Extended Interactions between Metal Ions

9 Structural and Magnetic Properties of Chromium(III) Dimers

Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0005.ch009

DEREK J. HODGSON U n i v e r s i t y of N o r t h C a r o l i n a , C h a p e l Hill, N.C. 27514

Introduction A number of s t r u c t u r a l i n v e s t i g a t i o n s in our l a b o r a t o r y and elsewhere have demonstrated that complexes of stoichiometry [Cu(L)OH]22+, where L is a bidentate l i g a n d , c o n t a i n a dimeric u n i t i n which two copper(II) centers are bridged by two hydroxo groups (1-5), and much of our recent research has been d i r e c t e d towards the c o r r e l a t i o n of the s t r u c t u r a l and magnetic p r o p e r t i e s of dimers of t h i s type (6,7). We have r e c e n t l y extended these s t u d i e s to chromium(III) complexes of the type [Cr(L) OH]2n+, where L i s again a bidentate l i g a n d , and i n t h i s paper I d e s c r i b e the r e s u l t s of our work i n t h i s area. The s t r u c t u r e s with which we are concerned are molecules or ions of the type shown i n f i g u r e 1, i n which we have two chromium ( I I I ) centers which are bridged by two hydroxo groups i n a planar array; the remaining c o o r d i n a t i o n s i t e s of the chromium octahedron are occupied by the bidentate l i g a n d s . For a symmetric bidentate l i g a n d , there are two p o s s i b l e geometries f o r t h i s dimer: if, i n f i g u r e 1, atoms AN(1) and AN(10), BN(1) and BN(10), e t c . form the c h e l a t e r i n g s , the dimer l a c k s an i n v e r s i o n center but has approximately D symmetry, but i f , f o r example, the c h e l a t i o n at C r ( l ) i s changed to AN(1)-BN(10) and BN(1)-AN(10) while that at Cr(2) i s unchanged the dimer has an i n v e r s i o n center and approximates C symmetry. Each of these geometries i s found. I t should be noted, however, that i n both of these geometries we maintain a planar b r i d g i n g u n i t and octahedral c o o r d i n a t i o n at the metal. Sinn (8) and G l i c k (9) have noted the i n f l u e n c e of the geometry at copper on the magnetic i n t e r a c t i o n s i n copper dimers, but here we are able to keep the geometry at the metal center approximately constant. 2

2

2h

The magnetic p r o p e r t i e s of these systems have been examined by my colleague, Professor W.E. H a t f i e l d . The Van Vleck equation f o r exchange coupled C r ( I I I ) ions (S = 3/2,3/2) can be w r i t t e n (10) as

94 Interrante; Extended Interactions between Metal Ions ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

9.

Chromium(IIl)

HODGSON

2

2

Ng $ kT

Dimers

2 exp(2J/kT) + 10 exy(6J/kT) + 28 exy(12J/kT) 1+3 exp(2J/kT) + 5 exp(6J/kT) + 7 exp(12JA!T)

95

(1)


where J i s the exchange coupling constant and -2J represents the energy d i f f e r e n c e between the s i n g l e t ground s t a t e and the t r i p l e t f i r s t e x c i t e d s t a t e . As a r e s u l t of the presence of a manif o l d of r e l a t i v e l y low-lying paramagnetic e x c i t e d s t a t e s , however, i t has been suggested (11) that the Van Vleck expression should be modified by the i n c l u s i o n of b i q u a d r a t i c exchange.This gives r i s e to the Hamiltonian

H

Q

z

=

-2J(S S )-é(S -S ) V

2

l

2

(2)

and the expanded expression becomes m

X

m

NgH

2

χ

kT

(3)

