CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS IN TRANSITION METAL COMPLEXES LUIGI SACCONI

Istituto di Chimica Generale ed Inorganica dell'Università di Firenze, LaboratorIo C.N.R., 41, Via J. Nardi, 50132 Firenze, Italy

ABSTRACT Iron(u), cobalt(ii) and nickel(ii) six- and five-coordinated complexes are reviewed, with particular attention to the factors influencing the spin multiplicity of the ground state. In the case of octahedral complexes, the Dq parameter and the effects of distortion are considered sufficient to interpret the experimental data, whereas in the case of five-coordinated complexes a new parameter, the overall nucleophilic reactivity constant n° is introduced in order

to account for the spin state of the complexes. In this latter case a detailed discussion of the influence of the geometry on the spin state is presented. The methods for rationalizing the magnetic behaviour of complexes showing spin state equilibria are critically discussed.

INTRODUCTION The simple measurement of the effective magnetic moment (Iteff) has been, for many years, a powerful method for determining the spin multiplicity of transition metal ions in coordination complexes1' 2 Very often some indication as to the structure is possible since the geometry of the chromophore

Lu

Strength of Ligcind field Figure 1. Sketch of crystal field ground levels for metal complexes

11

P.A.C.—27/1G

LUIGI SACCONI

is one of the factors which determines the spin orbit functions of the lowest. energy level. However, complexes of some metal ions which possess the same overall geometry, are sometimes found to be either high or low spin depending on the ligands. In the ligand field diagrams then (Figure 1), it happens that the fundamental level is high spin for a certain range of values of the parameters which represent, empirically, the perturbation potential of the donor atoms. For higher values the fundamental state becomes low spin. The point at which the two levels meet is called the crossover point for which the field strength has a corresponding value. The magnetic properties on either side of the crossover point are determined from the values ML and M5

of the function ,/i(L, ML, S, M) relative to the fundamental state. In the vicinity of the crossover point both the high and low spin levels will be statistically populated giving rise to unusual magnetic behaviour; i.e. they will have intermediate values of Peff and will not obey Curie's law.

This lecture deals with two topics: (1) The conditions which determine spin multiplicity and relative parameters; (2) Examples of unusual magnetic

behaviour in spin isomeric compounds. For this purpose I have chosen those five and six coordinate complexes of iron(n), cobalt(ii) and nickel(n), which have been found to exhibit various

spin multiplicities in the fundamental state and have, therefore, aroused much interest in the last few years.

HEXACOORDINATE COMPLEXES Iron(u) complexes It is well known that for octahedral complexes of iron(II), as for the other complexes possessing this geometry, the most meaningful parameter which determines spin multiplicity is Dq3. Furthermore, ligands have been arranged in the spectrochemical series according to their values of Dq, which empirically includes the effects of both c and it bonding.

The ligand field diagram reported by Tanabe and Sugano for iron(Ii) in °h symmetry is shown in Figure 2. It may be seen from this diagram that by gradually increasing the absolute value of Dq one passes from high to low spin complexes. Six-coordinate complexes of iron(I1) having the 5T2g funda-

mental state are obtained with ligands having low values of Dq, i.e.

H20, NH3 and pyridine. Whereas those with the 'A1g fundamental state are obtained with ligands having a high value for Dq, such as phen(1,1Ophenanthroline), bipy(2,2-bipyridyl), or CN -. In the series of complexes FeL2X2, where L = phen, bipy and X = halogen,

pseudohalogen or organic acid, the spin state has been found5 to depend on the nature of X. Several examples of complexes with spin states around the

crossover point are found in this series. These are summarized in Table 1 together with the /1eff at 293°C, the Weiss constant and the ground state.

When X = halogen, NCO, HCOO or CH3COO the complexes are high spin5. The dependence of the effective magnetic moment on temperature is more or less in agreement with the values predicted by ligand field theory for distorted 5T29 ground states. However, when X = fphen,

bipy, NO, CNO or CN the compounds are low spin with a small 162

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

magnetic moment which can be attributed to temperature independent paramagnetism9' . The compounds Fe(phen)2(NCS)2, Fe(phen)2(NCSe)2 and Fe(bipy)2(NCS)2 are high spin at room temperature with effective magnetic moments

50

0

30

20

10

0

2

—1

—3

Dq x

Figure 2. Energy levels for Fe(II) (d6) in °h symmetry4

of about 5.2 BM. On lowering the temperature (Figure 3) the magnetic moment at first obeys van Vieck's law for high spin complexes, then rapidly (within a few degrees) descends to around 2 BM6' 8• It then decreases slowly

with temperature. This dependence of the effective magnetic moment on temperature has been explainett in terms of a reversible transition from the 5T29 to the 'A19 state. Such a hypothesis gains further support from the 163

LUIGI SACCONI Table 1. Magnetic properties of Fe(phen)2X2 and Fe(bipy)2X2 complexes

i1efM) at

Compound

Weiss

293°K 5.13 5.24 5.18 5.20 5.07 5.27 5.34 5.17 4.98 1.00 0.98

Fe(phen)212 Fe(phen)2Br2 Fe(phen)2C12 Fe(phen)2(N3)2

Fe(phen)2(NCO)2 Fe(pheh)2(HCOO)2 Fe(phen)2(CH3COO)2

Fe(phen)2(NCS) Fe(phen)2(NCSe)

[Fe(phen)3]2 Fe(phen)2(N02)2 Fe(phen)2(CNO)2 Fe(phen)2(CN)2•2H20

—8 —8 —10

0 —9

+9

[Fe(bipy)3]2 Fe(bipy)2(CN)23H20

5T29 5T29 5T29 5T25 5T25 5T2g

6 7

7 7 7 7

'2

7c 8 8 9 10 11 12 5

5T20 1Aig 1A15 1A19

'A19 'A19 5Tig

0.68 5.17 5.23 1.00

Fe(bipy)2(NCS)

Ref.

5T29 t1Ai5

0.31

Fe(bipy)2C12

Ground state

9(°)

6, 13

5T29 tA,9

14

1A,9

0.61

12a

'A,9

At low temperature other polymorphic forms with slightly different magnetic moments have been observed.

