Metal Clusters and Extended Metal-Metal Bonding in Metal Oxide

18

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Metal Clusters and Extended Metal-Metal Bonding in Metal Oxide Systems ROBERT E. McCARLEY Iowa State University, Ames Laboratory and Department of Chemistry, Ames, IA 50011

The results of recent investigations of the synthesis and structures of ternary and quater nary molybdenum oxide compounds having metal -metal bonded cluster units or infinite chains are presented. In a l l cases the average oxi dation state of molybdenum is less than 4+, the lowest state previously known in structurally characterized oxide phases. New compounds con taining discrete clusters are LiZn Mo O and Ζn Μo O , with triangular units, and Ba . Mo O16 with two variants of rhomboidal tetranuclear units, one having regular and the other distorted rhomboidal geometry. Infinite metal-metal bonded chains consisting of Μο octahedral cluster units fused on opposite edges are found in the new compounds NaMo O , Ba (Mo O ) , Sc . Zn . Mo O , and Ti . Zn . Mo O . The ef fects of the differing metal-based electron counts in the repeat units of the infinite chains among these compounds are discussed with refer ence to metal-metal bond order and structural variations within the units. 2

3

3

3

8

8

1 14

8

6

4

0 75

1 25

4

7

6

0 5

4

5

1 5

4

6 8

7

Metal clusters in metal oxide systems have not been wellcharacterized or abundantly investigated up to the present time. Only isolated examples of metal-metal bonded units in oxide lattices have appeared from time to time. It will be the thesis of this presentation to show that highly unusual structures determined by strong metal-metal bonding will be found in ternary and quaternary metal oxide systems, and that oppor tunities abound for creative work on the synthesis, theory and structure-property relationships of such compounds. Because of the well-known correlation of d-electron population and d-orbital radial extension with metal-metal bond formation, 0097-6156/83/0211-0273$06.00/0 © 1983 American Chemical Society Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

we may expect that oxide systems c o n t a i n i n g the e a r l y 4d and 5d t r a n s i t i o n elements i n reduced o x i d a t i o n s t a t e s w i l l provide the most f e r t i l e area f o r new i n v e s t i g a t i o n s . We emphasize here our recent work with the reduced ternary oxides of molybdenum, but expect that r e l a t e d chemistry w i l l be observed f o r oxides of Nb, Ta, W, Tc and Re. The most f a m i l i a r examples of metal-metal bonding i n oxide compounds are found i n the d i o x i d e s , VO2, Nb02, M0O2> WO2 and a-Re02 > a l l of which adopt low-symmetry v a r i a n t s of the r u t i l e s t r u c t u r e ÇL-4). In each case the metal atoms occur i n chains of edge-shared ΜΟβ octahedra running p a r a l l e l to the r u t i l e ca x i s . Displacement of each metal atom towards one and away from the other of i t s nearest-neighbor metal atoms along the chain accounts f o r the a l t e r n a t e short and long d i s t a n c e s between metal atoms. The metal-metal bonds thus e s t a b l i s h e d may be r e garded as s i n g l e (VO2> NDO2) or double (M0O2» WO2 and a-Re02) (4). With the edge-shared b i o c t a h e d r a l arrangement of oxide l i g a n d s only 4 e l e c t r o n s may populate M-M bonding o r b i t a l s i n the c o n f i g u r a t i o n σ π (5). A d d i t i o n a l e l e c t r o n s then populate nonbonding (M-M) or antibonding (M-0) o r b i t a l s of 6 and 6 symmetry with respect to the M-M bond a x i s 0 5 , 6 ) . Thus, though 6 e l e c t r o n s r e s i d e i n metal-centered o r b i t a l s , the Re-Re bond i n a-Re02 i s best regarded as a double bond, with two a d d i t i o n a l e l e c t r o n s i n the δ*, e s s e n t i a l l y nonbonding o r b i t a l . The com pounds M0O2 and WO2 e x h i b i t m e t a l l i c c o n d u c t i v i t y because of overlap of the f i l l e d M-M π and empty M-0 π* valence bands, which then provide a p a r t i a l l y f i l l e d conduction band (4). More r e c e n t l y metal-metal doublets with m u l t i p l e bond character have been found i n Lai+ReeOig 05), La30s20io (7) and L a R e 0 (8). In the f i r s t two cases the M-M bonds, e s t a b l i s h e d i n an edgeshared b i o c t a h e d r a l arrangement, are a t best considered as double bonds, with e x t r a e l e c t r o n s consigned to M-0 π* del o c a l i z e d o r b i t a l s . The doublets of Re atoms i n La Re 0 o con s i s t of Re 0e u n i t s l i k e that of R e C l 8 ~ , i . e . two ReO^ f r a g ments h e l d together i n an e c l i p s e d c o n f i g u r a t i o n by a strong metal-metal bond. Here the Re atoms share 6 e l e c t r o n s i n a t r i p l e bond with the c o n f i g u r a t i o n σ τΛ, and the e c l i p s e d con formation of the Re 0s u n i t i s imposed by the arrangement of oxygen atoms i n the l a t t i c e . 2

