Nitrosyl Ligand Research Articles

Bis[1,3-bis(2,4,6-trimethylphenyl)-2,3-dihydro-1H-imidazol-2-ylidene]dichloridodinitrosyltungsten(II) tetrahydrofuran-d8monosolvate

The mol­ecular structure of the title compound, [WCl2(NO)2(C21H24N2)2]·C4D8O, displays a distorted octa­hedral arrangement around the W atom with two trans 1,3-bis­(2,4,6-trimethyl­phen­yl)imidazol-2-yl­idene (IMes) carbene ligands in axial positions. The four equatorial positions are occupied by nitrosyl and chloride ligands, which are trans to each other. The Ccarbene—W—Ccarbene bond angle of 173.44 (18)° and the Cl—W—Nnitros­yl bond angles of 171.34 (11) and 171.32 (13)° deviate only slightly from linearity. The distortion comes from the nitrosyl and chloride ligands which are not fully coplanar since the two N atoms deviate from the WCl2 plane by −0.279 (4) and 0.272 (4) Å, respectively. An inter­molecular C—H⋯O inter­action connects the organometallic mol­ecule and the tetra­hydro­furan-d 8 solvent mol­ecule.

Nitrosyl derivatives of the unsaturated dihydrides [Mn 2(μ-H) 2(CO) 6(μ-L 2)] (L 2 = Ph 2PCH 2PPh 2, (EtO) 2POP(OEt) 2)

The title dimanganese complexes react with NO (5% in N 2) at room temperature to give as major products the corresponding hexanitrosyl derivatives [Mn 2(NO) 6(μ-L 2)] in moderate yields, and they react rapidly with NO 2 to give the corresponding hydride derivatives [Mn 2(μ-H)(μ-NO 2)(CO) 6(μ-L 2)], these having a nitrite ligand bridging the dimetal centre through the N and O atoms. The dppm-bridged dihydride also reacts selectively at 273 K with (PPN)NO 2 to give first the nitro derivative (PPN)[Mn 2(μ-H)(H)(NO 2)(CO) 6(μ-dppm)], which then transforms into the nitrosyl complex (PPN)[Mn 2(μ-CO)(CO) 5(NO)(μ-dppm)] at room temperature or above (dppm = Ph 2PCH 2PPh 2; PPN + = [N(PPh 3) 2] +). The latter anion reacts with (NH 4)PF 6 to give the hydride-bridged nitrosyl complex [Mn 2(μ-H)(μ-NO)(CO) 6(μ-dppm)] and with [AuCl(PPh 3)] to give the trinuclear cluster [AuMn 2(μ-NO)(CO) 6(μ-dppm)(PPh 3)] (Mn–Au = ca. 2.68 Å; Mn–Mn = 2.879(2) Å). Both products are derived from the addition of the added electrophile at the intermetallic bond and rearrangement of the nitrosyl ligand into a bridging position. In contrast, methylation of the anion with CF 3SO 3Me takes place at the nitrosyl ligand to yield the unstable methoxylimide derivative [Mn 2(μ-NOMe)(CO) 6(μ-dppm)]. Analogous reactions at the nitrosyl ligand take place upon the addition of HBF 4·OEt 2 to the nitrosyl-bridged hydrides [Mn 2(μ-H)(μ-NO)(CO) n(μ-dppm) m] (n = 6, m = 1; n = 4, m = 2) to give the corresponding hydroxylimide derivatives [Mn 2(μ-H)(μ-NOH)(CO) n(μ-dppm) m]BF 4, which were also thermally unstable and could not be isolated nor fully characterized.

ChemInform Abstract: Synthesis and Crystal Structure of the Six-Coordinate Ruthenium Nitrosyl Complex ((η5-C5H5)Ru(PPh3)(NO)(Cl))PF6.

Abstract Reaction of [(η 5 -C 5 H 5 )Ru(PPh 3 ) 2 Cl] ( 1 ) with sodium nitrite in hot acidified ehtanol followed by anion exchange gave the red crystalline solid [(η 5 -C 5 H 5 )Ru(PPh 3 (NO)(Cl]PF 6 ( 2 ) in good yield. Compound 2 has been crystallographically characterised and shows a large interligand angle at ruthenium between the chloride and nitrosyl ligands. The dication {[(η 5 -C 5 H 5 )Ru(PPh 3 )(CN t Bu)(NO)][BF 4 ] 2 } ( 4 ) has been prepared from [(η 5 -C 5 H 5 )Ru(PPh 3 )(CN t Bu)(Cl)] ( 3 ) and nitrosinium tetrafluoroborate.