Mössbauer spectra of Fe(bipy)2(NCS)2, (Figure 4), which were measured at

room and liquid nitrogen temperatures13. The observed resonances split by quadrupole effects are characteristic of the quintuplet and singlet states respectively. The two doublets coexist only in the interval of temperature in which there is a distinct decrease in the effective magnetic moment.

The electronic spectra show essentially one band regardless of whether the complexes are high or low spin. Table 2 shows several frequencies and the 6.0

5.0

4.0

A 3.0 2.0

1,0 0 100

200

150

250

300

T, °K Figure 3. Magnetic moments versus temperature for some Fe(ti) complexes: A. Fe(phen)2 (NCS)2t"8; B, Fe(phen)2(NCSe)2'8; C, Fe(bipy)2(NCS)26

164

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

IS

(0)

.

I.

S

S

•S.,!•s•. • •• •.'.

. S.

,.4

IS.

S

Si

V 0 U

(b)

•.,..: ..

.•,

S

•

,e.

,,

'Si

-2.0

-3.0

0

-1.0.

+1.0

+2.0

+3.0

Velocity, mm/s Figure 4. Mössbauer spectra of Fe(bipy)2(NCS)2: (a), 293K; (b), 77K13

Table 2. Spectral properties of high and low spin Fe(II) complexes

Compound

Ref.

Frequency

Assignment

Aav(cm 1)

Fe(phen)2C12

{ 8470

5T2g 5Eg

9490

7b

Fe(phen)2Br2

{ 8470

5T2g

9610

7b

Fe(phen)2(NCO)2

{ 9250

5T2g -+ 5Eg

11350

7b

Fe(phen)2(NCS)2 Fe(phen)2(NCSe)2

11900 11900 10400 10400 12260 12500

5T2g 3 5Eg 5T2g 5Eg 1A1 —* 3T

11900 11900 16300 16300 18000 22470

8a 8a 8a 8a

Fe(phen)2(NCS) Fe(phen)2(NCSe)

[Fe(phen)3]2' Fe(phen)2(CN)2

(cm1)

10510 10750 11350

1A1g -+ 3T19 1A19 —÷ 3T1g 1A19 — 'T19

XOK;b insolution.

165

8a, 15, 16

12a

LUIGI SACCONI Table 3. Other hexa-coordinated Fe(ii) complexes showing anomalous magnetic behaviour Compound

Iieff(BM)

Ground state

Ref.

(room temp.)

X = C1, Br, 1

[Fe(2-pic)3]X2

4.2—5.0

3.65

[Fe(pyi)3]S043H20

4.67

[Fe(pyi)3]SO42H2O [Fe(pyi)3]C122.5H20 {Fe(pyi),]SeO4H2O [Fe(pyi)3]S2034H20 [Fe(pyi)3](SCN)22H2O [Fe(pyi)3](C104)2.2H20 [Fe(pyi)3](Bi4)23H2O

2.68

2.30 2.74 3.05

2.78

2.62

0'5.25

[Fe(pyim)3](Cl04)2 [Fe(me-phen)3]X2

X = 1,C104,

5.1—5.5

[Fe(papt)2] [Fe(pythiaz)2]X2 [Fe(HTPB)2]

X = C1O, BF

5.34—5.59

BF, BF

'T29 'T29

1A,9 1A,9

'T29

'A,9 'A,9

'T29

1A,9

5T29

'A,9 'A,9 'A,9 'A,9

'T29 'T29 5T29

18 18 18 18 19

'A,9 'T 'T29 'A,9

5.0 2.71

17 18 18 18 18

20 21 22 23

'A,9

'T9 3T,9 'T29

'A,9

pic = 2-picolylamine; pyi = 2-(2-pyridyl)imidazole; pyim = 2-(2-pyridyl)imidazoline; me-phen = 2-methyl-1,1O-phenanthroline; papt = 2-(2pyridylamino)-4-(2-pyridyl)thiazole; pythiaz = 2-4-bis(2-pyridyl)thiazole; HTPB = hydro-tns(1-pyra-

I

zolyl)borate.

assignment of these transitions, together with the estimated values of lODq for the ligands71" 8a, 15, 16• The value of 10 Dq at the crossover should fall between 11.9 and 16.3 kK and it has been estimated to be about 12.5kK. Other complexes of iron(I1) with ligands similar to phenanthroline'723 showing abnormalities in the effective magnetic moment are summarized in Table 3. Except for the complexes of 2-picolylamine'7, which show a A 0 3.0 C

2.0

1.0 120

180

240

300

T,°K Figure 5. Magnetic moment versus temperature for some Fe(u) complexes'8: B, [Fe(pyi),]C12 . 2.5H20; A, [Fe(pyi)3]S04. 31120; C, [Fe(pyi)]SeO,. H20; D, [Fe(pyi)3](SCN)2 2H20

166

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

900

-

(a)

0

0° 800 -

0 0

700 0

(b)

3000 -

•0

• Ccitc.

o Expt.

0

u 2000

>

0

.00 0 I

I

180

120

7

I 240

I

°K

Figure 6. Magnetic behaviour of [Fe(pyim)3]C12 2.5H2018. (a): AE = f(t) used for the calculation; (b): XM versus tnperature

change in 1eff with temperature similar to that already seen, the variation of the /2eff is found to be more gradual as depicted in Figure 5 for the pyridinimidazole derivative18. Attempts to calculate the variation of XM with temperature based upon a Boltzmann distribution between the 5T and 'Ajg Thble 4. Magnetic properties of some octahedral Fe(iI) complexes with a3 T1 ground state

Compound

Fe(phen)2F2 4H20 Fe(phen)2ox5H20 Fe(phen)2mal.7H20 Fe(bipy)2ox 3H20 Fe(bipy)2mal3H20 Fe(dmph)2ox4H20 Fe(dmph)2mal4H20

peff(BM)at 293°K

Weiss (°)

4.78 3.98 3.80 3.90 3.90 4.11 4.18

—15 —20 —12 —13 —9 —51

—19

Ref.