2

i f

2

lt

2

1 0

2

1

2

2

2

We n o t i c e that i n these examples o f t e r n a r y oxides the oxygen/metal (O/M) r a t i o i s h i g h . T h i s f o s t e r s small metalmetal bonded u n i t s because an octahedral arrangement of 0 atoms about each metal atom can be a t t a i n e d with a minimum of shared atoms, e.g. only two shared 0 atoms i n the O s 0 i o u n i t s o f L a 0 s 0 i o . I t i s reasonable to expect t h a t , as the O/M r a t i o i s decreased, higher degrees of aggregation w i l l ensue i n order f o r the metal atom to a t t a i n higher c o o r d i n a t i o n numbers. For a given electron/metal (e/M) r a t i o l a r g e r c l u s t e r u n i t s with M-M bonds of low bond order then w i l l be favored over the smal l e r u n i t s with M-M bonds of higher order. The best known 2

3

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Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

18.

Metal Oxide

MCCARLEY

Systems

275

examples showing e f f e c t s of t h i s kind are found i n metal h a l i d e systems, e.g. presumed dimers i n WC1J+, trimers i n N b C l ( 9 ) , and hexamers i n Z r C l i 2 (10). 3

8

6

Oxides with T r i n u c l e a r C l u s t e r s The f i r s t compound c o n t a i n i n g a w e l l - c h a r a c t e r i z e d c l u s t e r i n a metal oxide system was Ζ η Μ θ 3 θ , reported i n 1957 by M c C a r r o l l , Katz and Ward (11,12). In t h i s compound the M o 0 c l u s t e r u n i t s are constructed such that three M0O5 octahedra share common edges, and share 0 atoms with adjacent c l u s t e r u n i t s as r e f l e c t e d i n the c o n n e c t i v i t y formula Ζ η Μ ο 0 ^ 0 / Ο3/3· Thus the true c l u s t e r u n i t i s of the type M X as shown i n Figure 1. Although the e/Mo r a t i o i s the same as i n MoO^, a l l e l e c t r o n s are u t i l i z e d i n s i n g l e bonds i n the Mo 0 ~ c l u s t e r as opposed to the double bond i n the Mo dimers of Mo0 . Pos s i b l y t h i s d i f f e r e n c e i s d i c t a t e d by the higher 0 / t o t a l metal r a t i o i n Mo0 as opposed to Ζ η Μ ο 0 . ^_ Other compounds c o n t a i n i n g the M o 0 c l u s t e r u n i t s have been prepared as members of two s e r i e s . The f i r s t s e r i e s i s of the type M M o 0 (11) with M = Zn, Cd, Mg, Mn, Fe, Co and N i , and the second s e r i e s i s of the type LiM- ^- Mo 03 (13, 14,15) with M i l l = Ga, In, Se, Y, and r a r e e a r t h ions having atomic numbers greater than 62(Sm). The s t r u c t u r a l arrangement i n c r y s t a l s of the two s e r i e s however i s very s i m i l a r . In each case one-half of the c a t i o n s occupy t e t r a h e d r a l s i t e s and oneh a l f occupy octahedral s i t e s w i t h i n a close-packed oxide l a t t i c e . I n d i c a t i n g these as M° f o r ions i n octahedral s i t e s and M f o r those i n t e t r a h e d r a l s i t e s , we have the formulas Μ Μ ° Μ ο 0 and Li M°Mo 0e f o r the two s e r i e s . Our work was i n i t i a t e d on the reduced ternary molybdenum oxides with the thought that the metal c l u s t e r e l e c t r o n count (MCE) should be v a r i a b l e f o r the M o 0 c l u s t e r u n i t s . Based on Cotton's previous molecular o r b i t a l treatment of such c l u s t e r s (16) i t appeared that MCE s from 6 to 8 could be ac commodated, but i t was not c l e a r whether the seventh and e i g h t h e l e c t r o n s would occupy bonding or antibonding o r b i t a l s with respect to the M-M i n t e r a c t i o n s . We thus set about to determine t h i s from s t r u c t u r a l data on s u i t a b l e compounds. The attempted replacement of Z n with S c to secure the compound Zn Sc°Mo 03 conducted v i a the r e a c t i o n shown i n equation 1. 2

