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www.rsc.org/dalton

ISSN 1477-9226

DaltonTransactionsAn international journal of inorganic chemistry

1477-9226(2010)39:1;1-K

www.rsc.org/dalton Volume 39 | Number 3 | 21 January 2010 | Pages 657–964

Volume 39 | N

umber 3 | 2010

Dalton Transactions

Pages 657–964

PAPERManzano et al.Experimental and computational study of the interplay between C–H/p and anion–p interactions

COMMUNICATIONBu et al.Zinc(ii)-boron(iii)-imidazolate framework (ZBIF) with unusual pentagonal channels prepared from deep eutectic solvent

DaltonTransactions

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1

Stereospinomers of Pentacoordinate Iron Porphyrin Complexes:

The Case of the [Fe(porphyrinato)(CN)]- Anions.

Jordi Cireraa and Santiago Alvarezb

a) Department of Chemistry & Biochemistry, University of California, San Diego

Urey Hall 3210, San Diego, CA 92093-0314

b) Departament de Química Inorgànica and Institut de Química Teòrica i Computacional,

Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona (Spain).

Abstract

A computational study of the FeII porphyrinato complexes [Fe(tmp)CN]- and

[Fe(tmp)(CN)2]2- in the high- (S = 2) and low-spin (S = 0) states (tmp =

tetramethylporphyrinato) unravels the reasons for a thermally accessible high-spin for the

tetraphenylporphyrinato analogue of the former. It is shown that the different coordination

geometry of the high-spin stereospinomer is in large part responsible for its stability, while a

single strong-field cyanide ligand contributes to the stability of the low-spin stereospinomer.

The results are compared with the isoelectronic complexes [Ru(tmp)CN]- to evaluate the

influence of the two-electron terms on the relative stability of the two stereospinomers. Shape

and geometry ranges for the two spin states of [FeII(porphyrinato)(ligand)] complexes are

established and structural data for the deoxygenated form of hemo- and myogolbins are

analyzed from that point of view.

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Introduction

Transition metal complexes have been the focus of intense research for more than a

century, mostly due to their wide range of fascinating chemical and physical properties,

which make them suitable building blocks for technological innovation. The application of

Ligand Field and Molecular Orbital theories helped explain the properties of these

compounds through their molecular electronic structure. Both theories, when applied to the

octahedral coordination environment, predicted one of the most remarkable features of these

systems: the possibility of adopting more than one spin state under certain conditions.

Following this model, some general rules for the stereochemical preferences associated with

the electronic structure of the metal center have been established.1 For instance, it is well

know that four-coordinate d8 low-spin metal complexes adopt square planar geometries with

almost no exceptions. Less extended is the idea that significant structural changes can be

observed for alternative spin states in the same molecule.2,3 This phenomenon, known as

stereo-spin isomerism, associates a given spin state with a specific coordination geometry,

and can be exemplified by the [NiBr2(PBzPh2)2] molecule, that adopts a square planar

geometry in its S = 0 spin state, but a nearly perfect tetrahedral geometry for the S =1 state.4

This compound seems not to be a special case, since more examples of stereo-spin isomerism

have been recently reported.5

Among transition metal coordination complexes, those with porphyrinato ligands

occupy a central position due to their stability, the delocalized nature of the chemical bonding

within the macrocyclic framework and, foremost, their presence in a variety of biologically

relevant heme proteins.6 Scheidt and coworkers7,8 have recently added a remarkable FeII

pentacoordinate porphyrinato complex to an already interesting family developed by the same

group along the years.9 The new compound reported is a salt of the [Fe(tpp)(CN)]- ion, where

tpp stands for the tetraphenylporphyrinato dianion, that has been found in two polymorphic

forms. Its most salient feature is that it appears in two alternative spin states, S = 2 and S = 0

at high and low temperatures, respectively. Such a spin crossover behavior is well

substantiated by structural and Mössbauer data. The authors concluded that the data strongly

demonstrates the weaker-field nature of cyanide, a conclusion shared by Nakamura10 in a

highlight of Scheidt's paper, and by several other authors lately.11

In this paper we present computational evidence that the two spin states of

[Fe(porphyrinato)(CN)]- complexes constitute a case of stereo-spin isomerism, each of them

having distinct structural and stereochemical features. Such evidence can be obtained from