2 exp[(2eT-6.5j)/feff}f 10 exp[ (6J-13.5.1)/kT] + 28 exp[ (12e7-9.7)/kT] 1.0+3 exp[(2J-6.5j)/kT] + 5 exp [ (6J-13.5j) /kT]+7 exp [ (12J-9j)/kT] In t h i s modified form of the Van Vleck equation, the energy se p a r a t i o n between the s i n g l e t ground s t a t e and t r i p l e t f i r s t e x c i t e d s t a t e , ΔΕ, i s -2J + 6.5j. Since i t i s our aim to c o r r e l a t e s t r u c t u r a l and magnetic p r o p e r t i e s , t h i s d i s c u s s i o n deals only with complexes whose s t r u c t u r e s have been p r e c i s e l y determined and does not include the l a r g e number of complexes (12-16) f o r which only magnetic data are a v a i l a b l e . G l y c i n a t o Complex The f i r s t s t r u c t u r e which was determined was that of the g l y c i n a t o complex [Cr(gly)2OH]2 > which c r y s t a l l i z e s i n the mono c l i n i c space group Ρ2χ/η with two dimers Jn a c e l l of dimensions a = 5.691(3), b= 16.920(9), a - 7.900(4) A, and 3 - 79.90(3)° (17,18). With only two dimers i n the c e l l , i t i s apparent that there must be an i n v e r s i o n center i n the middle of the dimer, and that the molecule must be of the approximately type; an examination of the s t r u c t u r e , which i s shown i n f i g u r e 2, v e r i f i e s t h i s conclusion. The Cr-Cr and 0-0 separations i n the b r i d g i n g u n i t are 2.974(2) and 2.575(6) A, r e s p e c t i v e l y , and the s i m i l a r i t y of the two independent b r i d g i n g Cr-0 bond lengths of 1.966(4) and 1.968(4) A demonstrates that the b r i d g i n g i n t h i s u n i t i s symmetric. The s t r u c t u r a l parameter of greatest i n t e r e s t i s the value of the Cr-0-Cr b r i d g i n g angle, φ, and i n t h i s case i t i s 98.2(2)° (18).

Interrante; Extended Interactions between Metal Ions ACS Symposium Series; American Chemical Society: Washington, DC, 1974.


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IONS

Figure 1. Coordination about chromium(III) centers in a typical dihydroxo-bridged dimer. O(l) and G(2) are the oxygen atoms of the hydroxo bridges. Chelate rings are formed by joining AN(1) to AN(10), BN(1) to BN(10) etc. Data are for the [Cr(phen) OH], cation in [Cr(phen),OU],l, · 411,0. y

r

4

Inorganic Chemistry

Figure

2.

View

of the [Cr(gh/)^OH] atoms omitted

j molecule (18)

with

hydrogen


9.

Chromium(III)

HODGSON

Dimers

97

The low temperature magnetic s u s c e p t i b i l i t y data f o r [Cr( g l y ) O H ] are shown i n f i g u r e 3, i n which the dashed l i n e r e p r e sents the best l e a s t - s q u a r e s f i t to the unmodified form of the Van V l e c k expression (equation (1)) while the s o l i d l i n e represents the best l e a s t - s q u a r e s f i t to equation (3). I t i s evident t h a t , i n t h i s case, the observed s u s c e p t i b i l i t y data are much more r e a d i l y approximated by the s o l i d l i n e , i.e. the i n c l u s i o n of b i q u a d r a t i c exchange i s s i g n i f i c a n t i n t h i s case. The magnetic s u s c e p t i b i l i t y of [ C r ( g l y ) O H ] maximizes near 20°K. The l e a s t - s q u a r e s f i t t i n g process leads to values of 2J = -7.4 cm~l and j = 0.04 cm~l, or ΔΕ = -10.0 cm~l. These values are i n good agreement with the value of 2J p r e d i c t e d by Earnshaw and Lewis (12) on the b a s i s of high temperature s u s c e p t i b i l i t y data. 2