8


3

2

3

3

8

2

1 3

3

8

2

2

2

3

I I

2

8

3

2

8

8

1 1

3

8

t

c

3

fc

1:

3

8

t

3

3

8

1

2 +

w

a

3 +

s

3

2Sc 0 2

3

+ 4Zn0 + H M o 0

2

+ Mo

• 4ZnScMo 0 3

8

(1)

1100° X-ray powder patterns showed that the product of t h i s r e a c t i o n i s indeed isomorphous with Z n M o 0 and hence i s the d e s i r e d 7e l e c t r o n c l u s t e r d e r i v a t i v e . Unfortunately s i n g l e c r y s t a l s f o r a complete s t r u c t u r e determination have not been obtained. Sub sequent work (17) however showed that a d d i t i o n a l c a t i o n s could 2

3

8



276


Figure 1.

The M X cluster type as found in the oxides Zn Mo 0 , and Zn Mo 0 . (See Table I for atom numbering scheme.) 3

1S

3

2

3

3

8

LiZn Mo 0 ,

8


2

3

8

18.

MCCARLEY

277

Metal Oxide Systems

be introduced i n t o the l a t t i c e of a modified Z n M o 0 v i a high temperature r e a c t i o n s , equations 2 and 3. 2

L^MoO^ + 4ZnO + 4Mo0

2

+ IMo

3

2LiZn Mo 0 2

3

structure

8

(2)

8

1100° 6ZnO + 5Mo0

+ Mo

2

• 2Zn Mo 0 3

3

(3)

8

1100° Structure determinations of L i Z n M o 0 and Z n M o 0 (18) showed that these compounds are isomorphous but the p a t t e r n of o x i d e - l a y e r s t a c k i n g i s d i f f e r e n t from that of Z n M o 0 . Other wise the s t r u c t u r e s are c l o s e l y r e l a t e d and a l l contain the c l u s t e r u n i t s w i t h the same c o n n e c t i v i t y , [Μο 0^0 / 0 / ] ~. Thus we may compare s t r u c t u r a l data f o r Z n M o 0 (6MCE) with those f o r LiZn Mo 0 (7MCE) and Zn Mo 0 (8MCE) to d i s c e r n the e f f e c t s of adding successive e l e c t r o n s to the Mo 0^~ c l u s t e r u n i t . A comparison of Mo-Mo and Mo-O bond d i s t a n c e s i s given i n Table I f o r these compounds. I t i s c l e a r that a d d i t i o n of ο Table I. Comparison of Mo-Mo and Mo-O bond d i s t a n c e s (A) i n Z n M o 0 , L i Z n M o 0 and Z n M o 0 .


2

3

8

3

3

2

3

8

8

n

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8

2

3

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8

8

3

2

3

8

2

3

8

3

3

8

Zn Mo 0

Bond

Zn Mo 0

Mol-Mol

2.524(2)

2.578(1)

2.580(2)

Mol-01

2.058(10)

2.063(6)

2.100(9)

Mol-02

1.928(20)

2.003(8)

2.056(13)

Mol-03

2.128(30)

2.138(5)

2.160(8)

Mol-04

2.002(30)

2.079(7)

2.054(11)

Mo 1-0 (aver.)