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electronic structure calculations at Density Functional Theory level on a representative

[Fe(tmp)(CN)]- anion, where tmp stands for tetramethylporphyrinato. Insight into the factors

that favor the existence of the two stereospinomers (from here on we will use the term

"stereospinomers" to refer to stereo-spin isomers, i.e., molecules that differ in both

stereochemistry and spin state2) is obtained by a detailed analysis of the molecular orbitals of

the [Fe(tmp)(CN)2]2- and [Fe(tmp)(CN)]- anions, and comparison with its 4d congener

[Ru(tmp)(CN)]-. Before presenting the results and discussion, it is convenient to briefly

recap the principles on which the molecular orbital description of the electronic structure of

those complexes is based.

Splitting of the d Orbitals and Choice of the Spin State

A paradigm of coordination chemistry, originated from the early application of crystal

field theory, is that the set of valence d orbitals of a transition metal in an octahedral

environment is split into two subsets: a triply degenerate group of t2g orbitals and a pair of eg

orbitals at higher energy.12 Later application of molecular orbital theory to octahedral

complexes consistently showed the same splitting, but now ascribing metal-ligand

antibonding character to the eg set.13 With such a qualitative orbital ordering, complexes with

d2, d3 or d8 configurations are expected to have a ground state with a maximum of their d

electrons having parallel spins, according to Hund's rules.14 However, for d4 to d7

configurations, two alternative d orbital occupation schemes can be expected. If the splitting

of the two subsets of d orbitals is large compared to the so-called pairing energy (actually the

extra interelectronic repulsion resulting from two electrons occupying the same orbital,

combined with a lower exchange energy associated with electron pairing), a "low-spin"

situation is expected as the ground state. On the other hand, if the t2g-eg splitting is small

compared to the pairing energy, a "high-spin" state should be the most stable one.

The degree of splitting of the d orbitals in a homoleptic [MLn] complex can be

estimated, e.g., from its visible spectrum. Comparison of the values obtained for complexes

with the same metal ion and different sets of ligands or vice versa led to the two

spectrochemical series.15 One of those series organizes the metal ions according to the

relative magnitude of the d orbital splitting in a family of complexes with the same ligands

and varying metals. Conversely, the spectrochemical series of the ligands arranges them

according to the splitting found for various complexes with the same metal and different

ligands. If we restrict ourselves to homoleptic six-coordinate compounds, thus, the amount of

d orbital splitting is dependent on the nature of both the ligand and the metal ion. Therefore,

the fact that a given complex appears with a low-spin state cannot be solely attributed to the

strong-field nature of the ligands. In the same way, a high-spin state does not result only

because the ligands are weak-field ones. A classical example is that of CoII and CoIII

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complexes with the same ligands: while most of the six-coordinate CoII compounds have a

high-spin ground state, the CoIII analogues are low-spin complexes except with the harder O

and F donor atoms.16

All that has been said in the previous paragraph refer to homoleptic six-coordinate

complexes with octahedral geometry. If we wish to extrapolate our concepts of strong/weak-

field and high/low-spin states to a general transition metal complex that may have a different

coordination number and mixed ligands, we better rethink the whole story, due to the

combination of multiple effects occurring at the same time, that will ultimately lead to the

effective energy gap among the d based molecular orbitals.

