Square-planar Nickel Research Articles

Mixed Valence {Ni2+Ni1+} Clusters as Models of Acetyl Coenzyme A Synthase Intermediates.

Acetyl coenzyme A synthase (ACS) catalyzes the formation and deconstruction of the key biological metabolite, acetyl coenzyme A (acetyl-CoA). The active site of ACS features a {NiNi} cluster bridged to a [Fe4S4]n+ cubane known as the A-cluster. The mechanism by which the A-cluster functions is debated, with few model complexes able to replicate the oxidation states, coordination features, or reactivity proposed in the catalytic cycle. In this work, we isolate the first bimetallic models of two hypothesized intermediates on the paramagnetic pathway of the ACS function. The heteroligated {Ni2+Ni1+} cluster, [K(12-crown-4)2][1], effectively replicates the coordination number and oxidation state of the proposed "Ared" state of the A-cluster. Addition of carbon monoxide to [1]- allows for isolation of a dinuclear {Ni2+Ni1+(CO)} complex, [K(12-crown-2)n][2] (n = 1-2), which bears similarity to the "ANiFeC" enzyme intermediate. Structural and electronic properties of each cluster are elucidated by X-ray diffraction, nuclear magnetic resonance, cyclic voltammetry, and UV/vis and electron paramagnetic resonance spectroscopies, which are supplemented by density functional theory (DFT) calculations. Calculations indicate that the pseudo-T-shaped geometry of the three-coordinate nickel in [1]- is more stable than the Y-conformation by 22 kcal mol-1, and that binding of CO to Ni1+ is barrierless and exergonic by 6 kcal mol-1. UV/vis absorption spectroscopy on [2]- in conjunction with time-dependent DFT calculations indicates that the square-planar nickel site is involved in electron transfer to the CO π*-orbital. Further, we demonstrate that [2]- promotes thioester synthesis in a reaction analogous to the production of acetyl coenzyme A by ACS.

NiO nanoparticles by thermal decomposition of complex and evaluation of the structural, morphological, and optical properties

An efficient method for the synthesis of metal oxide nanoparticles (NPs) involves the solid-state thermal decomposition of precursor complexes. This study reports on the preparation of nanocrystalline nickel oxide (NiO) through a solid-state thermal decomposition of nickel(II) bis(N-methyl-N-paramethoxyphenyl dithiocarbamate) in a furnace. The novel Ni(II) complex was characterized using various analytical techniques. The single-crystal X-ray structure of the new nickel dithiocarbamate complex, C18H20N2NiO2S4, was determined and it showed a square-planar nickel coordination environment. The CH⋯S intramolecular hydrogen bond in this complex contributes to the planarity of the Nickel coordination environment. The complex underwent thermal decomposition in the air at about 750 °C for varying times of 90, 120, 150, and 180 min, resulting in the production of the oxide. Subsequently, the study explored how varying calcination times influenced the structural, morphological, and optical characteristics of the formed NiO nanoparticles. Face-centered cubic (FCC) structure of NiO NPs was confirmed whose crystallite size increased with an increase in the dwelling time of calcination and has been attributed to the possible coalescence of smaller crystallites to form larger ones. The confirmation of the face-centered cubic (FCC) structure of NiO nanoparticles was established, with the crystallite size observed to increase alongside longer calcination periods. This phenomenon has been linked to the potential amalgamation of smaller crystallites into larger ones. The optical energy bandgap decreased from 4.35 to 3.40 eV with an increase in the calcination time due to the increase in crystallite size occasioned by a longer reaction time from 90 to 180 min. These NPs will be useful as photocatalysts due to their high crystallinity and the capability to absorb within the visible region of the solar spectrum.

Interrelations between Linkage Isomers of an Efficient Square-planar Nickel(II) Nitrite Photoswitch in the Solid State.