2.017

2.058

2.088

2

3

a 8

LiZn Mo 0 2

3

3

8

3

8

Data taken from reference (12). e l e c t r o n s to the M o 0 c l u s t e r causes a net e l o n g a t i o n of both the Mo-Mo and Mo-O bonds. T h i s may be taken as evidence that the seventh and e i g h t h MCE's enter a molecular o r b i t a l which i s antibonding with respect to boçh Mo-Mo and Mo-O i n t e r a c t i o n s . Among the Mo-O bonds, those i n v o l v i n g the three-edge-bridging 0 atoms are the most s t r o n g l y a f f e c t e d . I t t h e r e f o r e appears that the set of three d - o r b i t a l s not involved i n e i t h e r Mo-O or Mo-Mo σ-bonding mix p r i m a r i l y with the set of three ρπ o r b i t a l s l o c a t e d on the edge-bridging 0 atoms to form a set of 3 bonding and 3 antibonding M0 s. The bonding set (a± + e) i s populated by e l e c t r o n s from the 0 atom lone p a i r s , and these probably l i e below the Mo-Mo σ-bonding o r b i t a l s ; the antibonding set ( a + e*) would then c o n s t i t u t e the LUM0 s of Z n M o 0 . Magnetic s u s c e p t i b i l i t y data f o r L i Z n M o 0 showed Curie-Weiss 3

8

f

f

x

2

2

3

3

8

8


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INORGANIC C H E M I S T R Y : TOWARD T H E 21ST

CENTURY

behavior over the temperature range 95-298 Κ with a very l a r g e Weiss constant θ = -545°. At 298 Κ, μ = 1.18 μ_, a low err D value caused by strong a n t i f e r r o m a g n e t i c c o u p l i n g . Magnetic measurements on Z n M o 0 , while l e s s d e t a i l e d , show only a weak paramagnetism, p f f = 0.6 μ , probably i n d i c a t i v e of Van Vleck paramagnetism. We thus i n f e r that the e l e c t r o n spins are p a i r e d i n Z n M o 0 , and that the a* l e v e l l i e s below the e* l e v e l . r

3

3

8

e

3

3

β

8

Oxides with T e t r a n u c l e a r C l u s t e r s In the s t r u c t u r e refinement of L i Z n M o 0 the L i ions were not l o c a t e d because of t h e i r apparent scrambling with Zn ions i n both the t e t r a h e d r a l and o c t a h e d r a l s i t e s . An e f f o r t to overcome t h i s d i f f i c u l t y was made with the attempted s y n t h e s i s of NaZn Mo 0 , where i t was expected that the N a ions would be confined only to the o c t a h e d r a l s i t e s . T h i s attempted s y n t h e s i s l e d i n s t e a d to the formation of the new compound NaMoÔg (19) which grew i n the r e a c t i o n mixture and on the w a l l of the moly bdenum container as t h i n needles with s i l v e r y m e t a l l i c l u s t e r . The s e r e n d i p i t o u s d i s c o v e r y of t h i s compound has proven to be extremely important to our v i s i o n s of p o s s i b i l i t i e s f o r new metal-metal bonded s t r u c t u r e s i n reduced oxide phases. In r e t r o s p e c t i t i s amazing that oxide phases c o n t a i n i n g molybdenum i n o x i d a t i o n s t a t e s l e s s than 4 were e s s e n t i a l l y unknown and c e r t a i n l y s t r u c t u r a l l y uncharacterized. The e x i s t e n c e of the p r e v i o u s l y mentioned s e r i e s M J M O 0 and L i M M o 0 should have been a t i p - o f f to an extensive chemistry f o r metal-metal bonded molybdenum oxide systems. Indeed, subsequent work has revealed a p l e t h o r a of new compounds a l l of which (where s t r u c t u r e has been determined) f e a t u r e strong metal-metal bonding i n e i t h e r d i s c r e t e c l u s t e r u n i t s or extended chain a r r a y s . +