For a general [MLmXn] complex, the degree of splitting of the d orbitals depends on

several factors, some qualitative and some quantitative:

1) The coordination number. It is well known, for instance, that the two-over-three pattern

(eg above t2g) of the d orbitals in an octahedral complex is inverted for a tetrahedral one (t2

above e). Also that for an eight-coordinate complex there are four antibonding and only one

non-bonding d orbital.17

2) The coordination geometry. Different stereochemistries produce different d orbital

splitting patterns, as for tetrahedral and square planar complexes. In this respect it must be

stressed that trigonal planar and linear complexes present a very small d-orbital splitting,

since the bonding with the ligands is essentially through sp2 and sp hybridizations,

respectively. Consistently, a high-spin ground state is found for low-coordinate complexes

with a variety of electron configurations, both experimentally18 and computationally.19

3) The energy match between the donor and acceptor orbitals. Or, in other words, the degree

of covalency of the metal-ligand bonds. This is probably the factor that is invoked when we

consider a ligand as being a strong or a weak donor.

4) The coordination site in mixed ligand complexes. Each ligand affects differently the

energies of different d orbitals, therefore the splitting cannot be attributed to only one of the

ligands. For instance, in [FeII(tpp)L], the axial ligand L affects the energy of the z2, and to

less extent the energies of the xy and xz orbitals, but it does not affect the energy of the x2-y2

orbital, and is therefore responsible only in part of the energy difference between these

orbitals (see below).

Then, for a given d orbital splitting pattern, the choice of the ground state, that may

correspond to high-, intermediate- or low-spin configurations, depends also on:

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5) The pairing energy. The two-electron (Coulomb and exchange) terms favor the highest

spin. We must recall here that two-electron terms are more favorable for metals of the second

and third transition series, that are found mostly in a low-spin configuration, even in the

presence of weak-field ligands.

6) The temperature. For systems in which states with different spin have similar electronic

energies, entropy favors the high-spin state and spin crossover behavior may appear, with the

low-spin state being observed at low, and the high-spin state at high temperature.20

From the enumeration of the factors that affect the d orbital splitting and the choice of

spin state, it is clear that one cannot blindly attribute the spin state of a given complex to the

strong-field or weak-field nature of a ligand without a careful analysis of all the other factors.

Molecular and Electronic Structure of [Fe(tmp)CN]-

To computationally study the high and low-spin states of the [Fe(tpp)CN]- anion, we

have used a slightly simplified model in which the tetraphenylporphyrinato ligand has been

replaced by its tetramethyl analogue, [Fe(tmp)CN]-. Although this might seem an

oversimplification due to the different electronic effects that the methyl and phenyl groups

have on the porphyrin ring, our results show that they barely affect the energy splitting of the

d-based molecular orbitals (see Supporting Information). Geometry optimizations for this

anion gave two energy minima corresponding to vacant octahedral (S = 0) and square

pyramidal (S = 2) stereospinomers (Table 1). The low-spin form has N-Fe-C bond angles of

94º and Fe-C and Fe-N bond distances of 1.870 and 1.971 Å, respectively, while the

corresponding parameters for the high-spin form are 102º, 2.100 and 2.100 Å, respectively, in

good agreement with the experimental data for the two spin states of the tpp analogue. In

contrast with the experimental structure, the low-spin isomer shows a loss of planarity of the

porphyrinato ligand, with an average deviation of the methine carbon atoms from the nitrogen

plane of 0.35 Å. Such a distortion is not unusual in both five- and six-coordinate iron

porphyrinato complexes, as verified through a CSD search.

Let us now briefly analyze the molecular orbitals of the two stereospinomers. To do

that we look first at the six-coordinate parent compound [Fe(tmp)(CN)2]2-, which is expected

to be a low-spin complex, with a calculated d orbital splitting of roughly 35,000 cm-1. This 21

result is consistent with the fact that the related [Fe(tMp)(CN)2]2- anion (tMp =

tetramesitylporphyrinato) and isonitrile analogues22 have been experimentally found to be

low-spin complexes. The approximately octahedral coordination is reflected in a two-over-

three d orbital pattern, as schematically shown in Figure 2, derived from the relative energies

of the Kohn-Sham orbitals in our DFT calculations.

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Figure 1. Optimized structures of the [Fe(tmp)CN]- anion in its S = 0 (vOC-5, left) and S = 2

( SPY-5, right) spin states.

