Nature of the Ru-NO Coordination Bond: Kohn-Sham Molecular Orbital and Energy Decomposition Analysis.

We have analyzed structure, stability, and Ru-NO bonding of the trans-[RuCl(NO)(NH3)4]2+ complex by using relativistic density functional theory. First, we focus on the bond dissociation energies associated with the three canonical dissociation modes leading to [RuCl(NH3)4]++NO+, [RuCl(NH3)4]2++NO, and [RuCl(NH3)4]3++NO-. The main objective is to understand the Ru-NO+ bonding mechanism in the conceptual framework of Kohn-Sham molecular orbital theory in combination with a quantitative energy decomposition analysis. In our analyses, we have addressed the importance of the synergism between Ru-NO+ σ-donation and π-backdonation as well as the so-called negative trans influence of the Cl- ligand on the Ru-NO bond. For completeness, the Ru-NO+ bonding mechanism is compared with that of the corresponding Ru-CO bond.

Introduction

Nitric oxide (NO) is involved in a large number of physiological and physiopathological processes, such as a biological messenger responsible for vasodilation, playing an important role in immune response, or acting as a neurotransmitter.1 Thus, owing to the importance of NO, which can either be beneficial or harmful depending on its concentration and bioavailability,1f studies concerning NO donors and scavengers have experienced large growth.2 Major advances in this area have been achieved with ruthenium tetraammine nitrosyl complexes trans‐[RuII(NO+)(NH3)4L], which appear to have a low cytotoxicity towards host cells, are water soluble, and stable against air oxidation. In such complexes, the role of the trans ligand L is crucial, because it modulates the reactivity of the trans ligand NO+.2h An example is the negative trans influence, that is, the bond strengthening effect, of L=Cl− on the Ru−NO+ bond in trans‐[RuII(NO+)(NH3)4Cl]2+,3 shown below:

In this context, the Ru−NO chemical bond can be described in terms of the {RuNO} notation, where n holds for the sum of the number of electrons in 4d orbitals of Ru, and π* orbital of NO, as proposed by Enemark and Feltham.3b The importance of this definition is based on the fact that NO is referred to as a “non‐innocent” ligand, that is, the oxidation state of this ligand cannot be defined in a clear way in these metal complexes.3c Another point is that this bond between ruthenium and the π acceptor ligand nitrosyl can be expected to have a synergism between two processes: σ‐donation from NO to dσ of Ru, plus π‐backdonation from dπ orbital of Ru to π* of NO.3a

The purpose of the present study is twofold. In the first place, we have analyzed the electronic structure and energetically preferred dissociation modes in order to determine the effective oxidation state and most suitable canonical form of the NO ligand in the complex. Thus, we have carried out an investigation based on three possible dissociation reactions for the trans‐[RuCl(NO)(NH3)4]2+ complex to give either 1) [RuCl(NH3)4]++NO+, 2) [RuCl(NH3)4]2++NO, or 3) [RuCl(NH3)4]3++NO−.

Next, we aim to shed light on the bonding mechanism of the Cl−Ru−NO unit in this complex through detailed quantitative Kohn–Sham molecular orbital and energy decomposition analyses. Here, we anticipate an important role of the synergism between σ‐donation and π‐backdonation between Ru and NO. Furthermore, we elucidate the mechanism behind the negative trans influence of Cl− on the Ru−NO coordination bond. Finally, we compare the nature of the Ru−NO bond with that of Ru−CO in the isoelectronic trans‐[RuCl(CO)(NH3)4]+ complex.