24 24,25 24,25 24 24 24 24

phen=1,1O-phenanthro!ine: bipy = 2,2-dipyridyl; dmph = 4.7-dimethyl-1,1O-phenanthroline; ox = oxalate; ma! = ma!onate.

167

LUIGI SACCONI

states separated by zlE, have given poor results8' 18• The failure of this method

of approach has been attributed to the invalid assumption that AE remains constant with temperature. The dependence of AE on temperature seemed to be supported by the structures of Fe(bipy)2(NCS)2 which were determined at 293°K and 1000K5. These showed that the bond distances Fe—N(aromatic) and Fe—NCS diminish by 0.14 and 0.08 A respectively, in passing from the quintuplet to the singlet fundamental state. For the Fe(pyridinimidazo1e) + Table 5. Some cobalt(Ii) octahedral complexes with anomalous magnetic moment

Ligand

Compound

[Co(PMI)3](BF4)2

[Co(BMI)3]12 H20 [Co(PdAdH)2]12 [Co(PBI)2]12

[Co(PPMI)2](C104)2 H20 [Co(DTPH)](C104)2 [Co(PvdH)3]Br2 [Co(GdH)3]Br2 [Co(terpy)2]F2.3.5H20 [Co(terpy)2]F2 4.5H20 [Co(terpy)2]C12 [Co(terpy)2]Cl2 H20 [Co(terpy)2]C12 .3.5H20

[Co(terpy)2]Br2 H20 [Co(terpy)2]Br2 3H20

.

[Co(terpy)2]12

[Co(terpy)2]I H20 [Co(terpy)2(C104)2

[Co(terpy)2](C104)2 H20 [Co(terpy)2](SCN)2 H2O [Co(terpy)2](N03)2

[Co(SNNMe)2(ClO4 H2O [Co(SNNMe)2]BF4

[Co(tetrapy)2]Br2 7H2O

[Co(tetrapy)2]12 4H20 [Co(tetrapy)2](NCS)2 H20

[Co(tetrapy)2(NCSe)2 H20 [Co(tetrapy)2](C104)2 H20

[Co(tetrapy)2(N03)26H20 [Co(TPT)2]C12 . 8.5HO

[Co(TPT)2](C104)2 H20

2-pyridinalmethylimine biacetylbis(methylimine) 2,6-pyridindialdihydrazone 2,6-pyridindialbis(benzylimine) 1,3-bis(2-pyridyl).azapropene l,12-bis(2-pyridyl)-1,2-l 1,12tetrakisaza-5,8-dithia-2,10dodecadiene pyruvaldihydrazone glyoxaldihydrazone 2,2',2"-terpyridine 2,2',2"-terpyridine 2,2',2".terpyridine 2,2',2"-terpyridine 2,2',2'-terpyridine 2,2',2"-terpyridine 2,2',2"-terpyridine

2,2',2"-terpyridine 2,2',2"-terpyridine 2,2',2".terpyridine 2,2',2"-terpyridine

2,2',2-terpyridine 2,2',2"-terpyridine 6-methylpyrid-2-yl.N-(methylthiophenyl)methyleneimine 6-methylpyrid-2-yl-N-(methylthiophenyl)methyleneimine 2,3,5,6-tetrakis(2-pyridyl)pyrazine 2,3,5,6-tetraKis(2-pyridyl)-

pyrazine 2,3,5,6-tetrakis(2-pyridyl)pyrazine 2,3,5,6-tetrakis(2-pyridyl)pyrazine 2,3,5,6-tetrakis(2-pyridyl)pyrazine 2,3,5,6.tetrakis(2-pyridyl)pyrazine 2,4,6-tris(2-pyridyl)1,3,5-triazine 2,4,6-tris(2-pyridyl)1,3,5-triazine

168

1(BM)

Ref.

300°K

Low temp.

4.31 2.91

2.16

2.85 3.72 3.64 2.63

2.22 2.38 1.90

27,28 27,28 28,29 28,30 28,30 28,31

4.23 3.18 2.15 4.00 4.65 2.1 4.17 2.7 2.94 3.97 3.40 4.3 3.74 4.01

— — — —

32 33 34 34

4.46

34

2.96

1.88

34 34

4.49

3.67

36

4.31

3.50

36

2.37

1.32

37

2.23

1.85

37

3.25

1.79

37

2.27

1.91

37

2.45

1.96

37

2.18

1.93

37

4.08

3.04

37

4.64

3.66

37

1.91

3.31

—

3.84

— —

2.31 2.03

—

3.23 2.82

28,35

34 28, 35

34

34 34 28, 35

28, 35

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

(Figure 6) complex the variation of itE with temperature has been assumed to be in such a way to obtain good agreement between calculated and experimental data'8 Other octahedral complexes, reported in Table 4, having magnetic moments between 3.8 and 4.8 BM and following the Curie law have been attributed24 to the 3T1g fundamental state. The effects of distortion and spin orbit coupling

5.5

5.0 -

4.5 4'

-B A 3.0

50

250

150

350

T °K Figure 7. Magnetic moments versus temperature for some Fe(II) complexes22: A, Fe(BPT)2(BF4)2; B, Fe(BPT)2(C104)2; BPT = 2,4-bis-(2-pyridyl)thiazole

are thought to be responsible for the stabilization of this state. In the case of the complexes Fe(2.4-bis(2-pyridyl)thiazole)X2 (X = C104, BF4) the existence of an equilibrium between the 3T1g and 5T2g ground states has been suggested22 in order to account for the large low temperature paramagnetic residue of about 3.3 BM (Figure 7).