2

3

8

+

2

3

8

+

I

I I I

3

8

3

8

A chart of these s t r u c t u r a l l y c h a r a c t e r i z e d compounds i s given i n Table I I , showing the c o r r e l a t i o n of average o x i d a t i o n s t a t e of the molybdenum with c l u s t e r formation or extended arrays. There are two notable e f f e c t s which are p e r t i n e n t to the nature of the metal-metal bonding, namely the e l e c t r o n / m e t a l (e/M) r a t i o and the oxygen/metal (O/M) r a t i o . With other f a c t o r s constant an increase i n e/M r a t i o should promote more extensive M-M bonding, and a decrease i n the O/M r a t i o should promote c l u s t e r s of i n c r e a s i n g n u c l e a r i t y , p r o g r e s s i n g from dimers through l a r g e r c l u s t e r s to systems with extended chains, sheets or 3-dimensional arrays (20, 21). The i n t e r e s t i n g compound Bai ii+Mo 0 (17,22) contains t e t r a n u c l e a r rhomboidal c l u s t e r u n i t s , t i e d i n t o i n f i n i t e chains by Mo-O-Mo bonding as represented i n the c o n n e c t i v i t y M o 0 0 / 0 / . The Ba + ions are stacked i n s i t e s along tunnels formed by weaving together the [MOÔQ ] chains, as shown i n Figure 2. In t h i s low-symmetry, metal-metal bonded adaptation of the well-known h o l l a n d i t e s t r u c t u r e the chains forming the four s i d e s of each tunnel are r e l a t e d i n p a i r s by Ρ 1 symmetry. 8

18

2

I+

2

8

2

6

3

qq


18.

MCCARLEY

Table I I .

C h a r a c t e r i s t i c s of metal-metal bonded ternary and quaternary molybdenum oxides. O.N.

Compound

a e /

Structural feature

Mo

3.72

2.28

2.0

rhomboidal c l u s t e r s

LiZn2Mo 0e

3.67

2.33

2.67

triangular

clusters

Zn Mo 0

3.33

2.67

2.67

triangular

clusters

2.75

3.25

1.50

i n f i n i t e chains

2.69

3.31

1.50


1.75


1.75


Ba

Mo

l.l*+ 8°16 3

3

3

8

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279

Metal Oxide Systems

Ba (Moi 0 ) 5

t

6

8

SCo . 7 5Ζηι . 25ΜΟ4Ο7

2.31

3.69

Tio.sZni.sMoitOy

?

Oxidation number (average) of molybdenum Average e l e c t r o n to metal r a t i o f o r metal-metal bonding Oxygen to molybdenum r a t i o

Figure 2. A three-dimensional perspective of the structure of Ba , Mo 0 as viewed down the unique axis parallel to the direction of chain growth and tunnel formation. 1A t

s

16


280


In one p a i r of chains the rhomboidal c l u s t e r u n i t s have f i v e Mo-Mo bonds of n e a r l y equal length ( r e g u l a r u n i t s ) , and i n the other p a i r the ( d i s t o r t e d ) rhomboidal u n i t s have three short and two long Mo-Mo bonds. Figure 3 shows how these rhomboidal u n i t s are bound together w i t h i n the i n f i n i t e chains. A comparison of Mo-Mo and s e l e c t e d Mo-0 bond distances f o r the r e g u l a r and d i s t o r t e d c l u s t e r u n i t s i s given i n Table I I I . Table I I I .

Selected bond d i s t a n c e s (A) w i t h i n the d i s t o r t e d and r e g u l a r c l u s t e r u n i t s of B a i . n*Mo 0i6. 8


Atoms

Distorted Cluster

Regular C l u s t e r

Mol-Mo2 Mol-Mo2 Mo2-Mo2

2.847(1) 2.546(1) 2.560(1)

2.616(1) 2.578(1) 2.578(1)

Mol-03 ï Mol-08 \ edge Mol-04 J

1.931(6) 1.894(6) 2.079(6)

1.936(6) 2.022(6) 2.143(6)

Mol-01 Ï Mol-02 I capping ΜΟ1-02Ί

2.082(6) 2.046(6) 2.104(6)

2.053(6) 2.055(6) 2.095(6)