Theoretical Methods

Computational Details

All calculations were performed with the Amsterdam Density Functional (ADF) software developed by Baerends and co‐workers.4, 5 Molecular orbitals (MOs) were expanded by using one large, uncontracted set of Slater‐type orbitals (STO): TZ2P for geometry optimization, vibrational analysis, and single‐point energy calculations.6 The TZ2P basis set is of triple‐ζ quality, augmented by two sets of polarization functions (d and f on heavy atoms; 2p and 3d sets on H). All electrons are included in the variational treatment (no frozen‐core approximation used). An auxiliary set of s, p, d, f, and g Slater‐type orbitals was used to fit the molecular density and to represent the Coulomb and exchange potentials accurately in each self‐consistent field (SCF) cycle.7

Energies and gradients were calculated by using the local density approximation (LDA: Slater8 exchange and VWN9 correlation) with gradient corrections,10, 11 owing to Becke (exchange) and Perdew (correlation) added self‐consistently. This is the BP86 density functional. Scalar relativistic corrections were included self‐consistently using the zeroth order regular approximation (ZORA).12 All open‐shell systems were treated with the spin‐unrestricted formalism. Vibrational analyses for all optimized geometries demonstrate that they are all energy minima at the level of theory applied here.

Energy Decomposition Analysis

All bonding analyses have been carried out at the ZORA‐BP86/TZ2P level of theory. The overall bond energy ΔE between [RuCl(NH3)4]+2‐ and NO (q=+1, 0, −1) is made up of two major components [Eq. (1)]:13

Here, the strain energy ΔE strain is the amount of energy required to deform the fragments NO, and [RuCl(NH3)4]+2‐, from their equilibrium structure to the geometry that they acquire in the overall trans‐[RuCl(NO)(NH3)4]2+ complex. The interaction energy ΔE int corresponds to the actual energy change when, for example, the geometrically deformed [RuCl(NH3)4]+ and NO+ are combined to form the overall complex.

Later on in our analysis, we focus on the interaction energy ΔE int. The latter can be further analyzed, in the framework of the Kohn–Sham molecular orbital (MO) model, using an energy decomposition analysis (EDA) into electrostatic interaction attraction, Pauli repulsion, and (attractive) orbital interactions [Eq. (2)]:4, 13

The term ΔV elstat corresponds to the classical electrostatic interaction between the unperturbed charge distributions of the prepared (i.e. deformed) [RuCl(NH3)4]+, and NO+. This term is usually attractive. The Pauli‐repulsion ΔE Pauli comprises the destabilizing interactions between occupied orbitals and is responsible for the steric repulsion. The orbital interaction ΔE oi in any MO model, and therefore also in Kohn–Sham theory, accounts for charge transfer (i.e., donor‐acceptor interactions between occupied orbitals on one moiety with unoccupied orbitals of the other, including the HOMO–LUMO interactions) and polarization (empty/occupied orbital mixing on one fragment due to the presence of another fragment). The orbital interaction energy can also be decomposed into the contributions from each irreducible representation Γ of the interacting system [Eq. (3)]:

Then, concerning the three possible dissociation reactions for trans‐[RuCl(NO)(NH3)4]2+, we must take into account that [RuCl(NH3)4]2+ and NO fragments [dissociation (2)] have one unpaired electron, whereas [RuCl(NH3)4]3+ and NO− [dissociation (3)] have two unpaired electrons each. The unpaired electrons in Ru fragments are located in dπ fragment molecular orbitals (FMOs), and for NO and NO− are located in π* FMOs (see also Figure 1).

Figure 1

Orbital interaction diagram with the processes of σ‐donation (blue) and π‐backdonation (red) for trans‐[RuCl(NO)(NH3)4]2+ composed of NO+ and [RuCl(NH3)4]+, emerging from our Kohn–Sham orbital analyses at ZORA‐BP86/TZ2P.

Analysis of the Charge Distribution

The electron density distribution is analyzed at ZORA‐BP86/TZ2P by using the Voronoi Deformation Density (VDD) method14, 15 for computing atomic charges. The VDD atomic charge Q A VDD is computed as the (numerical) integral16 of the deformation density Δρ(r)=ρ(r)‐ΣB ρ B(r) in the volume of the Voronoi cell of atom A [Eq. (4)]. The Voronoi cell of atom A is defined as the compartment of space bound by the bond midplanes on and perpendicular to all bond axes between nucleus A and its neighboring nuclei (cf. the Wigner–Seitz cells in crystals).15b

In Equation (4), ρ(r) is the electron density of the molecule and ΣB ρ B(r) the superposition of atomic densities ρ B of a fictitious pro‐ molecule without chemical interactions that is associated with the situation in which all atoms are neutral. The interpretation of the VDD charge Q A VDD is rather straightforward and transparent. Instead of measuring the amount of charge associated with a particular atom A, Q A VDD directly monitors how much charge flows, owing to chemical interactions, out of (Q A VDD>0) or into (Q A VDD<0) the Voronoi cell of atom A, that is, the region of space that is closer to nucleus A than to any other nucleus.