Table 1. Geometrical parameters for the two stereospinomers of the five-

coordinate iron(II) porphyrin complex anion [Fe(tmp)(CN)]-.

[vOC-5, S = 0] [SPY-5, S = 2]

Fe-C (Å) 1.870 2.100 Fe-N (Å) 1.971 (1) 2.100

L-Fe-N (º) 94.5 102 (1)

δa (Å) 0.154 0.447

S(SPY-5)b 1.12 0.07

S(vOC-5)b 0.10 1.28 a δ measures the displacement of the Fe atom out of the N4 plane.

b S(SPY-5) and S(vOC-5) are the continuous shape measures of the coordination sphere relative to the square pyramid and the vacant octahedron, respectively.

As expected, in the octahedral six-coordinate [Fe(tmp)(CN)2]2- molecule, the 3d orbital

splitting responds to the well know two-over-three pattern (non-bonding t2g and antibonding

eg subsets). Interestingly, the antibonding set is not degenerate and the z2 orbital is found to

be 2,315 cm-1 higher in energy than x2-y2. The higher energy of z2 should be attributed to its

stronger covalent interaction with the cyanides, compared to the interaction of x2-y2 orbital of

the nitrogen porphyrinic atoms. Upon removal of one of the axial cyanide ligands, the energy

of the z2 orbital (Figure 2a) suffers a significant decrease due to the loss of antibonding

character with the missing ligand. Moreover, the loss of inversion symmetry allows mixing

of the pz and z2 orbitals, whereupon the latter becomes strongly hybridized away from the

axial ligand (Figure 3) and, consequently, more stable. However, due to the strong donor

ability of the remaining cyanide, z2 is still sufficiently high in energy (some 26,000 cm-1

above the t2g-like orbitals in our calculations) as to warrant a low-spin ground state. Indeed,

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2

in spite of the favorable two-electron terms, the quintet state with the same geometry is found

to be 11,000 cm-1 higher in energy (Figure 4a). Consistently, this is the geometry found

experimentally at low temperature for [Fe(tpp)(CN)]- in the low-spin state,7 as well as in

related complexes with CO,23 CS,24 or NO - as axial ligands.25 This behavior is in sharp

contrast with that of five-coordinate iron porphyrins with other axial ligands, which are all in

the high-spin (S = 2) state,26 as found also for six-coordinate analogues with tetrahydrofuran,

ethanol or water as axial ligands.27 Thus, this results clearly show that one cyanide ligand

provides a stronger destabilization of z2 than, e.g., two O-donor ligands, due to its “strong

field” nature, as opposite to what has been proposed for these molecules.7 Much in the same

way, it is important to notice that removal of one axial ligand strongly affects the z2 orbital,

but barely changes the relative energy of the x2-y2 orbital (Figure 2b) with respect to the t2g

subset, because this orbital is in the porphyrin plane and its antibonding interactions with the

porphyrin N atoms remain almost invariant with respect to changes in the axial coordination

sites. Similarly, the t2g-like subset also remains almost unaltered because it is only affected

by changes in the weaker π-type interactions.

[Fe(tmp)(CN)2]2- [Fe(tmp)(CN)]-

OC-6 vOC-5 SPY-5

z2

x2-y2

10.000 cm`1

(a)

(b)

(c)

x2-y2

z2

xz,yz

xz,yz

xy xy

Figure 2. Energy ordering of the d-block molecular orbitals of FeII porphyrin complexes

[Fe(tmp)(CN)2]2- in an octahedral conformation (OC-6) and [Fe(tmp)(CN)]- in the vacant

octahedral (vOC-5) low-spin and square pyramidal (SPY-5) high-spin stereospinomers. The

orbital energies are relative to that of the lowest d orbital in each case.

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Figure 3. Isocontour (0.04) plot of the z2 orbital showing its hybridization away from the

cyanide ligand in the [Fe(tmp)(CN)]- anion thanks to the lack of an inversion center.