2


2

2

Phenanthroline Complexes The second complex whose s t r u c t u r e was determined was the 1,10-phenanthroline complex [ C r ( p h e n ) 0 H ] C l i * 6 H 0 . T h i s complex c r y s t a l l i z e s i n the t r i c l i n i c space group PI with two dimers i n a c e l l of dimensions a « 14.056(7), b = 11.296(6), ο = 18.990(9) A, α = 87.15(3), 3 = 107.63(2), and γ = 74.68(3)° (19). With two d i mers i n P I , no c r y s t a l l o g r a p h i c symmetry i s imposed on the system, and t h i s s t r u c t u r e i s an e i g h t y atom problem, not counting the hydrogen atoms! The s t r u c t u r e of the c a t i o n i s shown i n f i g u r e 4, and i t i s apparent that t h i s i o n c l o s e l y approximates Z? symmetry; there i s no i n v e r s i o n center, but there are three approximate two f o l d axes. I f the dimer attempted to adopt the approximately C h geometry found i n the g l y c i n a t o complex, there would be very se vere proton-proton i n t e r a c t i o n s across the dimer, e.g. between the phenanthroline group l a b e l e d G2 and that l a b e l e d G3. Hence, f o r the bulky phenanthroline l i g a n d , only the Z? geometry i s s t e r i cally feasible. The c o o r d i n a t i o n about the chromium(III) atoms i s shown i n f i g u r e 5. The Cr-Cr s e p a r a t i o n of 3.008(3) A i s a l i t t l e l a r g e r than that i n the g l y c i n a t o complex, and t h i s change i s due to an increase of approximately 4.5° i n the value of the b r i d g i n g angle, φ. Thus, i n t h i s phenanthroline complex the average value of φ i s 102.7°, while i n the g l y c i n a t o complex φ i s 98.2° (vide supra). The low temperature magnetic s u s c e p t i b i l i t y of [ C r ( p h e n ) OH] C l i · 6H 0 e x h i b i t s a maximum near 110°K, and the best l e a s t squares f i t to equation (3) gives a value of ΔΕ of approximately -55 cnT^ (20). S i n g l e c r y s t a l epr examinations of t h i s complex are c u r r e n t l y nearing completion. The magnetic data f o r t h i s c h l o r i d e s a l t of the phenanthro l i n e complex are of c o n s i d e r a b l e i n t e r e s t s i n c e , on the b a s i s of high temperature measurements, Earnshaw and Lewis (12) have c a l c u l a t e d that the corresponding i o d i d e s a l t has a ΔΕ of a p p r o x i mately -14 cm~l. Hence, i f our contention that the magnetic pro p e r t i e s of di-hydroxo-bridged dimers are p r i n c i p a l l y determined by the geometry of the bridge were c o r r e c t , i t appeared that the geo metry of the i o d i d e s a l t must be c o n s i d e r a b l y d i f f e r e n t from 2

2

+

2

c

2

2

2

2

2

+

2


98

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BETWEEN

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IONS


04

Inorganic Chemistry

Figure 3. Temperature variation of the magnetic susceptibility of [Cr(gly) OH] . Dashed line represents the best jit to equation (1); solid line represents the best fit to equation (3) (see text) (18). É

Â

Acta Crystaltographica

Figure 4. View the cation in [Cr(phen) OH] Ch, • 6H 0 (19) 2

2


2

of

9.

Chromium(III)