In addition, it is also possible to compute the charge rearrangement ΔQ A VDD with respect to an initial density that is the sum of polyatomic subsystems ρ i instead of the sum of monoatomic densities [Eq. (5)]:

Equation (5) offers direct insight into the redistribution of the electronic density caused by the formation of a bond between polyatomic subsystems.4 This last scheme will be used in the next section to analyze the electron‐density distribution of the isolated and deformed fragments that form the complex.

Results and Discussion

Accuracy and Relativistic Effects

Our computed ZORA‐BP86/TZ2P and BP86/TZ2P geometrical data and bond‐dissociation energies ΔE for the three dissociation schemes are summarized in Table 1, where we compare our calculated results, with and without ZORA relativistic correction, to the experimental values obtained from X‐ray diffraction.17 As can be seen, the computed geometries are in good agreement with the experimental ones. The slightly smaller deviation when ZORA is included demonstrates the significance of relativistic effects and justifies the effort of using the relativistic formalism ZORA‐BP86/TZ2P in all further analyses in this study.

Table 1

Geometric data and bond dissociation energies (−ΔE) for trans‐[RuCl(NO)(NH3)4]2+ complex.[a]

	ZORA	NR	EXP[b]
Bond distances [Å]
Ru−NO	1.838	1.825	1.799
N−O	1.150	1.147	1.026[c]
Ru−Cl	2.238	2.278	2.376
Ru−NH3	2.159	2.175	2.100[d]
Bond angles [°]
Ru−N−O	180.0	180.0	176.6
Cl−Ru−NO	180.0	180.0	177.9
NH3−Ru−NO	92.5	93.4	92.0[d]
NH3−Ru−Cl	87.5	86.6	88.1[d]
NH3−Ru−NH3	89.9	89.8	90.0[d]
−ΔE [kcal mol−1]
[RuCl(NH3)4]++NO+	56.6	48.3	[e]
[RuCl(NH3)4]2++NO	84.6	79.5	[e]
[RuCl(NH3)4]3++NO−	467.2	465.3	[e]

[a] Computed at BP86/TZ2P with relativistic effects (ZORA), without relativistic effects (NR), and experimental values (EXP). [b] Experimental values from Ref. 17a. [c] Not reliable due to disorder problems, as mentioned in Ref. 17c. [d] Average value of the experimental results from Ref. 17a. [e] Not available.

Structure and Stability

We first focused on choosing the most suitable canonical form for trans‐[RuCl(NO)(NH3)4]2+ complex based on two criteria: the energetically preferred dissociation mode and the geometry. To this end, Table 1 contains the bond dissociation energy (ΔE) for the three above proposed decomposition schemes. The first dissociation of trans‐[RuCl(NO)(NH3)4]2+ in [RuCl(NH3)4]++NO+ shows the lowest energetic value (56.6 kcal mol−1), as compared to the other two dissociation patterns (82.8 and 466.7 kcal mol−1, respectively). This is a direct consequence of a more favorable Coulomb energy as the two‐fold positive charge is separated over two singly positively charged fragments. Thus, trans‐[RuCl(NO)(NH3)4]2+ dissociates preferentially into [RuCl(NH3)4]++NO+, the latter fragment being isoelectronic with the CO ligand.