Cobalt(Ii) complexes

From the Tanabe—Sugano diagram (Figure 8) we can see that the

cobalt(II) ion in an octahedral ligand field can have a 4T1g or a 2E fundamental electronic state4. Although examples of both classes of compounds are known to exist, those which are high spin are more numerous26. Of these high spin complexes, some, e.g. Co(H2O) + and Co(o-phen) , have magnetic

moments between 4.80 and 5.30 BM. For the low spin complexes, e.g. Co(NO2) - and Co(DAS) + where DAS = o-phenylene-bisdimethylarsine,

values of the order of 1.70 to 2.0 BM have been found. In addition there exist numerous examples at room temperature where /1eff is intermediate between 169

LUIGI SACCONI 50

40

30

E

U

20

Dq x io Figure 8. Energy levels for Co(ii) (d7) in °h symmetry4

those for high and low spin27—37. Several examples of this type of complex are shown in Table 5. The group of the general formula Co(terpy)2X2 nHO where X = halogen, dO4, NCS, NO3 and Co(CN)4 has been studied most thoroughly28'34' 35• As a rule, these complexes do not follow the Curie—Weiss law, as can be seen from Figure 9. The values Of/1e and their variation with temperature depend markedly on the anion and on the degree of hydration. As for iron(u) complexes, the first attempts to justify theoretically the variation of the effective

magnetic moment with temperature took into consideration a Boltzmann distribution of the Zeeman levels derived from the perturbation of the spin orbit coupling, assuming that AE (between 4T1 and 3Eg levels) remains constant with temperature. Using this assumption the agreement with the experimental data was found to be entirely unsatisfactory28'38 170

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS 5.00

4.00 - 0

a)

2.00 A

1.OOo.

100

200

300

400

7°K Figure 9. Magnetic moments versus temperature for some Co(I1) complexes: A, [Co(terpy)2](N03)234; B, [Co(terpy)2](C104)228 C, [Co(terpy)2](SCN)2 H2O3'; D, [Co(terpy)2]C12 3.5H2O34

500

400

300

200

00

0

T, °K Figure 10. Calculated plots of 1/XM versus temperature38. Curves A to E correspond to different sets of values for , a, band c parameters.

171

LUIGI SACCONI

Later Stofer and co-workers38 suggested that AE is a function of temperature and developed it into a power series of the temperature. By terminating it at the third power, they showed that the general features of the variation of with temperature may be reproduced (Figure 10). Nevertheless, the significance of this method is limited by the high number of parameters used in the calculation. A new method for the interpretation of the variation of XM with temperature for systems in spin equilibrium has been introduced by Martin and co-workers39'41'42. They suggest that the

T, °K

Figure 11. l/ versus temperature for some Co(iI) complexes40: A, [Co(terpy)2](SCN)2 . 1.5H20; B, [Co(terpy)2](C104)2 l.5H20; D, [Co(terpy)2]Br2 2H20; C, [Co(terpy)]Iz 2H20; Circles, experimental data; E, [Co(terpy)2]C12 3.5H20. full lines, calculated patterns

bond distances in the high spin form are different, and in general greater, than in the low spin species. This appears to be reasonable in terms of molecular orbital theory. In fact, the filled molecular orbitals in high spin molecules will generally be different from those occupied in low spin complexes. This means that the spin equilibrium is associated with an equilibrium between species with different bond lengths. It follows then that for spin equilibria the Tanabe—Sugano diagram is not valid around the crossover point, since low spin complexes have a different Dq from those of high spin. Taking the equilibrium constant, K, as a parameter, it is possible to calculate the variation of XM with temperature from the usual Van Vieck 172

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

formulae for high and low spin ground states. The equilibrium constant is given by the equation39 K = GC exp (— E/k7), where G and C are the ratios of the spin multiplicities and of the vibrational partition functions of the two states respectively, and AE is the energy separation, which is kept constant with temperature. The curves of XM against temperature calculated in this way are in good agreement with those obtained experimentally40, as illustrated by Figure II. Nickel(II) complexes The nickel(ii) ion in strictly °h symmetry fields always has a 3A2g funda-

mental state irrespective of the field strength43 (Figure 12). Both high and

0,

D4h

Weak

Free o

Strong distortion

distortion -

b9

e9

b

3d 'Zg

___________

Ground states: 3A2g, {lAig or 3Big]. [1A,

eg

or 3A29]

Figure 12. 3d orbital splitting and ground states for a d8 configuration in °h and D4h symmetries

low spin configurations are possible when the degeneracy of the d orbitals is removed by tetragonal distortion44. This depends on the relative values of the spin pairing energy and on the energy separation between the highest orbitals, which depend mainly on the difference between the axial and equatorial fields. When the two energy values are ir close proximity spin equilibria may exist. In Table 6 some complexes in this unusual condition are

reported. Noteworthy, among these, are complexes with macrocyclic

ligands synthesized by Busch and co-workers47' 48 These ligands determine Table 6. Tetragonal Ni(iI) complexes for which spin-equilibria have been proposed

tff(BM)

Compound Ni(detu)4C12 Ni(dbtu)4C12 Ni(dbtu)4Br2

Ni(TAAB)C12H20 Ni(TAAB)Br2.H20 Ni(CRH)12

room temp.

other temp.

1.33

2.17 0.5 0.6

2.8 1.6 1.68 1.47 2.79

K 370

—

220

1.14

0.78

—

V

163 117

—

Ref. 45 46 46

47,48 47,48 47

detu N,N-diethyi-thiourea; dbtu = N,N-dibutyl-thiourea ;TAAB= tetrabenzo[b,fj,n]1,5,9,13-tetraazacyclo-hexadecane; CR11 = 2,12-dimethyi.3,7,11,17-tetra-aza-bicycio(I 1,3,1)-heptadeca.1(17),13,15.triene.

173

LUIGI SACCONI

a relatively rigid planar structure, so that by varying the donor power of the apical atoms, Y, complexes of high, low and intermediate spin multiplicities have been obtained. On the other hand the nickel—equatorial donor atom

distance is expected to be influenced by the nature of the apical donor atoms471' and therefore the assumption that in the plane Dq is independent of these variations, is not strictly true.

1600

1LOO —

1200 -

1 000

B

800 -

C

A

600

3

4

6

5

7

8

9

10

103/T,OK-l

Figure 13. Magnetic susceptibility versus temperature for some Ni(ii) complexes: A, Ni(detu)4C1245; B, Ni(TAAB)C12 . H2047'48;

C, Ni(TAAB)Br2 HO7'48

The variation of XM with the reciprocal of temperature for several of these complexes is reported in Figure 13. Little theoretical work has been carried

out on these complexes and no attempts to interpret the experimental curves have been reported.