HODGSON

Dimers

99

that of the c h l o r i d e . Hence, we a l s o examined the s t r u c t u r e of the i o d i d e s a l t , [ C r ( p h e n ) 0 H ] I i f 4 H 0 . The i o d i d e s a l t a l s o c r y s t a l l i z e s i n the t r i c l i n i c space group PI with two dimeric u n i t s i n a c e l l of dimensions a = 11.464(12), b = 9.893(11), ο = 22.757(25) Â, α = 90.06(2), 3 = 93.04(2), and γ = 82.82(2)° (21). The s t r u c t u r e of the c a t i o n , which i s shown i n f i g u r e 6, i s very s i m i l a r to that found i n the c h l o r i d e s a l t , with the bulky phenanthroline l i g a n d s again f o r c ing the dimer to adopt the roughly £> geometry. The inner coor d i n a t i o n sphere f o r t h i s dimer i s shown i n f i g u r e 1$ and a com p a r i s o n of t h i s f i g u r e with f i g u r e 5 demonstrates that the [Cr(phen) 0H] u n i t s i n these two s a l t s are s t r u c t u r a l l y sub s t a n t i a l l y s i m i l a r (21). Thus, f o r examgle, the Cr-Cr s e p a r a t i o n and Cr-O-Cr b r i d g i n g angle of 2.986(4) A and 102.2(2)°, r e s p e c t i v e l y , i n the i o d i d e s a l t probably do not d i f f e r s i g n i f i c a n t l y from the values (19) of 3.008(3) A and 102.7(5)° i n the c h l o r i d e analog. T h i s s t r u c t u r a l r e s u l t i s , c l e a r l y j i n c o n s i s t e n t with the magnetic p r o p e r t i e s reported by Earnshaw and Lewis (12), and so we have reexamined the magnetic s u s c e p t i b i l i t y of the i o d i d e s a l t (22). The low temperature s u s c e p t i b i l i t y data are shown i n f i g u r e 7, i n which the s o l i d l i n e represents the best f i t to the Van Vleck equation modified by the i n c l u s i o n of b i q u a d r a t i c exchange. The magnetic s u s c e p t i b i l i t y of [Cr (phen) 0H] Ii «4H 0 i s seen to maxmize near 110°K, and the l e a s t - s q u a r e s f i t to equation (3) y i e l d s 2J = -43.8 cm" , j = +1.5 cm , and ΔΕ - -53.6 cm" (22). These values are very s i m i l a r to those obtained (20) f o r the c h l o r i d e but are s u b s t a n t i a l l y d i f f e r e n t from the value of ΔΕ = -14 cm"" reported by Earnshaw and Lewis (12). Moreover, the sim i l a r i t y between these r e s u l t s and those f o r the c h l o r i d e s a l t i s c o n s i s t e n t with the s i m i l a r i t y of the two s t r u c t u r e s noted above. I t i s , however, noteworthy that while the magnetic pro p e r t i e s of the corresponding n i t r a t e s a l t , [Cr (phen) 0H] (N03)i* · 7H 0, with values of 2J - -42.2 cm" , J = 0.0 cm" , and ΔΕ -42.2 cm" (23) are s u b s t a n t i a l l y s i m i l a r to those of the c h l o ide and i o d i d e s a l t , the bromide s a l t , [Cr(phen^ OH^ Br^ ·8Η2θ, apparently undergoes a weaker i n t e r a c t i o n with values of 2J = -29.2 cm" , J = 0.5 cm" , and ΔΕ - -32.5 cm" (23). I t would appear, t h e r e f o r e , that the c a t i o n i n the bromide s a l t may i n deed be s t r u c t u r a l l y d i f f e r e n t from that i n the c h l o r i d e and i o d i d e cases, but no s t r u c t u r a l data are a v a i l a b l e to confirm or deny t h i s hypothesis. e

2

2

2

2

i++


2

2

2

1

2

+

2

-1

1

1

2

1

2

1

2

1

1

1

1

Oxalato Complex The f i n a l s t r u c t u r e of t h i s type, which has r e c e n t l y been completed i n our l a b o r a t o r i e s , i s that of the oxalato complex Na [ C r ( 0 X ) 0 H ] * 6 H 0 . T h i s m a t e r i a l c r y s t a l l i z e s i n the mono c l i n i c space group P 2 j / c with four dimers i n a c e l l of dimensions a = 19.530(12),b = 9.860(7), ο - 12.657(10) A, and 3 - 106.93(4)° 2

2

2

Q


100

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METAL


EXTENDED

Figure

6.

View

of the cation

in [Cr(phen),OII]J,,

4HjO


IONS

9.

HODGSON

Chromium(III)

101

Dimers

While no c r y s t a l l o g r a p h i c symmetry i s r e q u i r e d f o r four dimers i n t h i s c e l l , i t t r a n s p i r e s that each dimer s i t s on a c r y s t a l l o g r a p h i c i n v e r s i o n center so that there are, i n e f f e c t , two separate independent " h a l f - d i m e r s " i n the c e l l r a t h e r than one independent dimer. Hence, of course, the geometry of the anion must be of the ^2h y P r a t h e r than the D type. The s t r u c t u r e of the anion i s shown i n f i g u r e 8, the bond lengths and angles given being the average of the values obtained f o r the two independent halves; the agreement between these two independent measurements i s e x c e l l e n t (24). The values of the Cr-Cr s e p a r a t i o n of 3.000 A and the Cr-0Cr angle of 99.6(3)° are intermediate between those f o r the g l y c i n a t o and phenanthroline complexes. Hence, while the only magnet i c data a v a i l a b l e at present are the room temperature values (y= 3.43 μΒ) obtained on the t e t r a h y d r a t e (13), i t i s evident that the value of ΔΕ f o r t h i s complex must l i e between -10 and -53 cm" i f there i s a simple c o r r e l a t i o n between ΔΕ and φ. t