Figure 4. Relative energies of the low (S = 0) and high (S = 2) spin states of the

[Fe(tmp)(CN)]- anion: (a) in the vOC-5 geometry optimized for S = 0 and (b) in the SPY-5

geometry optimized for the S = 2 state.

To understand why the high-spin configuration of [Fe(tmp)CN]- may become

competitive with the low-spin one, as found at high temperatures for the tpp analogue, we

must consider the significant geometry differences between the two stereospinomers. In the

[SPY-5, S = 2] stereospinomer, there is first a displacement of the Fe atom above the N4

plane of the porphyrin, close to an ideal square pyramid, with N-Fe-C bond angles of 102º.

Such a displacement severely diminishes the overlap of the nitrogen donor orbitals with x2-

y2, which is therefore strongly stabilized in the square pyramid (Figure 2b). In addition to

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the changes in geometry and orbital overlap, the high-spin configuration, with one electron

occupying the Fe-C antibonding z2 orbital, induces a significant lengthening of the Fe-CN

bond (0.230 Å in both experiment and calculations). The outcome is an enhanced

stabilization of z2 due to its diminished σ* character (Figure 2c). In our DFT calculations

these changes are reflected in an inversion of the energy order of the spin states, with the

high-spin configuration appearing 3,400 cm-1 below the low-spin one if both are calculated

with the SPY-5 stereochemistry (Figure 4b).

In summary, the pyramidalization of the coordination sphere of Fe, with average C-Fe-

N bond angles changing from 94 to 102º, favors the high-spin (S = 2) state of [Fe(tRp)CN]-

(R = Ph, Me) whereas the strong field character of the cyano ligand, together with a vacant

octahedral coordination of the Fe atom, stabilize its low-spin (S = 0) state. The similar

stability of these two stereospinomers results in the observed spin crossover behavior. It must

be stressed that even above room temperature a significant fraction of the [Fe(tpp)CN]- anion

should be in the low-spin form according to the experimental magnetic data.7

Although the differences in geometry between the low- and high-spin states are not

dramatic, a look at the magnetic and structural data for five-coordinate FeII porphyrinato

complexes7,26 indicates that they correspond to two distinct situations best described as

stereospinomers. The geometric signature of each stereospinomer is given by three

parameters (Table 2): (i) the Fe-N bond distance to the porphyrinic nitrogen atoms, with a

borderline between low and high-spin complexes at around 2.00 Å, (ii) the degree of

pyramidalization at the Fe atom, calibrated by the average L-Fe-N bond angle, which is

smaller than 96º for the low-spin but higher than 98º for the high-spin isomers, and (iii) the

displacement of the Fe atom out of the N4 plane, δ, smaller than 0.22 Å in the low-spin case

but higher than 0.31 Å in the high-spin one. A simple way to summarize the geometry of the

coordination sphere is through its Continuous Shape measures (CShM) relative to reference

polyhedra.28 In the present case, both the square pyramid (SPY-5) and the vacant octahedral

(vOC-5) shape measures clearly distinguish the low and high-spin isomers (Table 2 and

Figure 5), as indicated by small values of the shape measures relative to the corresponding

ideal polyhedron.

If we apply the same geometrical criteria for distinguishing the two stereospinomers

(Table 2) to a sample of more than two hundred heme groups in deoxymyo- and

hemoglobin29,30 (see Supporting Information for more details), we find all of them to

correspond to the stereochemistry of the high spin form with S(vOC-5) larger than 0.31, with

an average of 1.5 (5). The only heme groups that become close to the geometry of the low

spin stereospinomer are found in the low humidity form of deoxyhemoglobin, with S(vOC-5)

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values between 0.31 and 0.59, and whose geometries were reported to be closer to that in

oxyhemoglobin than to the deoxy form.30

Table 2. Limiting values of the geometric and shape parameters of the low and high-spin

stereospinomers in five-coordinate iron(II) porphyrin complexes.