Trigonal Bipyramidal {Fe(NO)}7 Complex [(NO)Fe(SC9H6N)2] Containing an Equatorial Nitrosyl Ligand: The Critical Role of Chelating Ligands in Regulating the Geometry and Transformation of Mononitrosyl Iron Complex (MNIC)

AbstractTrigonal bipyramidal {Fe(NO)}7 complex [(NO)Fe(SC9H6N)2] (2) with NO ligand occupying equatorial position, obtained from nitrosylation of complex [(Cl)Fe(SC9H6N)2]− (1) accompanied by elimination of [Cl]−, was characterized by IR, UV‐Vis, EPR, SQUID and single‐crystal X‐ray diffraction. Compared to the S = 3/2 ground state displayed by the trigonal bipyramidal complex [Et4N][(ON)Fe(N‐(CH2CH2S)3)], the chelating ligands ([SC9H6N]−) play a key role in modulating the position of exogenous NO ligand and controlling the spin state of complex 2 (S = 1/2). In contrast to nitrosylation of [Fe(NO)(SPh)3]− MNIC yielding [Fe(NO)2(SPh)2]− DNIC, the inertness of complex 2 toward NO(g) was observed. Reaction of [NO][BF4] and complex 2 in a 1:1 stoichiometry led to the formation of homodinuclear {Fe(NO)}7‐{Fe(NO)}9 complex [(ON)Fe(‐SC9H6N)2Fe(NO)2][BF4] (3) characterized by IR, UV‐vis, 1H NMR and single‐crystal X‐ray diffraction. Antiferromagnetic coupling between the S = 1/2 {Fe(NO)}7 [S2Fe(NO)2] core and the S = 1/2 {Fe(NO)}7 [(NO)FeS2N2] core may account for the diamagnetism and the EPR silence of complex 3. This study provides the evidence that the geometric and electronic structures of {Fe(NO)}7 MNICs modulated by the coordination environment of iron center determines the nitrosylation products.

Electronic structure and dynamics of nitrosyl porphyrins.

Nitric oxide (NO) is a signaling molecule employed to regulate essential physiological processes. Thus, there is great interest in understanding the interaction of NO with heme, which is found at the active site of many proteins that recognize NO, as well as those involved in its creation and elimination. We summarize what we have learned from investigations of the structure, vibrational properties, and conformational dynamics of NO complexes with ferrous porphyrins, as well as computational investigations in support of these experimental studies. Multitemperature crystallographic data reveal variations in the orientational disorder of the nitrosyl ligand. In some cases, equilibria among NO orientations can be analyzed using the van't Hoff relationship and the free energy and enthalpy of the solid-state transitions evaluated experimentally. Density functional theory (DFT) calculations predict that intrinsic barriers to torsional rotation are smaller than thermal energies at physiological temperatures, and the coincidence of observed NO orientations with minima in molecular mechanics potentials indicates that nonbonded interactions with other chemical groups control the conformational freedom of the bound NO. In favorable cases, reduced disorder at low temperatures exposes subtle structural features including off-axis tilting of the Fe-NO bond and anisotropy of the equatorial Fe-N bonds. We also present the results of nuclear resonance vibrational spectroscopy measurements on oriented single crystals of [Fe(TPP)(NO)] and [Fe(TPP)(1-MeIm)(NO)]. These describe the anisotropic vibrational motion of iron in five- and six-coordinate heme-NO complexes and reveal vibrations of all Fe-ligand bonds as well as low-frequency molecular distortions associated with the doming of the heme upon ligand binding. A quantitative comparison with predicted frequencies, amplitudes, and directions facilitates identification of the vibrational modes but also suggests that commonly used DFT functionals are not fully successful at capturing the trans interaction between the axial NO and imidazole ligands. This supports previous conclusions that heme-NO complexes exhibit an unusual degree of variability with respect to the computational method, and we speculate that this variability hints at a genuine electronic instability that a protein can exploit to tune its reactivity. We anticipate that ongoing characterization of heme-NO complexes will deepen our understanding of their structure, dynamics, and reactivity.

