Synthesis, characterization, and catalytic alkyne polymerization results for the first trianionic pincer alkylidyne complex, [(t)BuOCO]W≡CC(CH(3))(3)(THF)(2) (6), are described. Complex 6 is a highly active catalyst for the polymerization of acetylenes and exhibits a high turnover number (4371), activity (1.05 × 10(6) g(PPA) mol(cat)(-1) h(-1)),and yield (87%) for the polymerization of 1-ethynyl-4-fluorobenzene.

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.

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.

The tris(phosphinoamide)-bridged Fe(II)Fe(II) diiron complex Fe(μ-(i)PrNPPh2)3Fe(η(2)-(i)PrNPPh2) (1) can be reduced in the absence or presence of PMe3 to generate the mixed-valence Fe(II)Fe(I) complexes Fe(μ-(i)PrNPPh2)3Fe(PPh2NH(i)Pr) (2) or Fe(μ-(i)PrNPPh2)3Fe(PMe3) (3), respectively. Following a typical oxidative group transfer procedure, treatment of 2 or 3 with organic azides generates the mixed-valent Fe(II)Fe(III) imido complexes Fe((i)PrNPPh2)3Fe≡NR (R = (t)Bu (4), Ad (5), 2,4,6-trimethylphenyl (6)). These complexes represent the first examples of first-row bimetallic complexes featuring both metal-ligand multiple bonds and metal-metal bonds. The reduced complexes 2 and 3 and imido complexes 4-6 have been characterized via X-ray crystallography, Mössbauer spectroscopy, cyclic voltammetry, and SQUID magnetometry, and a theoretical description of the bonding within these diiron complexes has been obtained using computational methods. The effect of the metal-metal interaction on the electronic structure and bonding in diiron imido complexes 4-6 is discussed in the context of similar monometallic iron imido complexes.

To understand the metal-metal bonding and conformational flexibility of first-row transition metal heterobimetallic complexes, a series of heterobimetallic Ti/M and V/M complexes (M = Fe, Co, Ni, and Cu) have been investigated. The titanium tris(phosphinoamide) precursors ClTi(XylNPPr) (1) and Ti(XylNPPr) (2) have been used to synthesize Ti/Fe (3), Ti/Ni (4, 4), and Ti/Cu (5) heterobimetallic complexes. A series of V/M (M = Fe (7), Co (8), Ni (9), and Cu (10)) complexes have been generated starting from the vanadium tris(phosphinoamide) precursor V(XylNPPr) (6). The new heterobimetallic complexes were characterized and studied by NMR spectroscopy, X-ray crystallography, electron paramagnetic resonance, and Mössbauer spectroscopy, where applicable, and computational methods (DFT). Compounds 3, 4, 7, and 8 are C-symmetric with three bridging phosphinoamide ligands, while compounds 9 and 10 adopt an asymmetric geometry with two bridging phosphinoamides and one phosphinoamide ligand bound η to vanadium. Compounds 4 and 5, on the other hand, are asymmetric in the solid state but show evidence for fluxional behavior in solution. A correlation is established between conformational flexibility and metal-metal bond order, which has important implications for the future reactivity of these and other heterobimetallic molecules.