An efficient nitrite nickel(II) photoswitch, with the 1-phenyl-2-hydroxyimino-3-[(2'-dimethylamino)ethyl]imino-1-propanone moiety used as the ancillary ligand, is reported. In the ground-state ('dark') crystal structure, the studied compound exists predominantly as the nitro-(η1 -N(O)2 ) isomer, however, traces of the exo- and endo-nitrito-(η1 -ONO) forms are detected both at 100 K (4-5 % each) and under ambient conditions (~9 % each). When excited with the 405-530 nm LED light, the nitro-to-nitrito isomerization takes place. The total conversion exceeds 90 %. The exo-nitrito linkage isomer constitutes the dominant photo-generated form, whereas the relative population of both nitrito species depends on temperature. The reaction is fully reversible and reproducible. The photo-products are stable up to 200 K. The system constitutes a good model case for the reaction mechanism studies. Thus, experimental and theoretical investigations on the photo-isomerism were conducted and are presented in detail. Eventually, the nitro→exo-nitrito→endo-nitrito reaction pathway is proposed.

Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes.

Square-planar NiII complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic PtII luminophores. Metal-to-ligand charge transfer (MLCT) excited states and their deactivation pathways are particularly relevant in these contexts. We sought to extend the lifetimes of 3MLCT states in square-planar NiII complexes by creating coordination environments that seemed particularly well adapted to the 3d8 valence electron configuration. Using a rigid tridentate chelate ligand, in which a central cyclometalated phenyl unit is flanked by two coordinating N-heterocyclic carbenes, along with a monodentate isocyanide ligand, a very strong ligand field is created. Bulky substituents at the isocyanide backbone furthermore protect the NiII center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into 1MLCT absorption bands and ultrafast intersystem crossing to the 3MLCT excited state, the latter relaxes onward into a metal-centered triplet state (3MC). A torsional motion of the tridentate ligand and a NiII-carbon bond elongation facilitate 3MLCT relaxation to the 3MC state. The 3MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar NiII complexes with enhanced photophysical properties. The longest 3MLCT lifetime reached in solution at room temperature is 48 ps, which is longer by a factor of 5-10 compared to previously investigated square-planar NiII complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.

Biosynthesis and Function of the Nickel‐Pincer Nucleotide Cofactor

Lactate racemase interconverts the L‐ and D‐isomers of lactic acid, a central metabolic intermediate in cells. In 2015, the Hausinger laboratory discovered that the enzyme responsible for this activity in Lactobacillus plantarum, LarA, possesses a covalently‐tethered coenzyme they named the nickel‐pincer nucleotide (NPN) cofactor. This novel organometallic molecule contains a square‐planar nickel atom bound by one histidine residue and tri‐coordinated by a modified pyridinium mononucleotide forming C‐Ni and two S‐Ni bonds. In this seminar, Dr. Hausinger will describe an array of structure/function studies that characterize the three enzymes used for biosynthesis of the NPN cofactor. LarB is a carboxylase/hydrolase of nicotinic acid adenine dinucleotide that forms a dicarboxylated pyridine mononucleotide (P2CMN). LarE is an ATP‐dependent sacrificial sulfur insertase that converts P2CMN into a species with two thiocarboxylic acids (P2TMN). Finally, LarC is a CTP‐dependent nickel insertase or cyclometallase that transforms P2TMN into NPN. The NPN cofactor is incorporated into a variety of 2‐hydroxyacid racemases and selected sugar epimerases where it catalyzes proton‐coupled hydride transfer reactions.

Synthesis, crystal structures, optical properties, DFT and TD-DFT studies of Ni (II) complexes with imine-based ligands

Two new square-planar nickel (II) complexes were synthesized from (2-((E)-(9-ethyl-9H-carbazol-7-ylimino)methyl)phenol) and (1-((E)-(9-ethyl-9H-carbazol-7-ylimino)methyl)naphthalen-2-ol) ligands, for complex I and complex II, respectively. They were characterized by IR, UV-Vis spectroscopy and X-ray diffraction. Further measurements were also conducted as cyclic voltammetry and conductivity measurements. Both complexes crystallize in the monoclinic system with the space groups P21/n and P21/c for complex I and complex II, respectively. The electronic absorption spectrum of complex I exhibits the lowest-lying absorption band (LEB) at 381 nm and the highest energy absorption band (HEB) at 269 nm. For complex II, the LEB was located at 431 nm while the HEB was found to be at 260 nm. Theoretical structure geometries, vibrational frequencies and electronic properties of the two complexes were calculated, by means of the density functional theory (DFT). The basis set 6-31g+(d) was chosen, for non-metal atoms (C, H, N and O) and the Stuttgart/Dresden effective core potentials basis set (SDD), for Ni atoms. UV-Vis transitions nature was investigated via a time-dependent density functional theory (TD-DFT) calculation. Overall, theoretical calculations were in accordance with the experimental results.