1

1

See Figure 3 f o r atom numbering scheme. Since the Mol-Mo2 distance i n the d i s t o r t e d u n i t corresponds to a P a u l i n g bond order (23) of ca. 0.5 these bonds may be con sidered as one-electron bonds. Assuming the Mol-Mo2 and Mo2-Mo2 bonds are each normal two-electron bonds, there are c a . 8 MCE*s f o r the d i s t o r t e d u n i t s . In the r e g u l a r u n i t s the Mo-Mo d i s t a n c e s i n d i c a t e that these bonds should be considered as normal two-electron bonds and an MCE count of 10 i s _ o b t a i n e d . Thus the compound may be formulated as B a i . 14 (Moôl MMoi+Oe' ) to represent t h i s unbalanced e l e c t r o n d i s t r i b u t i o n . As noted i n the previous paper by P r o f e s s o r Chisholm the d i s t o r t e d Μοι^Οβ c l u s t e r u n i t has a r e c e n t l y discovered molecular analog i n WiÔEOie (24,25) • The d i s t o r t i o n i n these 8-electron rhom b o i d a l c l u s t e r s has been a t t r i b u t e d by Cotton and Fang (26) to a second order J o h n - T e l l e r e f f e c t . T h i s seems reasonable f o r the molecular Wit(0Et)i6 but i s questionable f o r the d i s t o r t e d u n i t i n Bai.ι^ΜοβΟιβ. Examination of the Mo-0 d i s t a n c e s i n Table I I I r e v e a l s a much s h o r t e r Mol-08 d i s t a n c e i n the d i s t o r t e d c l u s t e r as compared to that i n the r e g u l a r c l u s t e r . Thus i t appears that there i s a push-pull e f f e c t at work, such that e l e c t r o n density removed from Mol through weakening of the Mol-Mo2 bond i s compensated by increased π-bonding i n the Mol-08 bond of the d i s t o r t e d c l u s t e r . f

f

28

I t appears that other compounds having t h i s s t r u c t u r e type should be p o s s i b l e . In p a r t i c u l a r , by appropriate s u b s t i t u t i o n of B a by c a t i o n s of d i f f e r e n t charge the MCE count should be v a r i a b l e over some range, and conceivably even compounds having more than 10 MCE per c l u s t e r u n i t might be prepared, with ad2 +


Metal Oxide Systems

281


MCCARLEY

Figure 3. A view of one Mo O cluster chain in Βα^Μο^Ο^ which shows intrachain Mo-Mo and Mo-O bonding. (See Table 111 for atom numbering scheme.) k

%


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282

d i t i o n a l e l e c t r o n s e n t e r i n g a conduction band. Work along t h i s l i n e i s p r e s e n t l y i n progress i n our l a b o r a t o r y . Oxides with Extended Metal-Metal Bonding The remaining compounds l i s t e d i n Table I I a l l adopt s t r u c t u r e s with i n f i n i t e metal-metal bonded chains c o n s i s t i n g of o c t a h e d r a l c l u s t e r u n i t s fused on opposite edges. However, because of the l a r g e d i f f e r e n c e i n e f f e c t i v e i o n i c r a d i u s of the c a t i o n s concerned, very d i f f e r e n t l a t t i c e types are d i c t a t e d . The compounds NaMoi+Oe (19,22) and Β 3 ( Μ θ ι * 0 ) 8 (17) adopt tunnel s t r u c t u r e s with the N a or B a ions l o c a t e d i n s i t e s along the tunnels with 8 - f o l d c o o r d i n a t i o n by oxygen atoms. In the sodium and barium compounds there are 13 and 13.2 MCE*s per MoÔe- u n i t of the c h a i n , r e s p e c t i v e l y . Ordering of the c a t i o n vacancies along the tunnels i n Ba5(Moit06)8 r e s u l t s i n a s u p e r - l a t t i c e with a dimension e i g h t times that of N a M o i ^ along the unique chain (or tunnel) a x i s . Views of the N a M o i ^ (tetragonal) and Β35(Μοι»0β)8 (orthorhombic) s t r u c t u r e s as viewed down the c-axes are shown i n F i g u r e 4. C o n s t r u c t i o n of the [MoitOe]^ chains i s shown i n F i g u r e 5. 5