Infrared Spectroelectrochemical Reduction of Iron Porphyrin Complexes

The spectroelectrochemistry of iron porphinones and their nitrosyl complexes were examined by infrared spectroscopy, as well as ferrous octaethylporphyrin nitrosyl. With the use of d(8)-THF, the solvent was transparent down to 1200 cm(-1). For the porphinones, the reduction of the macrocycle ring could be observed by the changes in the nu(CO) band and, for the nitrosyl complex, the changes in the nitrosyl ligand were directly observable from the nu(NO) band. Formation of the ferrous complexes led to a small downshift in the nu(CO) band. Further reduction to the formal Fe("I") complex led to more complex spectra which were interpreted with the help of density functional theory (DFT) calculations. The reduction of Fe(OEP)(NO) and its porphinone analogues was also examined. The reduction of the iron porphyrin and porphinone nitrosyl complexes lead to substantial decreases in the nu(NO) band from 1665 to 1670 cm(-1) to 1442-3 cm(-1). The energy of the nu(NO) band in the reduced complex was unaffected by the presence of carbonyl groups on the porphinone ring, indicating little additional delocalization of the electron density of the Fe-NO moiety because of the carbonyl groups. The identity of the nu(NO) bands was confirmed with (15)N substitution of the Fe(OEP)(NO) complex. The nu(CO) band on the porphinone ring was found to be sensitive to the degree than electron density was delocalized to the ring.

Hydroperoxide-Initiated Intramolecular Insertions of NO into Metal−Carbon Bonds

The chemical transformations of metal-bound NO groups (i.e., nitrosyl ligands) have attracted considerable attention because of both their fundamental significance and their possible biological and environmental relevance. We now report our novel discovery that an oxygen-atom transfer reagent can initiate the subsequent intramolecular insertion of bound NO into an adjacent metal−ligand bond. Specifically, we have demonstrated that treatment of the dialkyl compounds Cp*Mo(NO)(CH2CMe3)2 (1) and Cp*W(NO)(CH2SiMe3)2 (3) (Cp* = η5-C5Me5) with cumene hydroperoxide induces the insertion of NO into one of their metal−carbon bonds and results in the clean formation of the η2-nitrosoalkane oxo complexes Cp*Mo(O)(η2-ONCH2CMe3)(CH2CMe3) (2) and Cp*W(O)(η2-ONCH2SiMe3)(CH2SiMe3) (4). Further treatment of complexes 2 and 4 with excess O2 results in liberation of the nitroalkanes O2NCH2EMe3 (E = C, Si) from the metals’ coordination spheres and formation of the dioxo alkyl complexes Cp*Mo(O)2(CH2CMe3) (5) and Cp*W(O)2(CH2...

Dynamics of NO motion in solid-state [Co(tetraphenylporphinato)(NO)

The temperature dependence of the crystalline phase of (nitrosyl)(tetraphenylporphinato)cobalt(II), [Co(TPP)(NO)], has been explored over the temperature range of 100-250 K by X-ray diffraction experiments. The crystalline complex is found in the tetragonal crystal system at higher temperatures and in the triclinic crystal system at lower temperatures. In the tetragonal system, the axial ligand is strongly disordered, with the molecule having crystallographically required 4/m symmetry, leading to eight distinct positions of the single nitrosyl oxygen atom. The phase transition to the triclinic crystal system leads to a partial ordering with the molecule now having inversion symmetry and disorder of the axial nitrosyl ligand over only two positions. At an intermediate temperature near the transition point, a transition structure in which the ordering observed at lower temperatures is only partially complete has been characterized. The increase in ordering allows subtle molecular geometry features to be observed. The transition of the reversible phase change begins at about 195 K. This transition has been confirmed by both X-ray diffraction studies and a differential scanning calorimetry study.

Probing Vibrational Anisotropy with Nuclear Resonance Vibrational Spectroscopy

A NRVS single-crystal study (NRVS=nuclear resonance vibrational spectroscopy) has provided detailed information on the in-plane modes of nitrosyl iron porphyrinate [Fe(oep)(NO)] (see picture; oep=octaethylporphyrin). The axial nitrosyl ligand controls the direction of the in-plane iron motion.

