In this review, the coordination chemistry of electron‐rich metal complexes with the simple aliphatic, anionic diphosphanylamido ligand {N(CH2CH2PR2)2}– is covered and compared with other commonly used, anionic PEP (E = C, N) pincer ligands. The strong π‐basicity of this ligand enables both the stabilization of electronically and coordinatively highly unsaturated complexes and their use as cooperating ligands in bifunctional stoichiometric bond activation reactions and catalysis. Versatile ligand backbone dehydrogenation gives access to related enamido and dienamido ligands {(R2PCHCH)N(CH2CH2PR2)}– and {N(CHCHPR2)2}–, respectively. This oxidative functionalization enables fine‐tuning of the ligand donor properties and thereby of the structural features, electronic structure, and reactivity of the respective complexes, which is discussed for several examples.

The reaction of [IrCl(COE) 2 ] 2 (COE = cyclooctene) with bulky PNP amino pincer ligand HN(CH 2 CH 2 P t Bu 2 ) 2 , (HPNP) tBu , results in high-yield formation of iridium(III) complex [IrH(C 8 H 13 )Cl(HPNP) tBu ] after oxidative addition of a vinylic COE C-H bond. Upon dissolving the product in polar solvents or chloride abstraction with TlPF 6 the cationic iridium(I) complex [Ir(COE)(HPNP) tBu ] þ is obtained, which is thermally unstable. This observation shows that the olefin isomer represents the thermodynamic minimum for the cationic complex, while the cyclooctenyl hydride isomer is trapped in nonpolar solvents by anion coordination. Owing to the lability of the olefin ligand, [Ir(COE)(HPNP) tBu ] þ undergoes facile intermolecular C-H activation. Fischer-carbene complex [Ir(H) 2 (dCO(CH 2 ) 3 )(HPNP) tBu ]BPh 4 was isolated in good yield upon R, R-dehydrogenation of THF. Mechanistic examinations suggest olefin dissociation to be rate-determining.

Squaring the circle: the novel dienamido pincer ligand N(CHCHPtBu(2))(2)(-) affords the isolation of the unusual square-planar iridium(II) and iridium(III) amido complexes [IrCl{N(CHCHPtBu(2))(2)}](n) (n=0 (1), +1 (2)). In contrast, the corresponding iridium(I) complex of the redox series (n=-1) is surprisingly unstable. The diamagnetism of 2 is attributed to strong N→Ir π donation.

The iridium dihydride [Ir(H)2 (HPNP)](+) (PNP=N(CH2 CH2 PtBu2 )2 ) reacts with O2 to give the unusual, square-planar iridium(III) hydroxide [Ir(OH)(PNP)](+) and water. Regeneration of the dihydride with H2 closes a quasi-catalytic synthetic oxygen-reduction reaction (ORR) cycle that can be run several times. Experimental and computational examinations are in agreement with an oxygenation mechanism via rate-limiting O2 coordination followed by H-transfer at a single metal site, facilitated by the cooperating pincer ligand. Hence, the four electrons required for the ORR are stored within the two covalent MH bonds of a mononuclear metal complex.

Potassium N-isopropyl-2-(isopropylamino)troponiminate, K{(iPr)2ATI}, and potassium N-cyclohexyl-2-(cyclohexylamino)troponiminate, K{(Cy)2ATI}, were synthesized by treatment of the neutral ligands with an excess of KH in THF. Reaction of the potassium reagents with [AuClPPh3] resulted in the gold complexes [Au{(iPr)2ATI}PPh3] and [Au{(Cy)2ATI}PPh3]. The solid-state structures of both compounds, in which the ligands are arranged in plane, show distorted trigonal planar coordinated gold atoms. Potassium 2-(isopropylamino)troponate (K(iPrAT)) and the cyclohexyl analogue (K(CyAT)) were obtained by deprotonation of corresponding aminotropones with KH. In an analogous fashion the gold complexes of composition [Au(iPrAT)PPh3] and [Au(CyAT)PPh3] were prepared by reaction of K(iPrAT) and K(CyAT) with [AuClPPh3], respectively.