e


2

1

Interpretation The s t r u c t u r a l and magnetic data described above are c o r r e l ated i n f i g u r e 9, i n which the s i n g l e t - t r i p l e t s p l i t t i n g ΔΕ i s p l o t t e d against the b r i d g i n g angle φ; a s i m i l a r p l o t f o r the analogous copper(II) dimers [ C u ( L ) 0 H ] i s a l s o included i n figure 9 examination of the chromium data suggests that the oxalato complex Na^[Cr(0X) 0H] *6H 0, which has a φ of 99.6° (vide supra), should have a ΔΕ of approximately -25 cm . F i g u r e 9 i s noteworthy f o r two separate reasons: f i r s t l y , because f o r a given metal there i s apparently an almost l i n e a r c o r r e l a t i o n between ΔΕ and φ, and secondly because the slope of the ΔΕ v s . φ p l o t f o r the chromium(III) complexes i s c o n s i d e r a b l y smaller than that f o r the copper(II) complexes. Each of these f e a t u r e s i s r e a d i l y explained i n terms of simple bonding theory. 2 +

2

#

2

2

2

-1

The c o r r e l a t i o n between ΔΕ and φ. The c o r r e l a t i o n noted can be explained i n terms of valence bond theory and the p r i n c i p l e s of super exchange(25).If the o r b i t a l s used by the b r i d g i n g oxygen atoms are pure ρ o r b i t a l s , the bond angle i s expected to be 90° and the ground s t a t e i s p r e d i c t e d to be a t r i p l e t (i.e. ΔΕ > 0); i f the o r b i t a l s are purely s , the ground s t a t e i s pre d i c t e d to be a s i n g l e t (i.e. ΔΕ < 0). Hence, s i n c e an increased value of the b r i d g i n g angle i m p l i e s greater s character i n the b r i d g i n g o r b i t a l s , we would expect a decrease i n ΔΕ as the b r i d ging angle i s increased from 90°. For the s i x copper and three chromium cases which have been studied i n d e t a i l , t h i s trend i s observed (_2). T h i s c o r r e l a t i o n can a l s o be expressed i n terms of molecular o r b i t a l theory. The M-O-M-O r i n g i s of approximate Z? symmetry i n these molecules, with the x-axis defined as the Cu-Cu d i r e c t ion and y - a x i s p a r a l l e l to the 0-0 v e c t o r (26).Neglecting oxygen s o r b i t a l s , the eight σ-orbitals i n t h i s system transform i n D h 2h

2



102

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X 006 |-

120 TEMPERATURE,

Figure

7.

ϊ·0 Κ

Temperature variation of the magnetic susceptibility 4H 0. Solid line represents the best fit to equation 2

of [Cr(phen) OH] (3) (see text). É

Figure 8. View of the anion in Na [Cr(ox) OH] · 6H 0. Data are the average values of the two crystallographically independent dimers. r

2

2

2

Oi


I

g

5


9.

HODGSON

Chromium(III)

Dimers

103

-500 h Figure 9. M-O-M bridging angle, φ (abscissa) v s . the singlet-triplet splitting energy, A E (ordinate) for dihydroxo-bridgecl complexes of Cu (steeper line) and Cr (flatter line) B2u

Big / A g ^ J

- dxy \

\\ V \

/

w w \\ w

/

\/

'dx^y metal ion

2

Px

\

\

/

\ \

/

\

11

\ \ \

/

// / /

liqand σ orbitals

/

/

/ /

B2u/ /

Agi - t molecular

orbitals

Figure 10. Molecular orbital diagram for the σ-orbitals in the CrO-Cr ring, assuming D symmetry and a Cr-O-Cr angle of 90° 2ll


104

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INTERACTIONS

BETWEEN

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IONS

Γ \

/

/

*

\B2u

/-

/ / \

Ν \

~^ 3u\\ B

—\ d x y

W W

y

^d 2- 2 \\ x


metal

ion

•i b a s i s

set

y

>w

symmetry and a Cr-O-Cr angle considerably greater than .90 ~ h

DO DO Cu(II) d

Cr(III) d

Figure 12. Comparison of the overlap between the metal orbitals containing the unpaired spin and the ρ orbitals on the bridging ligand for copper(II) and chromium(IU)


9.

