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Unprecedented metalated phosphonite boranes were prepared from PH-substituted precursors and silyl amides. Althoughp otassium derivatives were thermally stable and could even be isolated and structurally characterised,l ithiated analogues proved to be unstable towards self-condensation under cleavage of LiOR at ambient temperature. Reaction studies revealed that the metalated phosphonite boranes exhibit ambiphilic character. Their synthetic potential as nucleophilic building blocks was demonstrated in the synthesis of the first stannylated phosphonite representing an ew structural motif in phosphine chemistry.

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KEYWORDS N-heterocyclic Phosphenium complexes -NO complexes -π-acceptor ligandselectronic structure -ligand centered reactivity ABSTRACT. N-heterocyclic phosphenium (NHP) and nitrosonium (NO + ) ligands are often viewed as isolobal analogues which share the capability to switch between different charge states and display thus redox 'non-innocent' behavior. We report here on mixed complexes [(NHP)M(CO)n(NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)3(NO)] or PPN[Cr(CO)4(NO)], respectively, and fully characterized. The structural and spectroscopic (IR, UV-VIS) data support the presence of carbene-analogue NHP ligands with overall positive charge state and π-acceptor character. Even the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that the NO units exhibit nitroxide (NO -) character. This assignment was validated by computational studies, which reveal also that the electronic structure of iron NHP/NO-complexes is closely akin to that of the Hieber anion, [Fe(CO)3(NO)] -. The electrophilic character of the NHP units is further reflected in the chemical behavior of the mixed complexes. Cyclic voltammetry and IR-SEC studies revealed that complex [(NHP)Fe(CO)2(NO)](4) undergoes chemically reversible one-electron reduction. Computational studies indicate that the NHP unit in the resulting product carries significant radical character and the reduction may thus be classified as predominantly ligand-centered. Reaction of 4 with sodium azide proceeded likewise under nucleophilic attack at phosphorus and decomplexation, while super hydride and methyl lithium reacted with all chromium and iron complexes via transfer of a hydride or methyl anion to the NHP unit to afford anionic phosphine complexes. Some of these species were isolated after cation exchange, or trapped with electrophiles (H + , SnPh3 + ) to afford neutral complexes representing the products of a formal hydrogenation or hydrostannylation of the original M=P double bond.

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