HVE COORDINATION The field of five-coordination chemistry is relatively new49, but nevertheless a large amount of data is now available for nickel(ii) and cobalt(u). Depending on the separation of the two highest energy orbitals, the ions with d6, d7 and d8 configurations may be for either C4,, or D31' geometry, in either the high or low spin state5° (Figure 14). Experimentally it has been shown that whereas

ligands containing soft donor atoms, such as C, P or As form low spin 174

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

Ion

d orbitoLs splitting

Configuration

Ground

[

C,,.,,

Fe -j

[

b1

Co f

[

Ni -[

e

L

D3h

Fe

[ Co -r [ e'

(e)4(b2)(a1)

(e)3(b2)(a1)(b1) (e)4(b2)2(a1)

2A1

(e)4(b2)(a1)(b1)

'A2

(e )4(b2)2(a1)

A1

( e )(b2.)2(a1)(b1)

3B1

(e")4(e')2

3A

(e'i3(e')2(aç)

5E"

(e")(e')3

2E'

(e")'(e')2(oç) (e")4(e')4

Ni L

(e")4(e')3(a)

Figure 14. Electronic configurations for Fe(u) (d6), Co(ii) (d7) and Ni(ii) (d8) in five-coordinate complexes with C4 and D3h symmetries

spin complexes are usually obtained with hard 57—60 In both types of complexes we find S, Se, donor atoms like N or halogens and isothiocyanate as donors. If the total strength of the donor set is progressively increased one passes from the high to the low spin region so locating the crossover point49'6 1• Unfortunately, however, it is impossible complexes49' 51—56, high

ix

Me X,Y, V. Z: Donor atoms

Aliphotic oraromatc chains Figure 15. Skeletons of tn- and tetra-dentate ligands giving five-coordinate complexes

175

LUIGI SACCONI

in practice to find a large number of strictly C4 and D3h symmetrical chromo-

phores with different donor atoms, since five-coordinated complexes are formed preferentially with polydentate ligands; besides, with monodentate ligands solid state forces often distort the geometry49'6 62—6

The polydentate ligands having the greatest capacity for forming five-

coordinated complexes are tn- and tetra-dentate ligands of the type illustrated in Figure 15. Considering tripod-like tetradentate ligands it is reasonable to 66

(Ni(TAP)CNJ

ICo(Me6tren)BrJ

JN

_Ni ,

AsZ

Co I

Nap—Co—Br

1800

As 178°

P—Ni—C

Eq. angles 120°

LCo(QP)Cl)

69

Eq.ang1es119° 68

67

(Ni(NP3) 11+

I

Ct

Co—Ct 171°

N—Ni—I 180°

Eq. angles = 108, 113, 137°

Eq. angLes 120°

Figure 16. Typical structures of five-coordinate complexes with tetradentate ligands

expect a generally tngonal bipyramidal structure, as shown in Figure 16 for the complexes Co(Me6tren)Cl66, Ni(QP)C167, Ni(NP3)168 and Ni(TAP)CN69.

However, with tridentate ligands, an intermediate structure between a tetragonal pyramid and a trigonal bipyramid is expected as found for the Co(Me5den)C1270, Ni(PNP)Br27 1, Ni(SPS)C1272 and Ni(As3)(As2)73 cornplexes (Figure 17). In order to correlate the spin stafte with the nature of the donor atoms it may

be assumed, to a first approximation, that: (1) the geometries are the same for the two classes of compounds and independent of the donor atoms; (2) variations in symmetry, caused by different donor atoms in the same 176

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

chromophore, have little effect on the spin multiplicity of the ground state74. In several cases where detailed structural information is available, it has been possible to correlate spin state with variation in geometry. Iron(iI) complexes

Very few five-coordinatcd complexes of iron(II) have been reported

(Table 7). However, chromophores of the type Fe(P4)X where X = halogen and P4 is a tripod-like ligand containing four soft P atoms75 are low spin

with a triplet ground state, while chromophores with four hard nitrogen donor atoms are high spin76. The substitution of a single P atom in P4 with a nitrogen donor is sufficient to give a high spin complex with a quintuplet ground state.77. The crossover point for trigonal bipyramidal complexes Table 7. Trigonal bipyramidal Fe(ii) complexes77

Spin state

Chromophore [Fe(P4)X] + [Fe(NP3)X] + {Fe(N4)XJ +

X = C1 Br, I X = Cl, Br,! X = Br

Low spin (triplet) High spin (quintuplet) High spin (quintuplet)

must therefore occur between the P4 and the NP3X donor sets. Pentacoordinate complexes of iron(ii) with tridentate ligands showing spin equilibria have been recently prepared78a. Cobalt ('I) complexes Complexes with trident ate ligunds

The chromophores, CoN3X2, obtained with diethylenetriamines58'70' are high spin (Figure 18). Complexes of N-diphenylphosphinoethyl-N'-diethylethylenediamine, with an N2PX2 donor set, show analogous magnetic behaviour77. When X = NCS the complexes have a magnetic moment of 3.94 BM at room temperature which varies reversibly from 78b,

2.16 at 77°K to 4.32 BM at 418°K80. This behaviour has been rationalized8' in terms of a spin equilibrium between the 2A2 and 'A1 states in C4 symmetry

according to Martin's method39 (Figure 19). This equilibrium can be followed in the reflectance spectra measured between —160°C and + 90°C,

illustrated in Figure 20. The assumption that the doublet and quadruplet states have different bond lengths is supported by the infra-red spectra (Figure 21), which show two N—H vibrations whose intensities are temperature dependent80. When two P atoms are present in the ligand, i.e. NP2X2 as donor set, the spin multiplicity depends not only on X but also on whether

the nitrogen is aromatic or aliphatic82' 83 In fact, with 2,6-bis(-2-diphenylphosphinoethyl)-pyridine, the chiorocomplex is high spin, the iodo-complex is low spin and the bromo-complex is spin isomeric with a doublet-quadruplet temperature dependent equilibrium82. If the nitrogen atom is aliphatic, as in bis(2-diphenylphosphinoethyl)R-amine, the complexes are low spin". The crossover point occurs therefore, when the donor sets are N2P(NCS)2 and P2NBr2. 177

