A new tris(phosphaalkene)phosphine ligand (1) was synthesized via phospha-Wittig methodology. Metalation of 1 with [RhCl(C 2 H 4 ) 2 ] 2 and [IrCl(COE) 2 ] 2 (COE = cyclooctene) produced trigonal bipyramidal metal chlorides 2a (M = Rh) and 2b (M = Ir) in which the ligand coordinates in a tetradentate fashion. X-ray crystallographic studies on 1·1.5THF, 2a·5CHCl 3 , and 2b·2.5CHCl 3 combined with DFT calculations revealed a pronounced change in hybridization of the phosphaalkene phosphorus atoms upon coordination to the Rh/Ir centers, resulting in highly sterically congested metal complexes. Nucleophilic substitution on 2a with NaN 3 afforded Rh−N 3 complex 3; computational analysis, IR spectroscopy, and 15 N{ 1 H} NMR spectroscopy on isotopologue 15 N-3 provided additional structural insights. Halide abstraction of the chloride in 2b with AgOTf in the presence of acetonitrile afforded cationic Ir−NCMe complex 4. Evidence of the bound acetonitrile unit was obtained by 2D NMR spectroscopy and deuterium labeling studies. ■ INTRODUCTIONTetradentate ligands 1 featuring sterically demanding groups are becoming increasingly popular, 2 as the resulting metal center contains a highly crowded, but well-defined binding pocket for the activation of small molecules. 3 Schrock and co-workers successfully exploited bulky TREN ligand derivatives (TREN = 2,2′,2′′-triaminotriethylamine; HIPT = hexaisopropylterphenyl, shown in Scheme 1) to direct dinitrogen coordination into a sterically protected trigonal bipyramidal (TBP) Mo pocket. 4 Subsequent treatment of the bound N 2 unit with a proton source and reducing agent resulted in the first example of homogeneous and catalytic ammonia production. 5The guiding principle of using sterics to dictate metalcentered reactivity 6 is evolving as ligands capable of electronically influencing reactivity have experienced a surge of interest. 7 The most common examples of ligands engaging in this redoxactive/noninnocent behavior 8 are pincer-derived metal complexes, which undergo metal−ligand cooperativity via dearomatization/aromatization processes 9 or by ligand-centered reduction through easily accessible, low-lying, and extended π networks. 10 Recently though, the Peters lab has directly observed this electronic influence with tetradentate XP 3 -supported (X = Si, 11 C, 12 B, 13 and N 14 ) TBP first-row metal complexes designed to catalyze dinitrogen reduction (Scheme 1). 15 Fe 16 and Co complexes 17 ligated by the BP 3 ligand facilitated the catalytic conversion of N 2 to NH 3 (in the presence of HBAr F and KC 8 ), while the performance of SiP 3 and CP 3 analogues was stoichiometric at best; no N 2 reduction has been reported with the NP 3 ligand set. 17 Trivalent boranes (BAr 3 ) are often counted as zero-electron ligands, 18 but recent evidence has suggested that the tricoordinate boron unit (BAr 3 ) in BP 3 can act as a zero-, one-, or two-electron ligand depending on the oxidation state of the metal complex. 19 This