The coordination chemistry of an N-heterocyclic phosphenium (NHP)-containing bis(phosphine) pincer ligand has been explored with Pt(0) and Pd(0) precursors. Unlike previous compounds featuring monodentate NHP ligands, the resulting NHP Pt and Pd complexes feature pyramidal geometries about the central phosphorus atom, indicative of a stereochemically active lone pair. Structural, spectroscopic, and computational data suggest that the unusual pyramidal NHP geometry results from two-electron reduction of the phosphenium ligand to generate transition metal complexes in which the Pt or Pd centers have been formally oxidized by two electrons. Interconversion between planar and pyramidal NHP geometries can be affected by either coordination/dissociation of a two-electron donor ligand or two-electron redox processes, strongly supporting an isolobal analogy with the linear (NO(+)) and bent (NO(-)) variations of nitrosyl ligands. In contrast to nitrosyls, however, these new main group noninnocent ligands are sterically and electronically tunable and are amenable to incorporation into chelating ligands, perhaps representing a new strategy for promoting redox transformations at transition metal complexes.

A series of V/Fe heterobimetallic complexes supported by phosphinoamide ligands, [Ph 2 PN i Pr] À , is described. The V(III) metalloligand precursor [V( i PrNPPh 2 ) 3 ] can be treated with Fe(II) halide salts under reducing conditions to afford [V( i PrNPPh 2 ) 3 FeX] (X ¼ Br (2), I (3)). These complexes feature multiple bonds between Fe and V, leading to an intermetallic distance of $2.07 Å. Exploration of the oneelectron reduction of complex 3 allows isolation of [V( i PrNPPh 2 ) 3 Fe(PMe 3 )] ( 5), which also features metal-metal multiple bonding and a nearly identical Fe-V distance. Mössbauer spectroscopy of complexes 2 and 5 suggest that the most reasonable oxidation state assignments for these complexes are V III Fe I and V III Fe 0 , respectively, and that reduction occurs solely at the Fe center in these bimetallic complexes. A theoretical investigation confirms this description of the electronic structure, providing a description of the metal-metal bonding manifolds as (s) 2 (p) 4 (Fe nb ) 3 and (s) 2 (p) 4 (Fe nb ) 4 for complexes 3 and 5, consistent with a metal-metal bond order of three. One electron-oxidation of complex 3 results in halide abstraction from PF 6 À , forming FV( i PrNPPh 2 ) 3 FeI (6). Complex 6 has a much weaker V-Fe interaction as a result of axial fluoride ligation at the V center.

Single electron transfer from the Zr(III)Co(0) heterobimetallic complex (THF)Zr(MesNP(i)Pr2)3Co-N2 (1) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph2CO)Zr(MesNP(i)Pr2)3Co-N2]2 (2) via coupling of two ketyl radicals. In this work, thermolysis of 2 in an attempt to favor a monomeric ketyl radical species unexpectedly led to cleavage of the C-O bond to generate a Zr/Co μ-oxo species featuring an unusual terminal Co═CPh2 carbene linkage, (η(2)-MesNP(i)Pr2)Zr(μ-O)(MesNP(i)Pr2)2Co═CPh2 (3). This complex was characterized structurally and spectroscopically, and its electronic structure is discussed in the context of density functional theory calculations. Complex 3 was also shown to be active toward carbene group transfer (cyclopropanation), and silane addition to 3 leads to PhSiH2O-Zr(MesNP(i)Pr2)3Co-N2 (5) via a proposed Co-alkyl bond homolysis route.

The tris(phosphino­amide)-linked heterobimetallic Co/Zr complex (THF)­Zr(Mes­NP i Pr2)3­CoN2 (1) has been investigated as a catalyst for the hydrosilylation of ketones with PhSiH3. Catalytic activity superior to monometallic Co or Zr analogues has been observed, demonstrating the importance of cooperative reactivity between Co and Zr. Upon examining stoichiometric reactions, complex 1 was found to be unreactive toward PhSiH3, implying that the mechanism diverges from the typical Chalk–Harrod-type hydrosilylation pathway. In contrast, 1 reacts readily with ketones, and in the case of benzophenone, a radical coupling product [(Ph2CO)­Zr(MesNP i Pr2)3­CoN2]2 (3) was isolated, implying the intermediacy of a Zr-bound ketyl radical fragment. A radical-based hydrosilylation mechanism is proposed involving hydrogen atom transfer from PhSiH3 to the Zr-bound ketyl-radical.

A series of tris(phosphinoamide) heterobimetallic Cr-M (M ¼ Fe, Co and Cu) complexes has been investigated in an effort to probe and contribute to the understanding of the electronic structure and metal-metal bonding in heterobimetallic complexes of the first row transition metals. The chromium tris(phosphinoamide), [Cr( i PrNPPh 2 ) 3 ] (1), is a useful isolable precursor and can be treated with MI 2 under reducing conditions to form [Cr( i PrNPPh 2 ) 3 M-I] (M ¼ Fe (2), Co (3)). Both of these complexes can be reduced by one electron to generate [Cr( i PrNPPh 2 ) 3 M-PMe 3 ] (M ¼ Fe (4), Co ( 5)). The Cr-Cu complex [Cr( i PrNPPh 2 ) 3 Cu-I] (6) has also been synthesized for comparison. The solid state structures of 2-6 have been determined crystallographically, revealing relatively short metal-metal interatomic distances. M össbauer spectroscopy, cyclic voltammetry, and computational methods have been used to evaluate the electronic structure and metal-metal interactions in these unique bimetallic complexes in an effort to uncover the underlying factors that affect metal-metal bonding between elements of the first row transition series.