A New Hexanuclear Iron−Selenium Nitrosyl Cluster: Primary Exploration of the Preparation Methods, Structure, and Spectroscopic and Electrochemical Properties

A new hexanuclear iron-selenium nitrosyl cluster, [(n-Bu)(4)N](2)[Fe(6)Se(6)(NO)(6)] (1), and a hexanuclear iron-sulfur nitrosyl cluster, [(n-Bu)(4)N](2)[Fe(6)S(6)(NO)(6)] (2), were synthesized by the solvent-thermal reactions of [(n-Bu)(4)N][Fe(CO)(3)NO] with selenium or sulfur in methanol, while a tetranuclear iron-sulfur nitrosyl cluster, (Me(4)N)[Fe(4)S(3)(NO)(7)] (3), was also prepared by the solvent-thermal reaction of FeCl(2).4H(2)O with thiourea in the presence of (CH(3))(4)NCl, NaNO(2), and methanol. Complexes 1-3 were characterized by IR, UV-vis, (1)H NMR, electrochemistry, and single-crystal X-ray diffraction analysis. IR spectra of complexes 1 and 2 show the characteristic NO stretching frequencies at 1694 and 1698 cm(-1), respectively, while the absorptions of complex 3 appear at 1799, 1744, and 1710 cm(-1). The UV-vis spectra of complexes 1-3 show different bands in the range of 259-562 nm, which are assigned to the transitions between orbitals delocalized over the Fe-S cluster, the ligand-to-metal charge transfer, pi*(NO)-d(Fe), and the metal-to-ligand charge transfer, d(Fe)-pi*(NO). Single-crystal X-ray structural analysis reveals that complex 1 crystallizes in the monoclinic P2(1)/n space group with two molecules per unit cell. Two parallel "chair-shaped" structures, consisting of three iron and three selenium atoms, are connected by Fe-Se bonds with an average distance of 2.341 A; each iron center is bonded to three selenium atoms and a nitrogen atom from the nitrosyl ligand with a pseudotetrahedral center geometry. Cyclic voltammograms of complexes 1 and 2 display two cathodic and three anodic current peaks with an unusually strong cathodic peak. Further electrochemical investigations demonstrated that the intensity of the unusually strong peak is a result of at least three processes. One is the quasi-reversible reduction, and the other two are from an irreversible electrochemical process, in which the compound goes through a typical electron transfer and chemical reaction mechanism. Compound 3 shows three quasi-reversible reductions.

Preparation and structure of [Ru II/III2(O 2CMe) 4] 2[Fe(CN) 5NO] and magnetically ordered H x[Ru II/III2(O 2CMe) 4] 3−x[Cr(CN) 5NO] possessing interpenetrating lattices

Reaction of [Ru 2(O 2CMe) 4]Cl with K 3[Cr(CN) 5NO] in water forms H x [Ru II/III 2(O 2CMe) 4] 3− x -[Cr(CN) 5NO]· zH 2O ( x = 0.2) that magnetically orders at 4.0 K and possesses an interpenetrating body centered cubic [ a = 13.2509(2) Å] structure with random locations of the bridging nitrosyl ligands, and x/3 vacant cation sites. Similarly, the aqueous reaction of [Ru 2(O 2CMe) 4]Cl with Na 2[Fe(CN) 5NO] forms paramagnetic [Ru 2(O 2CMe) 4] 2[Fe(CN) 5NO]·H 2O, which has a similar tetragonal interpenetrating structure [ a = 13.0186(1) Å, c = 13.0699(2) Å] where the NO ligands are presumably nonbridging and 1/3 of the expected cation sites are unoccupied. The presence of uncoordinated NO sites in addition to missing neighboring [Ru 2(O 2CMe) 4] + units, results in significant vacancies (or holes) in the lattice.

Nitrosyltris(pyridine-2-thiolato-κ2N,S)molybdenum(II) dihydrate

In the title compound, [Mo(C5H4NS)3(NO)]·2H2O, the Mo atom is coordinated by a nitrosyl ligand and three monoanionic N,S-bidentate ligands in a distorted MoN4S3 penta­gonal-bipyramidal mol­ecular geometry. The pyridine N atom of one pyridine-2-thiol­ate (pyt) ligand is coordinated to the Mo atom in the trans position relative to the NO ligand [N(pyt)—Mo—N(NO) = 170.62 (19)°]. The compound has C s symmetry, with a mirror plane that includes one pyt ring and the NO group. The S—Mo—N(NO) and N(pyt)—Mo—N(NO) angles [97.24 (12) and 91.87 (8)°, respectively] are large relative to the ideal angles of 90°. In the crystal, the mol­ecules pack in a zigzag arrangement. The cavities between the mol­ecules are occupied by disordered water mol­ecules of crystallization.