[vOC-5, S = 0] [SPY-5, S = 2]

Fe-N < 2.00 Å > 2.04 Å

L-Fe-N < 96.4º > 98.7º

δa < 0.23 Å > 0.31 Å

S(SPY-5)b

S(vOC-5)b

> 1.12

< 0.32

< 0.61

> 0.82 a δ is the out of plane displacement of the Fe atom.

b S(SPY-5) and S(vOC-5) are the continuous shape measures of the coordination sphere relative to the square pyramid and the vacant octahedron, respectively.

Figure 5. Structures of [FeII(porphyrinato)L] complexes plotted in a shape map relative to

the vacant octahedron (vOC-5) and the square pyramid (SPY-5). Squares represent low-spin,

circles high-spin complexes, and triangles correspond to the optimized structures of

[Fe(tmp)CN]- with S = 0 (left) and S = 2 (right). Numerical data provided as Supporting

Information.

To test the influence of the two-electron terms on the relative stability of the two

stereospinomers we can compare the isoelectronic complexes [Fe(tmp)CN]- and

[Ru(tmp)CN]-. It must be stressed that the optimized stereochemistries of the low- and high-

spin forms of the latter (Table 3) are coincident with those of the iron complexes (Tables 1

and 2). However, their relative energies are very different. Thus, the optimized quintet state

appears in our calculations 30,000 cm-1 above the singlet for Ru, but the two spin states are

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[vOC-5, S = 0] [SPY-5, S =

2.156 Ru-C (Å) 1.929

Ru-N (Å) 2.044 (3) 2.251 (2)

L-Ru-N (º) 93.7 113.3 (3)

δa (Å) 0.132 0.891

S(SPY-5)b 1.30 0.62

S(vOC-5)b 0.07 4.38

nearly isoenergetic (the triplet 3200 cm-1 above the singlet) for Fe. While the splitting of the

d orbitals in the S = 0 stereospinomer is similar (26,000 and 28,000 cm-1, respectively) for the

two metals, the reduced interelectronic repulsion between paired electrons in the ruthenium d

orbitals compared to that in iron results in a much higher relative stability of the low-spin

state for Ru than for Fe.

Table 3. Geometrical and shape parameters of the two stereospinomers of the five-coordinate

ruthenium(II) porphyrin complex [Ru(tmp)(CN)]-.

2]

a δ is the out of plane displacement of the Fe atom.

b S(SPY-5) and S(vOC-5) are the continuous shape measures of the coordination sphere relative to the square pyramid and the vacant octahedron, respectively.

Conclusions

A remarkable finding of Scheidt and coworkers is that the two spin states found for the

[Fe(tpp)(CN)]- anion are associated with different stereochemistries around the iron atom.

We have presented here an orbital explanation. The low-spin state of [Fe(tpp)(CN)]- at low

temperature in a five-coordinate environment is due to the strong ligand field provided by just

one cyanide ligand along the axial direction. Pyramidalization of the Fe coordination sphere,

on the other hand, induces a stabilization of x2-y2 that makes the high-spin configuration

accessible, complemented with the additional stabilization of z2 due to the longer axial Fe-C

bond in this spin state.

The apparently subtle geometric differences between the vOC-5 and SPY-5 geometries

are enough to differentiate high from low-spin in the whole family of [FeII(porphyrinato)L]

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complexes, which can be calibrated by the corresponding shape measures of the coordination

sphere. This finding can be summarized by saying that there are two stereospinomers in the

family of [FeII(porphyrinato)L] complexes, a low-spin vacant octahedral [vOC-5, S = 0]

isomer, characterized by S(vOC-5) of less than 0.32 and shifts of the Fe atom out of the

porphyrin plane smaller than 0.23 Å, and a high-spin square pyramidal [SPY-5, S = 2]

isomer, identified by S(vOC-5) values higher than 0.82 and shifts of the Fe atom out of the

porphyrin plane of at least 0.31 Å. Such structural criteria apply well to a host of structures of

deoxyhemoglobin, which present stereochemical parameters typical of the high spin

stereospinomer. While most of the members of this family have been obtained only as one of

the two stereospinomers, the [FeII(tpp)CN]– anion is a remarkable example of the

identification of both isomers at different temperatures in a spin crossover process.