LUIGI SACCONI Ni (PNP)Br

Co(Me5den) C1°

Br 2.7

io

'

Br

Ni(SPS) 12

Ni(As)3 (As)3

2.8

Asj--As ) ' Ni Ni—As —2.3

Figure 17. Typical structures of five-coordinate complexes with tridentate ligands

Complexes with tetradent ate ligands

Table 8 shows the donor set of tripod-like tetradentate ligands which form five-coordinate complexes. Departing from the N4 high spin forming

set84 and introducing progressively less electronegative atoms one passes8 589 to the P4, low spm forming set90. In this way the crossover point is

found ardUnd the ligand NP3 (Figure 22). All the chioro- and bromo-

complexes with this ligand are high spin but the isothiocyanate and cyanate derivatives are low spin. The chromophore CoNP3I can be high or low spin, Ligand

Spin state

(R)2 NNN ( R)2 (Ph)2

PNN (Et)2

1] /J\

(Ph)2P

N

=H

X = NCS

X=C1 X

Br

P(Ph)2

x=

P(Ph)2

R = H,Me X = Br, I,NCS

R

(Ph)2P

::

Ref.

HS

HSLS HS HS—LS

82

LS

LS

Figure 18. Five-coordinate CoLX2 complexes with tridentate ligands

178

83

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

4.5 B A

4.0 3.5

3.0 2.5 2.0

200

400 300 1, °K Figure 19. Magnetic moments versus temperature for the Co(nnp) (NCS)280'8' complex: 100

A, calculated; B, experimental

2000 1500

Wavelength, 600

m

1000 800

500

400 D

d U

U,

> 0

/ // / /

.0

'I

D C

B A

5

10

15

20

25

30

Frequency, kK

Figure 20. Temperature dependence of solid state spectra of the Co(nnp) (NCS)2 complex80: A, 168°K; B, 223°K; C, 294°K; D, 338°K; E, spectrum of the five-coordinated (5-CI-SALenNEt2)Co complex

179

LUIGI SACCONI

'I1/ G

C—N stretch.

298

F 195

U

N—H stretch

U,

.ci

d 373 °K

U C

0

298\//'

E

77 °K

E

U)

C

0

195 °K B

77

3300 3200 3100 "

2120

2100 2080 2060201.0 2020

V, cm1 Figure 21. Temperature dependence of infra-reci spectra of the Co(nnp) (NCS)2 complex8°

Donor set NP3

Counterion X

Spin state

V

Cl

high

Br

high high

[I,PF6, BF4 NCS CN

tow low

ow

Figure 22. High and low spin [Co(NP3)X]Y complexes89

180

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

/PN1 -N

P

p

Br [Co(NP3)11191 Low spin

[Co(NP3)Br]PF693 High spin

Figure 23. Geometry of the low and high pin [Co(NP3)X] chromophores

Wavelenth, 2000 I

1500

1000

I

I

m 700

800

[Co(NP3)r] BPh4

High spin

150 ICo(NP3)I] I Cu

in nitroethane soin at various 7°C 100

50 [Co(NP3)I J I Low spin -

5.0

7.5

10.0

12.5

Frequency, Figure 24. Spectra of [Co(NP3)I] chromophores7789

181

15.0

LUIGI SACCONI Table 8. Five-coordinated [CoLX] + complexes with tetradentate tripod-like ligands

Ligand donor set

Spin state

Ref.

.

84 85

N4

NO+_x = lto3 NS4_x = lto3 N3P N3As N2OP NOP2 N2SP N2P2 NP3 P4

high spin

high and low low spin

85,86 87 87 77 88 77 87 89 90

depending on the nature.of the counterion89 which evidently modifies the lattice forces in such a way as to introduce a modification in the geometry, therefore causing a variation in the spin state. Structural x-ray investigations have shown (Figure 23) that whereas the low spin [Co(NP3)I]I is tetragonal pyramidal9t, the high spin [Co(NP3)I]BPh4 is trigonal bipyramidal, like

the chloro92 and bromo'93 analogues. This proves that the tetragonal pyramid favours the low spin state with respect to the trigonal bipyramid, as has been established from crystal field calculations62. In nitroethane (Figure 24) a temperature dependent spin doublet-quadruplet equilibrium

exists which presumably accompanies a conformational equilibrium between the two geometries89. The tendency for spin pairing seems to follow the series Cl = Br > I > NCS, which is, for halogens, contrary to that expected from the Dq values. Nickel(II) complexes Complexes with trident ate ligands and As356 For nickel(iI) complexes the ligands with sets give high and low spin complexes respectively, as shown in Figure 25.

The sets of donor atoms NNP, NNAs, ONP, ONAs, SNP and SNAs,

obtained from the ligands of type II, give high spin complexes with nickel bromide and iodide, except for Ni(NNP)12 and Ni(SNP)12, which are dia-

magnetic96. The crossover point seems to occur near the NiPNPCI2 chromophore (N aromatic) which is reported to show a singlet—triplet temperature dependent equilibrium97. When the chlorine atoms are replaced by bromine or iodine, or when the nitrogen in the PNP set is aliphatic as in

ligand 1V83, the complexes are low spin in common with the complexes V—V11198' For the compound exhibiting spin equilibrium, the variation of the magnetic moment with temperature has been rationalized in terms of a molecular distribution between the 3E1 and the 'A1 levels in D3,, symmetry separated by 300 cm (Figure 26)81. Complexes with tetradent ate ligands

Table 9 shows the following sets of donor atoms which give rise to high spin complexes84'87: N202X, N3OX, N4X, N3SX, N2S2X, NS3X, N3AsX 182

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS Spin

Ligand

state

Me N

1 (Me)2N

X C1,Br,l

N(Me)2

L=P,As rThL(Ph)2 Y=O CHN YNH L=P

2 Me 3 (Fh)2F

N

vs RF-{

N H

P(Ph)2

5(Ph)2As

N

As(Ph)2

/\

6 (Ph)2P

S

P(Ph)2

7(Ph)2As

S

As(Ph)2

8(Me)2As

As As (Ph)2

X Br

XI

XBr,I XBr x=l

94,95

high high Low

high

96

high Low

X=Br,I

Low

97

X=Br,I

Low

low

83

XI

low

98

xI

low

98,99

low

98

XCL,Br,1 low

56

xl

rTh

59, 78,

XCL high1ow

XI

RMe

4 (Ph)2P

/—\

-

I _ID

P(Ph)2

R

/__\

L=As

Y=S

X=Br,I

high

Ref.