Furthermore, the trans‐[RuCl(NO)(NH3)4]2+ compound assumes the geometry of a distorted octahedron with C 4 symmetry. As shown in Table 1, the NH3−Ru−NO (92.5°), and NH3−Ru−Cl (87.5°) angles deviate from 90°; and with Ru−NO, Ru−Cl, and Ru−NH3 bond lengths of 1.838, 2.238, and 2.159 Å, respectively. However, the most relevant data include the Ru−N−O bond angle of 180°, which has been previously attributed to trans‐[Ru(NH3)4(Cl)(NO)]2+, having predominantly [RuCl(NH3)4]++NO+ character, based on the fact that the overall complex is EPR silent.2h, 3d In addition, the reactions of these nitrosyl complexes with hydroxide ions, yielding trans‐[Ru(NH3)4(L)(NO2)]+, also confirm the nitrosonium character of the [NO] ligand.17b Further details of the optimized geometries are provided in the Supporting Information.

Our analyses show that the nature of trans‐[RuCl‐ (NO)(NH3)4]2+ is best represented by the [RuII‐NO+] canonical form. Therefore, we will cast our further analyses and discussion of the Ru−NO bonding mechanism in terms of the [RuCl(NH3)4]++NO+ fragments.

Ru−NO Bonding Mechanism in trans‐[RuCl(NO)(NH3)4]2+

The Ru−NO bonding analysis comprises a quantitative Kohn–Sham molecular orbital analysis complemented with an energy decomposition analysis on trans‐[RuCl(NO)(NH3)4]2+ formed from [RuCl(NH3)4]+ and NO+ fragments (I). This EDA is later complemented by two more EDA schemes: II) [Ru(NH3)4]2+ and NO+, to analyze the Ru−NO bond in absence of Cl−; and III) [Ru(NH3)4]2+ and Cl−, to investigate Ru−Cl in the absence of NO+. The combination of the three should give us a complete picture of the Ru−NO interaction in our trans‐[RuCl‐ (NO)(NH3)4]2+ complex, with emphasis on the synergy between σ‐donation and π‐backdonation in the Ru−NO interaction, and also on the negative trans influence of the Cl− ligand.

We first focus on the decomposition scheme that we proposed for our trans‐[RuCl(NO)(NH3)4]2+ complex, that is, [RuCl(NH3)4]+ and NO+ (Table 2). The bonding interaction is attractive (−64.6 kcal mol−1). Interestingly, this is exclusively attributed to the orbital interactions ΔE oi (−234.7 kcal mol−1). The latter compensate both the electrostatic term ΔV elstat (55.2 kcal mol−1), which is repulsive because of Coulomb repulsion between the two singly positively charged fragments that are formed, and the repulsive Pauli interaction (115.0 kcal mol−1).

Table 2

Analysis of Ru−L1 bond between [RuL2(NH3)4] and L1 in trans‐[Ru(L1)(L2)NH3)4] complex.[a]

	I	II	III
L1	NO+	NO+	Cl−	CO
L2	Cl‐	–	–	Cl‐
n	1	2	2	1
EDA [kcal mol−1]
ΔV elstat	55.2	171.7	−304.7	−136.8
ΔE Pauli	115.0	79.3	139.6	192.4
ΔE σ	−32.5	−43.0	−67.3	−52.1
ΔE π	−196.3	−151.9	−21.3	−72.6
ΔE A2,B1,B2	−5.9	−4.9	−3.7	−0.1
ΔE oi	−234.7	−199.8	−92.3	−124.8
ΔE int	−64.6	51.2	−257.5	−69.2
FMO energy [eV]
Ru:dσ*	−6.5	−14.2	−14.2	−6.9
Ru:dπ	−7.9	−14.3	−14.3	−8.1
Cl‐:pσ	–	–	0.6	–
Cl‐:pπ	–	–	0.6	–
L1:σ	−23.2	−23.2	–	−9.2
L1:π*	−15.6	−15.6	–	−2.4
FMO overlap, S
<Cl−:pσ | Ru:dσ*>	–	–	0.25	–
<Cl−:pπ | Ru:dπ>	–	–	0.11	–
<L1:σ | Ru:dσ*>	0.38	0.30	–	0.47
<Ru:dπ | L1:π*>	0.14	0.13	–	0.22
<Ru:dπ | L1:π>	0.09	0.08	–	0.10
FMO populations, P [e]
Ru:dσ*	0.32	0.37	0.53	0.56
Ru:dπ	1.43	1.53	1.97	1.73
Cl‐:pσ	–	–	1.43	–
Cl‐:pπ	–	–	1.92	–
L1:σ	1.71	1.64	–	1.48
L1:π	1.99	1.99	–	1.99
L1:π*	0.56	0.47	–	0.27
VDD charge rearrangement	ΔQ [a.u.][b]
[RuL2(NH3)4]n+	+0.892	+0.712	−0.226	+0.320

[a] Computed at ZORA‐BP86/TZ2P. [b] Based on Equation (5).

