As nickel carbenes are generally nucleophilic, we also investigated the protonation of compound and). In contrast, the reaction of 2 with one equivalent of HCl (diethyl ether solution, Scheme 1, C) in d8-THF at room temperature led to the rapid formation of a stoichiometric amount of 6 together with a mixture of paramagnetic complex (PPP p-tol )Ni(Cl) (8) and diamagnetic complex (PPP p- tol )NiCl2 (9). EPR analysis confirmed the presence of complex 8, and 31 P{ 1 H} NMR analysis confirmed the presence of complex 9 (Figures and). To account for these observations, we propose the initial formation of a Ni II (alkyl) complex (either 5 or 7), which undergoes homolytic cleavage of the nickel alkyl ligand bond (Scheme 1B and 1C) to form a Ni I center and a stabilized alkyl radical that ultimately dimerizes to form 6. This contrasts with the reactivity of the dtbpe analogue, for which the Ni II (alkyl) complex resulting from protonation is stable and can be isolated. DFT calculations provide more insight into the nature of the nickel-carbene interaction in compound 2. First, the geometry of a slightly truncated model of 2 bearing phenyl instead of ptolyl and p-anisyl substituents is optimized at M06L-D3/6-31g(d,p) (Figure ). The nickel carbene 2 presents a distorted tetrahedral geometry where the calculated Ni-C bond length is somewhat longer (1.854 Å, Table ) than the analogous bond of the optimized (dtbpe)NiCPh2 (1.805 Å, Figure ) and the corresponding X-ray crystal structure (1.836 (2) Å). A conformational search reveals alternative geometries, involving the coordination of only two of the three phosphine moieties of the ligand, which are significantly less stable (Figure ). To get more insight into the chemical bonding of (PPP Ph )NiCPh2, Quantum Theory of Atoms in Molecules (QTAIM) and Energy Decomposition Analyses (EDA) within the framework of quantitative Kohn-Sham molecular orbital theory are performed. QTAIM is a topological analysis of the electron density that can provide information about the nature of the Ni-carbene bond as donor-acceptor interaction (Fischer carbenes, prone to nucleophilic attack at carbon) or covalent delocalized bonds (Schrock carbenes, carbon nucleophiles). Analysis of the Laplacian map of the π-plane at the carbene carbon (perpendicular to the CPh2 plane) of (PPP Ph )NiCPh2 shows an area of charge depletion in the direction of the pπ orbital of the carbon atom of the carbene. This indicates that the electrons of the carbene fragment are primarily located in the sp 2 -hybridized orbital, engaging in a donoracceptor interaction with nickel, and the pπ-like orbital is depleted (Figure ). Notably, the same feature is also observed for (dtpbe)NiCPh2 (Figure ). In contrast, analysis of the Laplacian map of the π-plane of Piers' carbene pincer complex (PCcarbeneP)NiPPh3 shows a charge concentration around the carbon atom showing a delocalized Ni-C bonding reminiscent of Schrock carbenes (Figure ). These results suggest that, even though Ni carbenes exhibit nucleophilic reactivity (see above), their electronic structure can resemble that of a Fisher carbene. Similarly, Grubbs' Ru-carbene catalysts have been shown to present characteristics of both Fischer and Schrock carbenes. The EDA is used to analyze and compare the bonding mechanism between the Ligand-Ni and the CPh2 fragments in the Nicarbene complexes (dtpbe)NiCPh2 and (PPP Ph )NiCPh2 (Table ). In this method, the interaction energy (DEint) is decomposed into four physically meaningful terms: the classical electrostatic interaction (DVelstat), the steric (Pauli) repulsion (DEPauli) arising from the repulsion between occupied closedshell orbitals of both fragments, the orbital interaction (DEoi) that accounts for donor-acceptor interaction between the fragments and polarization within a fragment, and the dispersion energy (DEdisp). The EDA results reveal that the Ni-carbene bond is for our newly synthesized (PPP Ph )NiCPh2 less stabilizing than for (dtpbe)NiCPh2, that is, the interaction energy is -83.9 kcal mol -1 for (PPP Ph )NiCPh2 and -98.6 kcal