Crystal structure of [3,10-bis-(4-fluoro-pheneth-yl)-1,3,5,8,10,12-hexa-aza-cyclo-tetra-deca-ne]nickel(II) diperchlorate.

The square-planar nickel(II) title complex, [Ni(C24H36F2N6)](ClO4)2 or [NiL](ClO4)2 (L = 3,10-bis-(4-fluoro-pheneth-yl)-1,3,5,8,10,12-hexa-aza-cyclo-tetra-deca-ne) was synthesized by a one-pot reaction of template condensation and its X-ray crystal structure was determined. The nickel(II) ion lies close by a twofold axis and the complex displays whole-mol-ecule disorder. Ligand L, a hexa-aza-cyclo-tetra-decane ring having 4-fluoro-phenethyl side chains attached to uncoordinated nitro-gen atoms, adopts a trans III (R,R,S,S) configuration. The average Ni-N bond distance is 1.934 (9) Å, which is quite similar to those of other nickel(II) complexes with similar ligands. The nickel(II) ion is located 0.051 (7) Å above the least-squares plane through the four coordinated N atoms. The average C-N bond distance and C-N-C angle involving uncoordinated nitro-gen atoms are 1.425 (12) Å and 118.0 (9)°, respectively, indicating a significant contribution of sp 2 hybridization for these N atoms. The inter-molecular N-H⋯O, C-H⋯O/F hydrogen bonds of the complex form a network structure, which looks like a seamless floral lace pattern.

Photocrystallographic and spectroscopic studies of a model (N,N,O)-donor square-planar nickel(II) nitro complex: in search of high-conversion and stable photoswitchable materials.

A new, cheap, easy-to-synthesize and air-stable photoswitchable nickel(II) complex, QTNiNO2, is reported. The metal centre in QTNiNO2 is coordinated by a nitro group and a [2-methyl-8-amino-quinoline]-1-tetralone ligand. The compound crystallizes in the tetragonal space group I41/a with one complex molecule comprising the asymmetric unit, and the crystals are stable under ambient conditions. Irradiation of the solid-state form of QTNiNO2 with 530-660 nm LED light at 160 K converts the ambidentate nitro moiety fully to the nitrito linkage isomer which is stable up to around 230 K, as indicated by IR spectroscopy measurements. The structures of all species present in the examined crystals and their thermal stability were confirmed via X-ray multi-temperature and photocrystallographic experiments. The impact of temperature on the (photo)isomerization reaction taking place in a single crystal was additionally investigated. The experimental results are supported by computational analyses of crystal packing and intermolecular interactions that influence the isomerization process studied.

Dynamic Covalent Chemistry Approach toward 18-Membered P4N2 Macrocycles and Their Nickel(II) Complexes.

A dynamic covalent chemistry approach was used for the stereoselective synthesis of 1,10-diaza-3,8,12,17-tetraphosphacyclooctadecanes via condensation reaction of 1,4-bis(organylphosphino)butane, formaldehyde, and primary amines. The obtained 18-membered P4N2 macrocycles were isolated in pure form as meso- (RPSPSPRP) or rac- (RPRPRPRP/SPSPSPSP) isomers. The structural features of the individual stereoisomers were revealed by NMR spectroscopy and X-ray structure analysis. All P4N2 macrocycles form square-planar nickel(II) complexes with the RPSPSPRP isomer only, in which the orientation of the lone pairs of electrons at phosphorus favors this coordination mode, independent of the initial configuration of the ligand, indicating the ability of the 18-membered P4N2 macrocycles to stereoisomerize in the course of the complexation.