We would like to insist in the importance of being very cautious when using the terms

"high-spin" and "low-spin", because they mean different things for different coordination

geometries. It is also important to realize that "low-spin complex" and "strong field ligands"

are not synonyms. Extending the use of these terms beyond the usually well understood case

of octahedral complexes may result in confusing and unjustified statements. Paraphrasing

Lewis Carroll, when we use a word we should know precisely what we mean, and we must

avoid using words that can take different meanings.31

The proposal10 that [FeII(tpp)L] complexes could provide an idealized framework to

evaluate the ligand field strengths of L is too simplistic. On one hand, it neglects the effect of

the coordination geometry, which may be more important than that of a single axial ligand, as

shown in this work for L = CN-. On the other hand, a single axial ligand essentially affects

the energy of only one d orbital and is not by itself responsible for the whole splitting pattern

of the five d orbitals.

Six different factors affect the splitting pattern of the d block orbitals (hence the choice

between high and low-spin configurations) in a transition metal complex: (1) coordination

number of the metal atom (2) geometry of the coordination sphere, (3) degree of covalency of

the metal-ligand bonds, (4) the coordination site occupied by each ligand in mixed ligand

complexes, (5) the pairing energy, and (6) the temperature and the associated entropy factor.

Therefore, it is not straightforward to associate the spin state of [FeII(porpyrinato)L]

complexes only to the position of L in the spectrochemical series. In particular, in

[Fe(tpp)(CN)]- other factors are responsible for the relatively low stability of a high-spin

state: a coordination number smaller than six, a square pyramidal coordination geometry with

a relatively large pyramidality angle, the poorer σ-donor character of the basal porphyrinic N

atoms with respect to the axial cyanide ligand, its occupation of an axial coordination site that

makes it non-interacting with respect to x2-y2, the relatively high pairing energy for the 3d

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transition metal electrons, and the high temperatures that entropically favor the high-spin

state. Against such a plethora of factors pressing the complex to be a high-spin one, the

strong field character of cyanide alone is still capable of stabilizing the low-spin state if only

the entropy factor is diminished by lowering the temperature down to 100 K. Also the

analogous RuII monocyano complex is seen by calculations to have a similar degree of

splitting of the d orbitals than the FeII one, yet it is accessible only as a low spin

stereospinomer because of its higher pairing energy.

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Computational Details

All Density Functional Calculations were carried out with Gaussian09 (revision C.01)32

using the TPSSh hybrid functional with a 10-6 convergence criterion in the density matrix.33

This functional has been reported to perform accurately for iron electronic energy

differences.34 We have compared our results with those obtained with the B3LYP functional

to ensure that our results are independent of the choice of functional.35 For some spin states,

the quadratic convergence approach has been requested. A triple-ζ all electron Gaussian

basis set36 was employed for all elements but Ru. For the [Ru(tmp)CN]- anion, a

combination of triple-ζ all electron Gaussian basis set together with the SDD

pseudopotentials for Ru37 has been used. All optimized geometries have been checked to be

energy minima through the corresponding vibrational analysis.

Continuous shape measures, path deviation functions and generalized coordinates were

calculated with the SHAPE program.38 Searches of structural data for five-coordinate iron(II)

porphyrin complexes were carried out in the Cambridge Structural Database39 with the help

of the ConQuest software.40 The oxidation and spin states assigned in the Cambridge

Database were verified for each of the reported structures included in the statistical analysis

of Table 1. Extensive searches were also carried out in the PDB for deoxy heme groups

(HEM ligand) in hemoglobins and myoglobins, and the shape measures and geometrical

parameters are provided as Supporting Information.

Acknowledgments

This work has been supported by the Ministerio de Invesigación, Ciencia e Innovación

(MICINN), project CTQ2011-23862-C02, and by Generalitat de Catalunya, grants 2009SGR-

1459 and XRQTC.

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