Binding of O2, NO, and CO to model active sites in ferrous heme proteins: Ligand geometry, electronic structure, and quadrupole splittings

The electronic structures of model oxy-, nitrosyl-, and carbon monoxy ferroporphyrin compounds with N-methylimidazole as an axial ligand are calculated for a number of different conformations of the O2, NO, and CO ligands using the iterative extended Huckel method. For the dioxygen ligand a bent end-on (Pauling) geometry with a low-energy off-axis displacement is favored. The “anomalously large” electric field gradient at the iron nucleus can be accounted for by a paired iron(II)-dioxygen configuration. The carbon monoxide ligand is linear in model compounds but bent or tilted in protein. The reduced affinity observed for the intact protein is explained by these results. The nitrosyl ligand is bent both in model compounds and in the protein. The calculated electronic structure is consistent with the spin density observed in hyperfine splitting in electron spin resonance spectra and gives a quadrupole splitting in excellent agreement with experiment.

Substitution of chloride by nitrosyl ligand in a scorpionate ruthenium(III) compound: A theoretical study

A theoretical study of the ruthenium(III) complex [RuCl 2(pz 2CHSO 3)(en)] and of its nitrosyl-substituted product [Ru(NO)Cl(pz 2CHSO 3)(en)] + is presented, based on density functional calculations. Several isomers of each compound differing in the position of the anionic tail of a bis(3,4-dimethyl-1-yl)methanesulfonate scorpionate ligand, pz 2CHSO 3 −, relative to the monodentate ligands have been optimized. A two-step mechanism is proposed for the ligand substitution reaction that is consistent with the computational results and the weak coordination of the sulfonate group.

The synthesis and X-ray structural characterization of mer and fac isomers of the technetium(I) nitrosyl complex [TcCl 2(NO)(PNPpr)

The nitrosyl complex H[TcNOCl 4] reacts with the tridentate ligand bis[(2-diphenylphosphino)propyl]amine (PNPpr) to yield a mixture of the mer or fac isomers of [TcCl 2(NO)(PNPpr)]. In acetonitrile, where the ligand is freely soluble, reaction occurs at room temperature to yield mostly the mer isomer with the linear nitrosyl ligand cis to the amine ligand; and the phosphine ligands arranged in a mutually trans orientation. The reaction in methanol requires reflux to dissolve the lipophilic ligand and generates the fac isomer of [TcCl 2(NO)(PNPpr)] as the major product, with the tridentate ligand in a facial arrangement, leaving the chlorides and nitrosyl ligand in the remaining facial sites. The steric bulk of the tridentate ligand’s diphenylphophino-moieties results in a significant distortion from octahedral geometry, with the P–Tc–P bond angle expanded to 99.48(4)°.The infrared spectra display absorptions from these nitrosyl ligands in the 1700 and 1800 cm −1 regions for the fac and mer isomers, respectively. The ESI(+) mass spectra each display the parent ion at 647 m/ z.

Synthesis, Structure, and Reactivity of Nitrosyl Pincer-Type Rhodium Complexes

Pincer-type linear nitrosyl Rh(I) complexes, Rh(PCPtBu)(NO)][BF4] (2) and [Rh(PCPtBuCH2)(NO)][BF4] (8), are reported (PCPtBu = 1,3-bis[(di-tert-butylphosphino)methyl]-2,4,6-trimethylbenzene). Complex 2 was synthesized by reaction of the Rh(I) dinitrogen complex Rh(PCPtBu)N2 (1) with NOBF4, while treatment of the methyl chloride complex Rh(PCPtBu)(CH3)Cl (7) with NOBF4 led to the formation of 8. Upon addition of CO, a linear ↔ bent nitrosyl equilibrium was established, both in solution and in the solid state, between the linear nitrosyl Rh(I) complex 2 and the bent nitrosyl Rh(III) complex [Rh(PCPtBu)(NO)(CO)][BF4] (3). Addition of LiCl to complex 2 resulted in the quantitative formation of the bent nitrosyl complex Rh(PCPtBu)(NO)(Cl) (4). An IR study of solvent interactions of the nitrosyl ligand of complex 2 in various solvents is also presented, showing a linear ↔ bent nitrosyl equilibrium induced by solvent coordination. Treatment of 4 with HBF4·O(C2H5)2 led to chloride abstraction, with formation of c...