Heterometallic metallacyclophanes constructed from side-off bicompartmental ligands

Two metallacyclophanes have been assembled by self-assembly processes involving the trianionic compartmental ligand (L3-), nickel(II) and sodium or potassium ions: [LNiNa]2·C2H5OH·2H2O (1) and [LNiK]2·C2H5OH·2H2O (2) (the Schiff-base proligand, H3L, is obtained by condensation of o-vanillin and DL-2,3-diaminopropionic acid). The structures of 1 and 2 can be viewed as resulting from two enantiomeric fragments, {LNiNa/K} connected to each other by the two carboxylato groups. The intramolecular distance between the phenyl rings is 3.64 Å. The sodium/potassium ions are coordinated by five oxygens in a planar pentagon. Weak intermolecular sodium/potassium···CH interactions are observed in the packing diagrams of the two compounds. The intra- and intermolecular non-covalent interactions are analyzed through several methods based on electron density, Bader’s QTAIM, non-covalent interactions index and natural bond orbitals. They confirm the in-plane five coordination of the alkali ion, which is of an electrostatic, non-covalent nature and the intermolecular interaction with σ C-H orbitals as well. The diffuse reflectance spectra of the two compounds show characteristic features of square-planar nickel(II) chromophores.

Unconventional access to a solvatochromic nickel (II) dye featuring a coordination-induced spin crossover behavior

We report the synthesis of a square-planar nickel complex in which the metal center is located in a NOON environment. Promoting its axial ligation triggers a spin state switching process which is accompanied by a change of the absorption properties depending on the solvent. Indeed, a strong solvatochromic shifts could be observed in the presence of DMSO or pyridine, highlighting the key interplay between the donor strength of the solvent and the optical properties of the complex. The optical and magnetic properties of this compound in coordinating media have been carefully investigated through combined experimental/theoretical approaches.

New square-planar nickel(II)-triphenylphosphine complexes containing ONS donor ligands: Synthesis, characterization, electrochemical and antioxidant properties

Five new mixed ligand nickel(II) complexes with the general formula [Ni(L)(PPh3)], [C1–C5], were prepared from the reaction of triphenylphosphine (PPh3) with ONS donor ligands (L) [where R = Η (L1); 4-OCH3 (L2); 4-OCH2(CH2)6CH3 (L3); 4-OCH3, 4′-CH3 (L4) and 4-OCH2CHCH2 (L5)] derived from 2-hydroxybenzophenones and thiocarbohydrazide. The structures of the complexes were characterized by elemental analysis, IR, 1H and 31P NMR spectroscopy, conductivity measurements. The crystal structures of ligand (L2) and complex (C2) were determined by single crystal X-ray diffraction method. The ligand is coordinated to the nickel core through the phenolate oxygen, azomethine nitrogen and sulfur atoms, while the fourth coordination site is occupied by the P atom of a triphenylphosphine ligand. X-ray analysis indicated distorted square-planar structure for the complex C2. Electrochemical behaviors of thiocarbohydrazone ligands (L1-L5) and their nickel(II) complexes (C1–C5) were studied and effects of the type of substituents were compared. All ligands illustrated two irreversible reductions and one irreversible oxidation. Complexing of the ligands with Ni(III) ion considerably influenced their redox behaviors. Electron transfer process of the complexes get easier and became more reversible when compared with those of the corresponding ligands. Moreover, Ni(II) complexes illustrated NiII/NiI and NiII/NiIII processes in addition to the ligand-based electron transfer reactions. In-situ spectroelectrochemical responses of the ligands considerably changed after formation of Ni(II) complexes. Total antioxidant capacity and free radical scavenging activity of the compounds have been tested using CUPRAC and DPPH methods, respectively. The results of Ni(II) complexes were compared with a standard antioxidant compound.

Chirality Control in Crystalline Ni(II) Complexes of Thiophosphorylated Thioureas

Chirality control over the formation of Ni(II) complexes with chiral thiophosphorylated thioureas was achieved via breaking the symmetry of nickel coordination geometry by the introduction of the pyridine ligand, while centrosymmetric meso-complexes are formed from racemic ligands in case of square-planar nickel coordination. Centrosymmetric heterochiral arrangement is observed in crystals of ligands themselves through N–H⋅⋅⋅S hydrogen bonds in intermolecular dimers. Molecular homochirality in tetragonal pyramidal complexes is further transferred to supramolecular homochiral arrangement via key–lock steric interactions.