+

6

2 +

The compounds Sco . 7 δΖηχ . 5M04O7 and Tio . s Z n i . 5M04O7 are the most recent a d d i t i o n s to the f a m i l y of reduced, ternary or quaternary oxides (27). Once again they were obtained as unexpected products i n experiments designed f o r another purpose. Attempts to o b t a i n s i n g l e c r y s t a l s of ScZnMo 03 produced only m a c r o c r y s t a l l i n e powders i n the range 1100-1400°C. At vL450°C the l a t t e r compound decomposed and b e a u t i f u l gem-like c r y s t a l s of the new compound were found i n the product mixture. Analyses of s e v e r a l c r y s t a l s f o r Sc, Zn and Mo by electron-microprobe x-ray f l u o r e s c e n c e provided c o n s i s t e n t r e s u l t s and the average atomic r a t i o s Sc:Zn:Mo of 0.75:1.25:4. These data, combined w i t h the subsequent x-ray s t r u c t u r e determination, e s t a b l i s h e d the composition S C Q · ι ^ τ ΐ γ . 2 5 ^ 0 ^ 0 7 . The composition of c r y s t a l s of T i . sZni. 5MOI+07, formed i n a r e a c t i o n between T i 0 , ZnO, Mo0 and Mo at 1450°C, was e s t a b l i s h e d i n l i k e manner. Although these new compounds proved to be isomorphous, d i s t i n c t d i f ferences were found i n the metal-metal bonding c h a r a c t e r i s t i c s , e v i d e n t l y caused by d i f f e r i n g MCE counts i n the repeat u n i t s of the i n f i n i t e chains. Because the o x i d a t i o n s t a t e of T i i n T i . s ^ l · 5 ° t + 0 7 has not been e s t a b l i s h e d , the assessment of the MCE count and i n t e r p r e t a t i o n of bonding d i f f e r e n c e s with S C Q . 7 5Zn!. 2 5Moi 0 i s u n c e r t a i n . Fcr t h i s reason only the s t r u c t u r e and bonding f e a t u r e s of the scandium compound w i l l be described here. 2

3

0

2

2

n

M

0

+

7

A view down the c-axis ( p a r a l l e l to the chain axes) of the scandium compound i s shown i n Figure 6. I t can be seen that the [MoÔy] chains are bound together through Mo-O-Mo bridge bonding to form l a y e r s which are, i n t u r n , bound together through O-Sc-0 and O-Zn-0 bonding. The s i t e s f o r these metal ions are of two types: t e t r a h e d r a l s i t e s occupied only by Z n i o n s , and o c t a h e d r a l s i t e s occupied by e i t h e r Z n or S c . In 2 +

2 +


3 +

Metal Oxide Systems


MCCARLEY

Figure 4. A three-dimensional perspective of the structures of NaMofie (top) and Ba (Mo 0 ) (bottom) as viewed down the unique axis of chain growth and tunnel formation. 5

lt

6

8


283


284


Figure 5.

Segment of one (Mo Oi)^ chain in NaMo 0 which shows intrachain Mo-Mo and Mo-O bonding. Key: %, Mo; and O, oxygen. k

k

6


McCARLEY

Metal

Oxide

Systems


18.


285

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CENTURY

t h e o c t a h e d r a l s i t e s t h e m e t a l i o n s a p p e a r t o be s t a t i s t i c a l l y d i s o r d e r e d w i t h 0 . 2 5 Z n + and 0 . 7 5 S c * f r a c t i o n a l o c c u p a t i o n . B a s e d on t h e s e c o n s i d e r a t i o n s and t h e s h a r i n g o f 0-atoms a l o n g and b e t w e e n c h a i n s , t h e compound c a n be f o r m u l a t e d a s 2