Mapping NO movements in crystalline [Fe(Porph)(NO)(1-MeIm)

Orientational disorder of the distal nitrosyl (NO) ligand in iron porphyrinates is a common phenomenon. We present an analysis of multitemperature crystallographic data for the order/disorder phenomenon. The observed temperature-dependent order/disorder and variable rotational orientations of nitrosyl ligands for six different six-coordinate iron porphyrinates have been examined in terms of the nonbonded contacts found in the solid state. Favorable orientations for NO can be identified either by calculation of the close nonbonded contacts or by evaluation of the geometry-dependent potential energy using semiempirical nonbonded potential functions. The nonbonded contacts display temperature-dependent differences consistent with observed structural differences. The motion of NO appears to be controlled by intermolecular interactions that allow a limited set of orientations, and under some conditions, only a single NO orientation is allowed. In some cases, the equilibria involving the orientations of NO can be analyzed using the van't Hoff relationship, and the free energy and enthalpy of the solid-state transitions can be evaluated. The intrinsic barriers to rotation of the NO were examined using a fine-meshed series of DFT calculations. The calculations also showed the detailed effects of the variation of the NO orientation on the equatorial bond distances.

Tetragonal to triclinic--a phase change for [Fe(TPP)(NO)

The temperature dependence of the crystalline phase of (nitrosyl)(tetraphenylporphinato)iron(II), [Fe(TPP)(NO)], has been explored over the temperature range of 33-293 K. The crystalline complex is found in the tetragonal crystal system at higher temperatures and in the triclinic crystal system at lower temperatures. In the tetragonal system, the axial ligand is strongly disordered, with the molecule having crystallographically required 4/m symmetry, leading to eight distinct positions of the single nitrosyl oxygen atom. The phase transition to the triclinic crystal system leads to a partial ordering with the molecule now having inversion symmetry and disorder of the axial nitrosyl ligand over only two positions. The increase in ordering allows subtle molecular geometry features to be observed; in particular, an off-axis tilt of the Fe-N(NO) bond from the heme normal is apparent. The transition of the reversible phase change begins at about 250 K. This transition has been confirmed by both X-ray diffraction studies and a differential scanning calorimetry study.

含ピリジン配位子を有するルテニウム錯体の酸化還元と含窒素化合物の反応

In this accounts, syntheses and reactions of ruthenium complexes bearing pyridyl-containing ligands have been described. The properties and reactivity of complexes are related to their geometrical configuration around the metal center, and regulated by the combination of supporting ligands such as bpy, pyca, tpy, and bpya. Ruthenium complexes containing a nitrosyl ligand, which functions as a regulator of reactivity, have been synthesized and their structures and redox properties are investigated. We conclude that the interaction of the nitrosyl ligand through the ruthenium ion strongly depends on an electronic feature of the co-existing ligand in the series of nitrosylruthenium complexes. Conversion reactions of nitrogen-containing compounds such NO2- (NO), N3-, CH3CN, and NO3- are performed under mild conditions to afford nitrogen-containing ligands. The reactions of N3- give two different type of ligands, imine (NH=CHR) and bridging nitrido (-N3--), depending on supporting ligands. These reactions are important as a synthetic route of nitrogen-containing complexes. A geometrical isomerization and ligand-dissociation reactions are induced by redox of ruthenium comeplxes. The stabilities and reactivities of the ruthenium complexes have been explained by considering the electronic interaction of metal-ligand and ligand-ligand through the ruthenium center.

Theoretical investigations of the structure and bonding of several transition metal complexes to probe their carbon monoxide releasing properties

AbstractThe structural and bonding characteristics of several CO‐releasing molecules have been investigated using density functional theories. Based on the nature of the physiological buffer compositions used in experiments, several isodesmic reactions involving chloride (Cl−), hypophosphate (HPO), and nitrosyl (NO) ligands are set up to find out whether the ligand‐substituted products, which could have direct influence in CO releasing mechanism, are thermodynamically feasible. The isodesmic reactions involving the Cl− and HPO ligands are exothermic whereas it is endothermic with the NO ligand. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009