The ΔE oi term (Figure 1) can be further decomposed into σ and π contributions, which amount to −32.5 and −196.3 kcal mol−1, respectively. Note that the π orbital term is by far the dominant contributor of stabilization in the Ru−NO bond. From the gross Mulliken populations of the fragment molecular orbitals (FMOs), we observe σ‐donation from NO+:σ (P=1.71 e) to Ru:dσ* (P=0.32 e) and π‐backdonation from Ru:dπ (P=1.43 e) to NO+:π* (P=0.56 e). The origin of the stronger π‐backdonation is the smaller energy difference between the interacting π FMOs compared to the σ ones (7.7 vs. 16.7 eV, respectively). The corresponding FMO overlaps support both σ‐donation and π‐backdonation interactions; the former being larger (0.38) compared to the latter (0.14). Finally, according to the VDD charge analysis, such larger π‐backdonation also translates into a net transfer of 0.892 e to the NO+ ligand (see Table 2).

Next, we studied the synergism between the σ‐donation and the π‐backdonation processes present between [RuCl(NH3)4]+ and NO+ fragments. Thus, if we want to only focus on the σ‐donation, we have to remove π virtual orbitals of NO+. On the other hand, if we are interested in π‐backdonation, σ virtual orbitals of Ru fragment must be deleted. Note that this deletion of virtuals on one fragment also suppresses polarization between the occupied and virtual orbitals within the same fragment.

When π‐backdonation is turned off (see Table 3), ΔE σ is only slightly reduced, by 1.9 kcal mol−1. On the other hand, when σ‐donation is turned off, ΔE π experiences a considerable weakening by 28.1 kcal mol−1. This reduction in ΔE π is also observed in the gross Mulliken populations, which show a smaller π charge transfer (Ru:d=1.55 and NO+:π*=0.44). Therefore, the synergy effect amounts to ΔE synergy=ΔE oi(σ‐donation + π‐backdonation)−ΔE oi(σ‐donation only)−ΔE oi(π‐backdonation only)=−23.5 kcal mol−1. Furthermore, concerning the overall charge transfer between the two fragments, when σ‐donation is turned off, the VDD charge rearrangement for NO+ is only slightly reduced to ΔQ=−0.839 e, whereas turning‐off π‐backdonation causes NO+ to adopt a positive charge of ΔQ= +0.022 e.

Table 3

Analysis of the synergy between σ‐donation and π‐backdonation orbital interactions in [RuCl(NH3)4]++NO+ [kcal mol−1].[a]

OI allowed[b]	ΔE σ	ΔE π	ΔE A2,B1,B2	ΔE oi	ΔE int
σ‐don. + π‐back.	−32.5	−196.3	−5.9	−234.7	−64.6
σ‐don.	−30.6	0.0	−5.6	−36.2	133.9
π‐back.	0.0	−168.2	−6.8	−175.0	−4.8

[a] Computed at ZORA‐BP86/TZ2P level. [b] OI=orbital interactions; σ‐don.=σ‐donation; π‐back.=π‐backdonation.

In this way, concerning the ΔE int between the two fragments, if σ‐donation is cancelled, we still have an attractive interaction by −4.8 kcal mol−1. However, this is not the case if π‐backdonation is suppressed, as we would get a dramatically repulsive interaction that amounts to +133.9 kcal mol−1. Thus, π‐backdonation is not only larger than the σ‐donation Ru−NO bonding mechanism, but it is crucial to have an attractive interaction between [RuCl(NH3)4]+ and NO+ fragments in this trans‐[RuCl(NO)(NH3)4]2+ complex. We conclude that the σ–π synergism is necessary in order to yield a stabilizing Ru−NO bond in trans‐[RuCl(NO)(NH3)4]2+.