HODGSON

Chromium(III)

Dimers

105

symmetry as 2Ag + 2 £ i + 2 B + 2 £ , and there are bonding and anti-bonding combinations with a l l r?our of these symmetries. On the assumption that only oxygen ρ o r b i t a l s p a r t i c i p a t e , the 4 and molecular o r b i t a l s would have i d e n t i c a l energies when the Cr-O-Cr angle was 90°, as would the # i and £ orbitals.This s i t u a t i o n i s depicted i n f i g u r e 10, which demonstrates that t h i s ten e l e c t r o n system ( f o r chromium) must give r i s e to a t r i p l e t ground s t a t e . As the Cr-O-Cr angle i n c r e a s e s , however, the overlap of the i4g and £ ^ c o m b i n a t i o n s increases r e l a t i v e to that of the #3 and £ combinations. Hence, the o r b i t a l degenercies i n f i g u r e 10 are l i f t e d , and at s u f f i c i e n t l y l a r g e values of Φ the molecular o r b i t a l diagram shown i n f i g u r e 11 becomes o p e r a t i v e . At these l a r g e r b r i d g i n g angles, the s p l i t t i n g of the #3 * i ^g* o r b i t a l s i s s u f f i c i e n t to overcome the p a i r i n g energy, and the s i n g l e t s t a t e becomes the ground s t a t e . T h i s molecular o r b i t a l view, t h e r e f o r e , i s analogous to the valence bond approach above, and the experimental data may be i n t e r p r e t e d on t h i s b a s i s ; presum ably, f i g u r e 11 becomes appropriate at φ values greater than approximately 97.6°, while at angles between 90° and 97.6° the s p l i t t i n g between the #3 * and A * o r b i t a l s i s l e s s than the p a i r i n g energy, and so the t r i p l e t s t a t e remains lower i n energy than the s i n g l e t (i.e. ΔΕ remains p o s i t i v e ) (27). Unfortunately, f o r chromium(III) complexes of t h i s general type there are no examples of p o s i t i v e ΔΕ v a l u e s , but the presence of the t r i p l e t ground s t a t e complexes i n the copper(II) s e r i e s lends strong support to t h i s hypothesis. Moreover, of course, examination of the copper(II) l i n e i n f i g u r e 9 suggests that the ground s t a t e i s the t r i p l e t i f φ < 97.6°; hence, s i n c e f o r the chromium complexes the s m a l l e s t value of φ yet obtained i s 98.2°, none of the chrom ium complexes examined i n d e t a i l i s p r e d i c t e d to have a p o s i t i v e value of J (or Δ Ε ) . g

2 u

3

g

g

2 u

U

2 u

a n (


U

U

The Slopes of the Cr and Cu L i n e s . In the copper(II) complex es the unpaired s p i n r e s i d e s p r i n c i p a l l y i n atomic o r b i t a l s which point d i r e c t l y at the b r i d g i n g oxygen l i g a n d s (the d y o r b i t a l s i n f i g u r e 10, but more c o n v e n t i o n a l l y d 2 _ 2 s i n c e the usual a x i a l system i s d i f f e r e n t from that f o r c e d upon us i n P ^ symme t r y ) , while i n the chromium(III) complexes the unpaired s p i n i s i n t g o r b i t a l s which p o i n t between the b r i d g i n g atom; t h i s d i s p a r i t y i s demonstrated p i c t o r i a l l y i n f i g u r e 12. Hence, s i n c e the overlap between the b r i d g i n g o r b i t a l s and the metal o r b i t a l s c o n t a i n i n g the unpaired s p i n i s much poorer f o r the chromium com plexes than f o r the copper complexes, we p r e d i c t that the magni tude of the s p i n - s p i n i n t e r a c t i o n should be greater f o r copper than f o r chromium. Hence, at a given value of φ, we p r e d i c t that the magnitude of ΔΕ f o r copper i s greater than that f o r chromium, i.e. that the slope of the l i n e f o r copper i s greater than that f o r chromium; t h i s r e s u l t , of course, i s e x a c t l y what i s seen i n f i g u r e 9. X

x

y

2

2


106

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Acknowledgments I t i s a pleasure t o acknowledge the a s s i s t a n c e which I have r e c e i v e d from my present and former colleagues, e s p e c i a l l y Professor W.E. H a t f i e l d , Dr. J.T. V e a l , Dr. D.L. Lewis, Dr. D.Y. J e t e r , Dr. R.F. Drake, Ms. E.D. Estes, Ms. D.E. H a r t i s , Mr. R.P. Scaringe, Mr. R.P. Eckberg, and Mr. K.T. McGregor.