mol -1 for (dtpbe)NiCPh2. This difference in interaction energy originates from (i) a more destabilizing steric (Pauli) repulsion; and (ii) less stabilizing orbital interactions for (PPP Ph )NiCPh2 compared to (dtpbe)NiCPh2. The former is a direct consequence of the increased steric repulsion between the phenyl groups of (PPP Ph )Ni and CPh2, which are, due to their spatial orientation, in closer proximity than the tert-butyl groups of (dtpbe)Ni and CPh2. This, in fact, not only amplifies the steric (Pauli) repulsion but also forces the Ni-carbene bond length to be longer for (PPP Ph )NiCPh2 (1.854 Å) compared to (dtpbe)NiCPh2 (1.805 Å). The reduction of stabilizing orbital interactions from -112.3 kcal mol -1 for (dtpbe)NiCPh2 to -94.2 kcal mol -1 for (PPP Ph )NiCPh2 can be directly understood by analyzing the molecular orbitals responsible for the interaction between the Ligand-Ni and the CPh2 fragments in the Ni-carbene complexes, that is, (i) s-bonding between the occupied sp 2 -hybridized orbital of CPh2 (HOMOCPh2) and the unoccupied a1-like orbital of Ligand-Ni (LUMOL-Ni); (ii) π-backbonding between the unoccupied pπ-like orbital of CPh2 (LUMOCPh2) and the occupied b2like orbital of Ligand-Ni (HOMOL-Ni) (Figure ). As proposed by Hillhouse and coworkers, 8 the orbitals of the L2Ni fragment are a perfect match for both s-bonding and π-backbonding interactions with the CPh2 fragment (Figure ). The perpendicular conformation of the carbene fragment in (dtpbe)NiCPh2 stand to maximize the π-backbonding interaction. In the case of (PPP Ph )NiCPh2, however, both the HOMO and LUMO, engaging in the s-bonding and π-backbonding interactions with the CPh2, are delocalized over the pincer ligand, which supresses stabilizing orbital interactions. Kohn-Sham molecular orbital analysis quantifies this and demonstrates that both the s-bonding and π-backbonding orbital interaction weaken from (PPP Ph )NiCPh2 to (dtpbe)NiCPh2, due to a loss of build-up in stabilizing orbital overlap as a result of both a smaller HOMOL-Ni and LUMOL-Ni lobe at the external face of (PPP Ph )Ni and a longer Ni-carbene bond length (Figure ). This further explains the facile reactivity of (PPP Ph )NiCPh2 with small molecules like CO. This weakening is a direct result of the increased steric repulsion between the phosphine ligands and CPh2, which are in closer proximity, as also reflected by the increase in steric (Pauli) repulsion. Introducing the apical phosphine ligand, hence having a complex that resembles (PPP Ph )NiCPh2, leads to a remarkable stabilization of the interaction energy from -62.1 kcal mol -1 for B to -71.8 kcal mol -1 for C. This stabilization can be traced back to the p-accepting character of the apical phosphine ligand, which, upon binding, pulls occupied orbital density away from the incoming CPh2 fragment, thereby lowering the destabilizing steric (Pauli) repulsion between the simplified (PPP Ph )Ni and CPh2 fragments (Figure ). This effect compensates the observed weakening of the stabilizing electrostatic and orbital interactions, corresponding to going from complex B to complex C. In conclusion, we report the synthesis and characterization of a stable tetracoordinated nickel carbene (PPP p-Tol )NiC(p-anisyl)2. The complex is reactive towards small molecules such as CO and CO2, and its nucleophilic character is evidenced by its reactivity towards acids. Computational methods support the tetracoordinated geometry of (PPP Ph )NiCPh2 as the most stable structure, with the Ni-C bond best viewed as a donoracceptor interaction. EDA provides a view of the interaction between (PPP Ph )Ni and CPh2 in this complex. This analysis shows that the key s-bonding and π-backbonding interactions are possible for (PPP Ph )NiCPh2 but somewhat weaker than for its 3-coordinate analogues. Interestingly, EDA also reveals that the third, apical phosphine ligand provides stabilization to the tetracoordinated nickel carbene by polarizing occupied orbital density away from its external face and thereby reducing the steric (Pauli) repulsion between (PPP Ph )Ni and CPh2.