Chloride trans Influence

Next, we investigate how and why the trans ligand Cl− affects the Ru−NO bond. To this end, we first analyze this bond in the absence of the chloride ligand, that is, we investigate the interaction between [Ru(NH3)4]2+ and NO+ in [Ru(NO)(NH3)4]3+ at the same geometry of the trans‐[RuCl(NO)(NH3)4]2+ complex. In [Ru(NO)(NH3)4]3+, at variance with trans‐[RuCl(NO)(NH3)4]2+, the Ru−NO interaction energy ΔE int (51.2 kcal mol−1) is clearly repulsive (see II in Table 2). This net repulsive character mainly originates from the large electrostatic repulsion between the two positively charged fragments (ΔV elstat=171.7 kcal mol−1) that, together with ΔE Pauli (79.3 kcal mol−1), overrule the attractive ΔE oi (−199.8 kcal mol−1).

The highly repulsive ΔV elstat is attributed to the lack of the negatively charged Cl− ligand. The absence of the trans Cl− ligand also goes with significant changes in the orbital interactions, as reflected by the ΔE oi term. Thus, if we take away the Cl− ligand, the σ component becomes more stabilizing (going from −32.5 to −43.0 kcal mol−1), but the π component is considerably weakened (going from −196.3 to −151.9 kcal mol−1). Analysis of the Kohn–Sham orbital interaction mechanism indeed reveals a stronger donor–acceptor mixing between the interacting σ FMOs and a weaker mixing between the π FMOs. This is also reflected by the gross Mulliken populations of the FMOs involved, which show larger σ‐donation from NO+:σ (P=1.64 e) to Ru:dσ* (P=0.37 e), but smaller π‐backdonation from Ru:dπ (P=1.53 e) to NO+:π* (P=0.47 e) in [Ru(NO)(NH3)4]3+, as compared to [RuCl(NO)(NH3)4]2+ (see Table 2). The stronger σ‐donation from, and weaker π backdonation to, NO+ translates into a less negative VDD charge (−0.712 e) of the ligand (see Table 2).

To precisely understand how introducing Cl− modifies the bonding capability of the ruthenium complex towards NO+, we undertake a bonding analysis of the Ru−Cl bond between [Ru(NH3)4]2+ and Cl− in [RuCl(NH3)4]+ (see III in Table 2). In this case, the ΔE int is largely attractive (−257.5 kcal mol−1), especially owing to the very attractive ΔV elstat (−304.7 kcal mol−1) term between the cationic and anionic fragments. The orbital interactions ΔE oi (−92.3 kcal mol−1) arise from the Cl− ligand σ‐donating charge to the [Ru] fragment (see Figure 2), from Cl−:pσ (P=1.43 e) to Ru:dσ* (P=0.53 e). There is no π‐backdonation (Ru:dπ P=1.97 e), as Cl− has no π acceptor orbitals. This is the reason why σ orbital interactions are larger than π. As a consequence, the Cl− ligand loses ΔQ=0.226 electrons, which are transferred to the [Ru] fragment (see Table 2). Importantly, the Cl−Ru orbital interactions push the Ru dπ orbitals up in energy in a Pauli repulsive Cl−:3pπ‐[Ru]:dπ closed‐shell interaction (see Figure 2). This is the main responsible mechanism that enhances the π‐backdonation capability of the resulting [RuCl(NH3)4]+ fragment towards NO+. In line with this mechanism, a trans F− ligand reinforces the Ru−NO+ coordination bond even more than trans Cl−, because of a stronger F−:2pπ‐[Ru]:dπ closed‐shell interaction (not shown in Figure 2). Thus, going from trans‐[RuCl(NO)(NH3)4]2+ to trans‐[RuF(NO)(NH3)4]2+, π‐backbonding interactions are enhanced. Accordingly, the net transfer of charge to the NO+ ligand increases from 0.892 to 0.904 e, the Ru−NO+ bond energy ΔE strengthens from −56.6 to −68.9 kcal mol−1, and the bond shortens from 1.838 to 1.766 Å (data for trans‐F− case not shown).