Syntheses and structural characterization of new trans-bis-[n-butyl-(p-methoxyphenyl) dithiophosphinato] nickel(II), the pyridine derivative thereof, and bis-{bis-[n-butyl-(p-methoxyphenyl) dithiophosphinato]cobalt(II)} complexes

Abstract Three new metal complexes of ammonium n-butyl-(p-methoxyphenyl) dithiophosphinate (NH4L) were synthesized. NH4L was treated with NiCl2·6H2O and CoCl2·6H2O in ethanol to obtain a new trans-bis-[n-butyl-(p-methoxyphenyl) dithiophosphinato]nickel(II) and bis-{bis-[n-butyl-(p-methoxyphenyl) dithiophosphinato]cobalt(II)} complexes, ([Ni(L)2] and [Co2(μ-L)2(L)2], respectively). Square-planar nickel(II) complex was furthermore reacted with excess pyridine (Py) and the octahedral {bis-pyridine-bis-[n-butyl-(p-methoxyphenyl)]}nikel(II) complex, ([Ni(L)2(Py)2]), was obtained. The structures of the complexes were analyzed by elemental analysis, 1H-, 13C-, 31P- NMR; and also by MS-, FTIR- and Raman spectroscopies. The magnetic susceptibilities of the Co(II) and the pyridine-Ni(II) complexes were measured to throw further light on the hybridization patterns. The solid-state structures of the nickel complexes were also done by X-ray crystallography. The P–S bonds in the Ni(II) complexes are essentially of the same lengths; indicating an effective electron delocalization around the central PS2 moiety.

Chameleon-like Nature of Anagostic Interactions and Its Impact on Metalloaromaticity in Square-Planar Nickel Complexes

Anagostic C–H···M (M = a metal center) intramolecular interactions, one of the most fundamental and elusive forces in organometallic chemistry, are intuitively considered as repulsive and purely el...

Tuning Semiconductor Performance of Nickel Complexes through Crystal Transformation.

Crystal transformation between two polymorphs (green, 1-G, and red, 1-R) of the square-planar nickel complex NiL2 (L = 2-ethoxy-6-( N-methyliminomethyl)phenolate) and their tuning effect to semiconductor properties were studied both experimentally and theoretically. When 1-G is heated to 413 K, it converts to 1-R, whereas soaking 1-R in several kinds of solvents causes it to revert to 1-G. Crystallographic and PXRD studies reveal the dramatic changes in crystal dimensions due to the changes of packing models. Heating device made from 1-G (D-1-G(298)) at 413 K significantly increases the electrical conductivity from 6.55 × 10-4 S cm-1 for D-1-G(298) to 1.11 × 10-3 S cm-1 for D-1-G(413), showing significant crystal form dependence. Heat-treating D-1-G and D-1-R devices at different temperatures clearly reveals the reason for the conductivity tuning. Thus, the conductivity of NiL2-based devices could be well tuned through crystal transformation by heating or by soaking in solvent. Theoretical calculations clearly revealed the reason for such conductivity changes and also predicted that both polymorphs are good p-type semiconductors with hole mobilities of 1.63 × 10-2 (1-G) and 2.11 × 10-1 cm2 V-1 s-1 (1-R).

Hydrogen-Atom Noninnocence of a Tridentate [SNS] Pincer Ligand.

Double deprotonation of bis(2-mercapto-4-methylphenyl)amine ([SNS]H3) followed by addition to NiCl2(PR3)2 in air-free conditions afforded [SN(H)S]Ni(PR3) (1a, R = Cy; 1b, R = Ph) complexes, characterized as diamagnetic, square-planar nickel(II) complexes. When the same reaction was conducted with 3 equiv of KH, the diamagnetic anions K{[SNS]Ni(PR3)} were obtained (K[2a], R = Cy; K[2b], R = Ph). In the presence of air, the reaction proceeds with a concomitant one-electron oxidation. When R = Cy, a square-planar, S = 1/2 complex, [SNS]Ni(PCy3) (3a), was isolated. When R = Ph, the bimetallic complex {[SNS]Ni(PPh3)}2 ({3b}2) was obtained. This bimetallic species is diamagnetic; however, in solution it dissociates to give S = 1/2 monomers analogous to 3a. Complexes 1-3 represent a hydrogen-atom-transfer series. The bond dissociation free energies (BDFEs) for 1a and 1b were calculated to be 63.9 ± 0.1 and 62.4 ± 0.2 kcal mol-1, respectively, using the corresponding p Ka and E°' values. Consistent with these BDFE values, TEMPO• reacted with 1a and 1b, resulting in the abstraction of a hydrogen atom to afford 3a and 3b, respectively.