Figure 25. Five-coordinate NiLX2 complexes with tridentate ligands

500

400

>

300 B

200 100

I 150

A

I

200

250

T°K Figure 26. l/XM versus temperature for the Ni(PNP)C12 complex: A, experimental97; B, calculated81

183

LUIGI SACCONI Table 9. Five-coordinated [NiLX] + complexes with tetradentate tripod-like ligands

Ligand donor set

Spin state

Ref. 84 85

N4

NO4_x=2,3 NS4x =1,2,3

85,86 87 87 87 88 98 77 89 90

high

N3P N3As

I

J

N2P2

NOP2

low

NAs3 NOAs2 NP3 P4

and N3PX, while the sets P4X, PAs3X, NP3X and N2P2X give low spin coniplexes77'8789 98-4o1• The crossover region, therefore, occurs between the N3PX and N2P2X sets.

Factors governing the spin state

The inadequacy of Dq values to correlate the spin pairing tendency of various donor atoms stimulated us to look for some other more suitable parameters. In this regard the nucleophilic reactivity constant n° for the donor groups as defined and determined by Basolo and Pearson'°2, has been proposed by us as a parameter. This constant is related to polarizability Table 10. Overall nucleophilic reactivity constants for five-coordinated Ni(II) complexes with tridentate ligands74

Set: ONNX2, NONX2, NNNX2, SNNX2, NSNX2. ONAsX2, NNPX2, NNAsX2, ONPBr2, SNAsBr2

n0 32.45, low spin.

and to the nephelauxetic series and also includes the çffects of electron delocalization and it bonding; thus n° may be considered as a measure of the 'softness' of the donor atoms. 184

CONFORMATIONAL AND SPIN STATE INTERC0NVERSIONS Table 11. Overall nucleophilic reactivity constants for five-coordinated Co(iI) complexes with tridentate ligands74 Set: NONX2, NNNX2, NSNX2, NNPX2, SNNX2 En° 33.15, low spin. X = Cl, Br, I, NCS

Tabulation of En° values for the sets of five donor atoms show that there is generally a distinct separation between high and low spin complexes for both nickel and cobalt in both types of geometries74. The high spin nickel series of complexes with D3X2 (Table 10), as donor set, goes to a value of 25.42 for En°, which corresponds to the SNAsI2 set; all the complexes above this value are high spin. An exception is the SPSC12 set which forms low spin nickel complexes but is situated in the high spin region. The crossover point then can be placed at the value of 25.5 for En°. Similarly (Table 11), the crossover point for cobalt can be placed between 28 and 29, i.e. between the two NNP(NCS)2 and PNPBr2 sets both of which are spin isomeric. Table 12. Overall nucleophilic reactivity constants for five-coordinated Ni(Ii) complexes with tetradentate ligands74 Set: N2O2X, N3OX, N4X, N3SX, N2S2X, NS3X, N3PC1:

Th° 26.82, low spin

X = Cl, Br, I, NCS.

Table 13. Overall nucleophilic reactivity constants for five-coordinated Co(ii) complexes with tetradentate ligands7' Set: N202X, N3OX, N4X, N3SX, N2S2X, N3AsX, NS3X, N3PX, N2OPX, N2SPX, N2P2X En° < 32.51, high spin Set

Zn°

Spin state

NP3C1

32.51

NP3Br

33.65

high high

Set

NP3I NP3NCS

Set: P4X:En° > 36.12, low spin. X = Cl, Br, I, NCS.

185

Zn°

Spin state

34.89 36.12

low, high

low

LUIGI SACCONI

Nickel complexes with D4X donor set (Table 12), are high spin up to

n0 = 22.31, which corresponds to the N2S2NCS set. The following PS3C1 set with n° = 26.02 begins the low spin region. Thus the crossover point

can be placed between these two values. For cobalt complexes (Table 13), the

crossover point is close to a value of 34.89, corresponding to the CoNP3I chromophore which is found to be both high and low spin. The influence of the geometry on the spin state

As already suggested the n° criterion is only considered to be valid for complexes having the same geometry. Variation in the geometry will change

the relative energy of the d orbitals and thus of n0 corresponding to the

-ii7 Figure 27. Distortion by elongation in a square pyramidal chromophore

crossover point. The fact that the chromophore CoNP3I is a tetragonal pyramid when low spin and a trigonal bipyramid when high spin indicates that the first geometry favours the low spin configuration. Furthermore, within the same stereochemistry geometrical distortions may have a decisive influence on the spin state. By means of ligand field calculations, Venanzi et al. have shown that a C2,, distortion in a trigonal bipyramid stabilizes the high spin states'°3.

oNi

1.3 -

ssr2 1.2

-

oCo

\oNOP2I

lcixl 'bcis

a

NOP2o\ N203

o oN4Br

PN PBr2—°\

N2CL3

1.1

LII N203

—

\

05

N4C[

0

N302

N /oN3C[2 NP3I(Co) 05N5° .N2S3

1.0 -

N3C12

N3CL

N5

0.9

95

0

100

S4NO 105

Apical angLe,a

Figure 28. Dependence of elongation on apical angle for square pyramidal geometries77

