It is well established that the active intermediate in reactions mediated by nickel tetracarbonyl (Ni(CO) 4 ) is the three-coordinate complex Ni(CO) 3 . 1À4 This coordinatively unsaturated binary carbonyl is formed by CO dissociation from its fourcoordinate, 18-electron precursor and is active toward ligand association 1À4 and oxidative addition 5À7 reactions. While too reactive to be isolated or observed in the condensed phase, pioneering gas-phase and matrix-isolation studies by Turner, DeKock, and Burdett succeeded in photolytically synthesizing and fully characterizing Ni(CO) 3 . 8À10 Along with these experiments, early theoretical predictions based on molecular orbital and angular overlap methods corroborated the fact that d 10 Ni(CO) 3 adopts a trigonal-planar coordination geometry. 11À13 This geometry is similar to three-coordinate, Ni(0) trisphosphine complexes (i.e., Ni(PR 3 ) 3 ), which are long known 14 and have been isolated when encumbering phosphines are employed. 15,16 Despite sharing three-coordinate, trigonal-planar geometries, Ni(CO) 3 is electronically distinct from Ni(PR 3 ) 3 complexes. This is a result of the stabilization of Ni-based, π-symmetry orbitals (e 0 and e 00 in D 3h symmetry) by back-donation to the CO ligands. Accordingly, the HOMO of D 3h -symmetric Ni(CO) 3 is d z 2 (a 1 0 ) in parentage, rather than the degenerate e 0 set (x 2 À y 2 , xy), as is widely accepted for Ni(PR 3 ) 3 complexes (Figure 1). 17 Indeed, stabilization of both the e 0 and e 00 orbital sets by the cylindrically symmetric CO π* orbitals serves to energetically isolate the Ni d z 2 orbital in Ni(CO) 3 (Figure 1) and renders the complex isolobal to σ-type Lewis bases such as phosphines and amines. In this respect, Ni(CO) 3 is also differentiated from the "planar" form of trisethylene Ni(0) (Ni(C 2 H 4 ) 3 ), 18 which possesses only the in-plane, e 0 -symmetry π-back-bonding interaction. However, the weaker π-acidity of ethylene relative to CO does not allow for an e 0 orbital Figure 1. (top) Qualitative molecular orbital diagrams for the D 3h -symmetric NiL 3 complexes Ni(C 2 H 4 ) 3 (left), Ni(PMe 3 ) 3 (center), and Ni(CO) 3 (right), reflecting the effect of π-back-donation on the d-orbital splitting pattern. (bottom) DFT-calculated HOMO for Ni(CO) 3 (left) and Ni(CN Me) 3ABSTRACT: Details are presented regarding a convenient synthesis of the nickel trisisocyanide complex Ni(CNAr Dipp2 ) 3 (Ar Dipp2 = 2,6-(2,6-[i-Pr] 2 C 6 H 3 ) 2 C 6 H 3 ). A previous synthesis of a Ni tris-isocyanide complex relied on a Tl(I) coordination-site protection strategy to discourage the formation of a tetrakis-isocyano complex. However, protectinggroup-free access to Ni(CNAr Dipp2 ) 3 is enabled by the encumbering m-terphenyl isocyanide CNAr Dipp2 . Treatment of Ni(COD) 2 with CNAr Dipp2 affords Ni(COD)-(CNAr Dipp2 ) 2 , which is readily oxidized to NiI 2 (CNAr Dipp2 ) 2 upon addition of I 2 . Reduction of NiI 2 (CNAr Dipp2 ) 2 with Mg metal generates Ni(CNAr Dipp2 ) 3 and does not require the addition of a third equivalent of C...

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