Figure 2

Orbital interaction diagram with the processes of σ‐donation (blue) and π‐backdonation (red) of Ru−NO present in [RuCl(NH3)4]+ composed of Cl− and [Ru(NH3)4]2+, emerging from our Kohn–Sham orbital analyses at ZORA‐BP86/TZ2P.

We conclude that the trans Cl− ligand reinforces the Ru−NO bond in trans‐[RuCl(NO)(NH3)4]2+ by reducing the electrostatic repulsion as well as by strengthening the π‐backdonation.

Ru−CO Interaction

To demonstrate that our findings are applicable more generally, we have substituted NO+ for the isoelectronic CO in the equivalent trans‐[RuCl(CO)(NH3)4]+ complex. In the case of the CO ligand, there is no discussion about the canonical form. The interaction between [RuCl(NH3)4]+ and CO has been analyzed by means of an EDA (see Table 2). One might expect that the Ru−CO bond is much stronger than the Ru−NO bond, because the latter occurs between two fragments of the same charge, whereas the former does not. Strikingly, however, Ru−CO (ΔE int=−69.2 kcal mol−1) is only 4.6 kcal mol−1 stronger than the Ru−NO bond (ΔE int=−64.6 kcal mol−1).

Indeed, the ΔV elstat term goes from repulsive in trans‐[RuCl‐ (NO)(NH3)4]2+ (+55.2 kcal mol−1) to very attractive in trans‐[RuCl(CO)(NH3)4]+ (−136.8 kcal mol−1). However, Pauli repulsion increases to 192.4 kcal mol−1, and ΔE oi weakens to only −124.8 kcal mol−1. The weakening in the orbital interaction terms from Ru−NO+ to Ru−CO results from a significant weakening in π‐backbonding to the neutral CO, which dominates the strengthening in σ donation. These trends also show up in the FMO mixing patterns, as reflected by the FMO population of Ru:dπ (P=1.73 e.).

Thus, despite the similar interaction energy between Ru and either NO or CO ligands in the complexes under analysis, their bonding character is rather different. Ru−NO is bonded thanks to a very attractive covalent π component, even though the interaction is electrostatically repulsive. On the other hand, the Ru−CO interaction is characterized by a more balanced attractive electrostatic and covalent σ and π components.

Conclusions

The trans‐[RuCl(NO)(NH3)4]2+ complex is best conceived as consisting of [RuCl(NH3)4]+ and the nitrosyl cation NO+ held together by a RuII−NO+ coordination bond. This follows from our relativistic density functional theory computations, which show that: i) this is the energetically by far preferred dissociation mode; ii) the Cl−Ru−NO angle is 180°; and iii) the Kohn–Sham molecular orbital electronic structure of the complex arises from RuII←NO+ σ‐donation and RuII→NO+ π‐backdonation.

Our quantitative bonding analyses show that there is synergy between σ‐donation and π‐backdonation, and that the latter is the dominant term in the RuII−NO+ coordination bond. Interestingly, the electrostatic interaction across RuII−NO+ is pronouncedly repulsive, owing the Coulomb repulsion between the two singly positively charged fragments of [RuCl(NH3)4]+ and NO+ that are formed in the lowest‐energy dissociation mode. It is the synergistic orbital interactions that compensate for both the electrostatic repulsion as well as the Pauli repulsion between closed shells, and thus keep this bond together.

Finally, we have identified, in detail, the mechanism behind the negative trans influence of the Cl− ligand. This ligand, which is oriented trans with respect to the NO+ ligand, reinforces the RuII−NO+ bond in two ways: i) it reduces the electrostatic repulsion by reducing the negative charge of the ruthenium fragment and ii) it strengthens π‐backdonation by pushing up the ruthenium dπ orbitals.

Conflict of interest

The authors declare no conflict of interest.

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