186

CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS

Numerous x-ray structures now available have shown that many tetragonal pyramidal complexes of cobalt and nickel are distorted towards a square planar geometry. This distortion corresponds to an elongation in the axial distance accompanied by a lowering of the metal atom, which normally lies above the basal plane, towards the plane itself (Figure 27). Concurrently,

this lowering corresponds to a decrease in the apical angle, that is, in the angle formed by the axis of the pyramid and the bonds between the metal and the four basal donor atoms. This angle thus tends towards 900. Figure 28 shows a plot of the relative elongation of the square pyramidal five-coordinated nickel and cobalt complexes against the apical angle. The

relative elongation is the ratio between the observed axial bond distance (lax) and the average basal bond distance found for the same donor atom Ubas) This figure shows that an increase in the axial ratio corresponds to a decrease in the apical angle and vice versa. A large portion of chromophores

are found to have an apical angle of 1000 and an elongation of one. This appears to be the standard geometry for tetragonal pyramidal coordination. At the top and to the left another group of donor sets is clustered, namely SPS1272, N0P21104, N4Br105 and N203106, which correspond to elongated pyramids. Such an elongation brings about a stabilization of the low spin levels with respect to the high spin levels both for nickel'°7 and cobalt'°8. This distortion NiN4Br chromophores

iiT

N

NI

I

-

flN Br

±.

.66 [Ni (Me6 tren) Br I high spin

.105

. (Ni(Macroc.Ligand)Br]+ : Low spin .

CoN2O3 chromophores

___

N

(_o _°—'

o

2.2

Co(SaL— NMe)2 : high spin 109

CoS&.—en:tow spin 106a,b

Figure 29. Influence of geometry on the spin state

187

LUIGI SACC0NI

may even induce a low spin ground state in complexes which, from the nature of the donor set, would be expected to be high spin. This is found to be the case for the NiN4Br and CoN2O3 chromophores. The N4Br set is

well within the high spin region, and accordingly the (NiMe6trenBr) complex66 shown in Figure 29 is high spin, but the nickel complex with the same donor set, Ni(macrocyclic ligand)Br 105, is low spin. Evidently the large nickel—bromine distance (2.79 A) decisively weakens the perturbation of bromine on the metal orbitals (Figure 29). The N203 set strongly favours the high spin state and, in fact, the dimeric

complex, CosalMe(set N203) is high spin'08. The complex, 'Cosalen', with the same donor set, but highly distorted towards a square planar geometrylo6a, is low spin1061' (Figure 29). The n0 criterion may then be considered a reasonable method for determining whether a complex is falsely or truly five-coordinated. In the high spin trigonal bipyramidal cobalt complexes another type of distortion, i.e. a tetrahedral distortion, often occurs. Here the cobalt atom is clearly removed from the equatorial plane and approaches the apical atom X, as shown in Figure 30. So the three X—Co-—equatorial donor atom

NiN 990

0i,P

Br

Co—N0z 2.10 11O

CL

Co—No 2.3

h'105°6°

X

XoCI; Co—N 2.67 92

XBr; Co—N2.76 Figure 30. Tetrahedral distortion in trigonal bipyramidal Co-complexes

angles become larger than 900. This distortion is hardly perceptible in the Co(Me6tren)Br complex109 (where the apical angle Br—Co—N is 99°) but it is clearly evident in Co(NP3)C192, Co(NOP2)77 and in the Co(NP3)Br93

complexes where the X—Co—D apical angles are 104—106° and the CONapicai distance goes up to 2.7 A.

This tetrahedral distortion probably favours a high spin configuration and could be decisive in determining the spin state even when the chromophore has a n° value corresponding to a low spin state, as does the Co(NP3)I chromophore.

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B. N. Figgis and J. Lewis, Modern Coordination Chemistry, p 415. Interscience: New York

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CONFORMATIONAL AND SPIN STATE INTERCONVERSIONS Y. Tanabe and S. Sugano, J. Phys. Soc. Japan, 9, 753, 756 (1954). E. Konig, Coord. Chem. Rev. 3, 471 (1968). 6 w A. Baker Jr and H. M. Bobonich, Inorg. Chem. 3, 1184 (1964). E. Konig, A. S. Chakravorty and K. Madeja, Theoret. Chim. Acta, 9, 171 (1967). K. Madeja and E. Konig, J. Inorg. Nuci. Chem. 25, 377 (1963). K. Madeja, W. Wilm and S. Schmidt, Z. Anorg. Ailgem. Chem. 346, 306 (1966). 8u E. König and K. Madeja, Inorg. Chem. 6, 48 (1967). E. König and K. Madeja, Chem. Commun. 61(1966). L. Cambi and A. Cagnasso, Atti Accad. Lincel, 19,458 (1934); S. Sugden, J. Chem. Soc. 328 (1943); H. Irving and D. H. Mellor, J. Chem. Soc. 5222 (1962). '° E. Kdnig, unpublished results; K. Madeja, Chem.Zvesti, 19, 186 (1965). " W. Becuand and E. Schnierer, Chem. Ber. 95, 3048 (1962). 12u A. A. Schitt, J. Am. Chem. Soc. 82, 3000 (1960). W. A. Baker Jr and H. M. Bobonich, Jnorg. Chem. 2, 1071 (1963); K. Madeja and E. König, J. Jnorg. NucI. Chem. 25, 377 (1963). 13 E. Konig, K. Madeja and K. J. Watson, J. Am. Chem. Soc. 90, 1146 (1968). 14 F. H. Burstall and R. S. Nyholm, J. Chem. Soc. 3570 (1952); S. Sugden, J. Chem. Soc. 328 (1943). 15 R. A. Palmer and T. S. Piper, Inorg. Chem. 5, 864 (1966). 16 E. König and H. L. Schiafer, Z. Phys. Chem. (Frankfurt), 34, 355 (1962). 17 G. A. Renovitch and W. A. Baker Jr, J. Am. Chem. Soc. 89, 6377 (1967). 18 R. J. Dosser, W. J. Eilbeck, A. E. Underhill, P. R. Edwards and C. E. Johnson, J. Chem. Soc. A, 810 (1969).

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