Literature Cited 1. Lewis, D.L., H a t f i e l d . W.E., and Hodgson, D.J., Inorg. Chem. (1974), 13, 147. 2. Lewis, D.L., H a t f i e l d , W.E., and Hodgson, D.J., Inorg. Chem. (1972), 11, 2216. 3. Casey, A.T., Hoskins, B.F., and W h i l l a n s , F.D., Chem. Commun. (1970), 904. 4. M i t c h e l l , T.P., Bernard, W.H., and Wasson, J.R., Acta Crystallogr. (1970), B26, 2096. 5. Majeste, R.J. and Meyers, E.A., J. Phys. Chem. (1970), 74, 3497. 6. McGregor, K.T., Watkins, N.T., Lewis, D.L., Drake, R.F., Hodgson, D.J., and H a t f i e l d , W.E., Inorg. Nucl. Chem. Letters (1973), 9, 423. 7. Estes, E.D., H a t f i e l d , W.E., and Hodgson, D.J., Inorg. Chem. (1974), in p r e s s . 8. Sinn, E. and Robinson, W.T., Chem. Commun. (1972), 359. 9. G l i c k , M.D. and L i n t v e d t , R.L., 167th. A.C.S. N a t i o n a l Meeting, Los Angeles, C a l i f o r n i a (1974). 10. Jezowska-Trzebiatowska, B. and Wojciedhowski, W., Transition Metal Chemistry (1970), 6, 1. 11. Ikeda, H., Kimura, I . , and Uryu, N., J. Chem. Phys. (1968), 48, 4800. 12. Earnshaw, A. and Lewis, J . , J. Chem. Soc. (1961), 396. 13. M o r i s h i t a , T., H o r i , Κ., Kyuno, Ε., and Tsuchiya, R., Bull. Chem. SOC. Japan (1965), 38, 1276. 14. Schugar, H.J., Rossman, G.R., and Gray, H.B., J. Amer. Chem. Soc. (1969), 91, 4564. 15. Kobayashi, H., Haseda, T., and M o r i , Μ., Bull. Chem. Soc. Japan (1965), 38, 1455. 16. J a s i e w i c z , Β., Rudolf, M.F., and Jezowska-Trzebiatowska, Β., Acta Physica Polonica (1973), A44, 623. 17. Hodgson, D.J., V e a l , J.T., H a t f i e l d , W.E., J e t e r , D.Y., and Hempel, J.C., J. Coord. Chem. (1972), 2, 1. 18. V e a l , J.T., H a t f i e l d , W.E., J e t e r , D.Y., Hempel, J.C., and Hodgson, D.J., Inorg. Chem. (1973), 12, 342. 19. V e a l , J.T., H a t f i e l d , W.E., and Hodgson, D.J., Acta Crystall ogr. (1973), B29, 12. 20. Eckberg, R.P., p r i v a t e communication. 21. Scaringe, R.P. and Hodgson, D.J., unpublished o b s e r v a t i o n s .


9.

HODCsox

Chromium(IH)

Dimers

107


22. Eckberg, R.P., Scaringe, R.P., Hodgson, D.J., and H a t f i e l d , W.E., unpublished o b s e r v a t i o n s . 23. Drake, R.F., Ph.D. D i s s e r t a t i o n , U n i v e r s i t y of North C a r o l i n a (1973). 24. Hodgson, D.J., and Scaringe, R.P., unpublished observations. 25. Goodenough, J.B., "Magnetism and The Chemical Bond", I n t e r s c i e n c e , New York, 1963. 26. Bertrand, J.Α., and Kirkwood, C.E., Inorg. Chim. Acta (1972), 6, 248. 27. Hodgson, D.J., Progr. Inorg. Chem. (1974), i n p r e s s .


Extended Interactions between Metal Ions

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