complex [(TIMMN mes )Co III -(NAd)](BPh 4 )( 3)w ith concomitant release of dinitrogen. The N-anchor in diamagnetic 3 features an unusual, planar tertiary amine,w hich results from repulsive electrostatic interaction with the filled d(z 2 )-orbital of the cobalt ion and negative hyperconjugation with the neighboring methylene groups.O ne-electron oxidation of 3 with [FeCp 2 ](OTf) provides access to the rare,h igh-valent cobalt(IV) imido complex [(TIMMN mes )Co IV (NAd)](OTf) 2 (4). Despite ah alflife of less than 1hat room temperature, 4 could be isolated at low temperatures in analytically pure form. Single-crystal Xray diffractometry and EPR spectroscopyc orroborate the molecular structure and the d 5 low-spin, S = 1 = 2 ,e lectron configuration. Ac omputational analysis of 4 suggests high covalency within the Co IV =NAdbond with non-negligible spin density located at the imido moiety,w hicht ranslates into substantial triplet nitrene character.

The first scandium phosphinoalkylidene complex was synthesized and structurally characterized. The complex has the shortest Sc-C bond lengths reported to date (2.089(3) Å). DFT calculations reveal the presence of a three center π interaction in the complex. This scandium phosphinoalkylidene complex undergoes intermolecular C-H bond activation of pyridine, 4-dimethylamino pyridine and 1,3-dimethylpyrazole at room temperature. Furthermore, the complex rapidly activates H under mild conditions. DFT calculations also demonstrate that the C-H activation of 1,3-dimethylpyrazole is selective for thermodynamic reasons and the relatively slow reaction is due to the need of fully breaking the chelating effect of the phosphino group to undergo the reaction whereas this is not the case for H.

The first phosphoniomethylidene complexes of scandium and lutetium, [LLn(CHPPh)X] (L = [MeC(NDIPP)CHC(NDIPP)Me]; Ln = Sc, X = Me, I, TfO; Ln = Lu, X = CHSiMe), have been synthesized and fully characterized. DFT calculations clearly demonstrate the presence of an allylic Ln, C, P π-type interaction in these complexes. X-ray diffraction indicates that the scandium iodide complex has the shortest Sc-C bond length to date (2.044(5) Å). These phosphoniomethylidene complexes readily convert into the ylide complexes, and the reactivity is affected by both X anion and Ln ion. The reaction of lutetium complex with imine shows a rapid insertion of imine into the Lu-C(alkylidene) bond. DFT calculations indicate that, although the bonding situation seems similar to that of the scandium analog, the strong negative charge at the alkylidene carbon is not sufficiently screened by one hydrogen in the lutetium complex because of a more ionic bonding, and therefore, the reactivity of the lutetium complex is much higher.