Variation of M···H-C Interactions in Square-Planar Complexes of Nickel(II), Palladium(II), and Platinum(II) Probed by Luminescence Spectroscopy and X-ray Diffraction at Variable Pressure.

Luminescence spectra of isoelectronic square-planar d8 complexes with 3d, 4d, and 5d metal centers show d-d luminescence with an energetic order different from that of the spectrochemical series, indicating that additional structural effects, such as different ligand-metal-ligand angles, are important factors. Variable-pressure luminescence spectra of square-planar nickel(II), palladium(II), and platinum(II) complexes with dimethyldithiocarbamate ({CH3}2DTC) ligands and their deuterated analogues show unexpected variations of the shifts of their maxima. High-resolution crystal structures and crystal structures at variable pressure for [Pt{(CH3)2DTC}2] indicate that intermolecular M···H-C interactions are at the origin of these different shifts.

Self-organized Tetranickel(II) Molecular Square with Imidazolinyl-imidazole Bridging Ligand

A novel square-planar cyclic tetranuclear nickel(II) complex coordinated by im-H2bizn− ligand (Him-H2bizn = 4,5-bis(4,5-dihydro-1H-imidazol-2-yl)imidazole), [NiII4(im-H2bizn)4(MeOH)4(py)4](ClO4)4·2ether (py = pyridine), has been synthesized. The tetranickel(II) complex crystallized in C2/m space group and exhibited antiferromagnetic intramolecular coupling with J = −24 K. The complex arrests ClO4− ion by moderate O–H⋯O and weak multipoint C–H⋯O hydrogen bonds.

Selective Transformation of CO2 to CO at a Single Nickel Center.

Carbon dioxide conversion mediated by transition metal complexes continues to attract much attention because of its future potential utilization as a nontoxic and inexpensive C1 source for the chemical industry. Given the presence of nickel in natural systems that allow for extremely efficient catalysis, albeit in an Fe cluster arrangement, studies that focus on selective CO2 conversion with synthetic nickel species are currently of considerable interest in our group. In this Account, the selective conversion of CO2 to carbon monoxide occurring at a single nickel center is discussed. The chemistry is based on a series of related nickel pincer complexes with attention to the uniqueness of the coordination geometry, which is crucial in allowing for particular reactivity toward CO2. Our research is inspired by the efficient enzymatic CO2 catalysis occurring at the active site of carbon monoxide dehydrogenase. Since the binding and reactivity toward CO2 are controlled in part by the geometry of a L3Ni scaffold, we have explored the chemistry of low-valent nickel supported by PPMeP and PNP ligands, in which a pseudotetrahedral or square-planar geometry is accommodated. Two isolated nickel-CO2 adducts, (PPMeP)Ni(η2-CO2-κ C) (2) and {Na(12-C-4)2}{(PNP)Ni(η1-CO2-κ C)} (7), clearly demonstrate that the geometry of the nickel ion is crucial in the binding of CO2 and its level of activation. In the case of a square-planar nickel center supported by a PNP ligand, a series of bimetallic metallacarboxylate Ni-μ-CO2-κ C, O-M species (M = H, Na, Ni, Fe) were synthesized, and their structural features and reactivity were studied. Protonation cleaves the C-O bond, resulting in the formation of a nickel(II) monocarbonyl complex. By sequential reduction, the corresponding mono- and zero-valent Ni-CO species were produced. The reactivities of three nickel carbonyl species toward various iodoalkanes and CO2 were explored to address whether their corresponding reactivities could be controlled by the number of valence d electrons. In particular, a (PNP)Ni(0)-CO species (13) shows immediate reactivity toward CO2 but displays multiple product formation. By incorporation of a -CMe2- bridging unit, a structurally rigidified acriPNP ligand was newly designed and produced. This ligand modification was successful in preparing the T-shaped nickel(I) metalloradical species 9 exhibiting open-shell reactivity due to the sterically exposed nickel center possessing a half-filled d x2- y2 orbital. More importantly, the selective addition of CO2 to a nickel(0)-CO species was enabled to afford a nickel(II)-carboxylate species (22) with the expulsion of CO(g). Finally, the (acriPNP)Ni system provides a synthetic cycle in the study of the selective conversion of CO2 to CO that involves two-electron reduction of Ni-CO followed by the direct addition of CO2 to release the coordinated CO ligand.