Sco.7 5 O - 2 5 l - O u t ( 4 8 / 2 2 / 2 ) ° 2 ] · E l e c t r o n t r a n s f e r f r o m t h e Sc and Zn t o t h e Μοι+Ογ u n i t s o f t h e c h a i n s r e s u l t s i n 1 4 . 7 5 MCE p e r u n i t . The a v e r a g e f o r m a l o x i d a t i o n s t a t e o f 2 . 3 1 + f o r Mo i n t h i s compound i s t h e lowest y e t a t t a i n e d i n a t e r n a r y o x i d e phase. A comparison o f Mo-Mo b o n d i n g a l o n g t h e c h a i n s b e t w e e n NaMoi+Og w i t h 1 3 MCE a n d SCQ.75Zni.25Μοι+θ7 w i t h 1 4 . 7 5 MCE i s i n s t r u c t i v e . Pertinent Mo-Mo bond d i s t a n c e s f o r t h e s e compounds a r e g i v e n i n T a b l e I V . The p r i n c i p a l b o n d i n g d i f f e r e n c e b e t w e e n t h e c h a i n s i n t h e two s t r u c t u r e s i s t h a t i n NaMoi+Og t h e s p a c i n g b e t w e e n Mo atoms a l o n g t h e c h a i n , b o t h a p e x - a p e x and w a i s t - w a i s t , i s p e r f e c t l y r e g u l a r . In c o n t r a s t , w h i l e the w a i s t - w a i s t s p a c i n g i s r e g u l a r , t h e apexapex d i s t a n c e s a l t e r n a t e between s h o r t ( 2 . 6 2 5 ( 2 ) Â ) and l o n g ( 3 . 1 3 9 ( 2 ) X) w i t h i n t h e c h a i n s o f S C Q · 7 5 M . 2 5Μθΐψθ7, a s shown i n F i g u r e 7. The sum o f P a u l i n g bond o r d e r s f o r t h e n e t o f two a p e x - a p e x bonds p e r Μ ο ^ 0 r e p e a t u n i t i n NaMoÔg i s 0 . 7 7 3 . F o r t h e n e t o f one s h o r t a n d one l o n g a p e x - a p e x bond p e r r e p e a t u n i t i n S c . 7 5 Ζ η . 25MO1+07 t h e sum o f P a u l i n g bond o r d e r s i s 1 . 0 9 2 . T h i s i n c r e a s e o f 0 . 3 1 9 f o r t h e a p e x - a p e x bond o r d e r sum i n g o i n g f r o m t h e f o r m e r t o t h e l a t t e r compound d i c t a t e s a n i n c r e a s e o f ca. 0.64 bonding e l e c t r o n s p e r repeat u n i t . Summed o v e r a l l Mo-Mo bonds o f t h e r e p e a t u n i t , t h e t o t a l bond o r d e r f o r NaMoi+Og i s 6 . 3 6 , and f o r S c . 5 n i . 2 5 °i+07 t h e sum i s 6 . 8 4 . T h e s e Z n

Z n


3

M o

0

0

Z R

6

0

χ

z

0

M

7

ο

Table IV.

A c o m p a r i s o n o f Mo-Mo bond d i s t a n c e s and Sc 7 Zn 25Mo 0 . 0 e

5

l e

4

if

(Mo-Mo) S C Q . 75^1. 5Μοι+0

NaMoi+Og

2

(waist-waist)

a

2.8618(2)

2.8854(5)

(waist-waist)

b

2.753(3)

2.817(2)

apex-waist

6

7

d Bond

(A) i n NaMo 0

2.780(2)

7

2.747(1) 2.782(1)

apex-apex

2.8618(2)

2.625(2) 3.139(2)

Bond d i s t a n c e p a r a l l e l t o c h a i n ^

Bond d i s t a n c e p e r p e n d i c u l a r

direction.

to chain

direction.

f

numbers s t r o n g l y i n d i c a t e t h a t a l l M C E s p a r t i c i p a t e i n b o n d i n g i n t e r a c t i o n s i n NaMoÔg and t h a t t h e a d d i t i o n a l e l e c t r o n s r e q u i r e d t o f o r m t h e c h a i n s i n SCQ . 75^1.2 5 f°7 f enhance t h e Mo-Mo b o n d i n g . Mot

u

r

t

n

e

r


Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 0 1!i

U

k

1

Figure 7. A view perpendicular to the direction of chain growth in Sc . Zn 2!>Mo O showing the mode of interchain Mo-O-Mo linking and alternately short and long apex-apex Mo-Mo bonds.



288

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Several important questions then a r i s e . What range of e l e c t r o n counts per repeat u n i t i s permitted i n chain s t r u c t u r e s of t h i s type? What changes may be expected i n the p a t t e r n of metal-metal bonding w i t h i n the u n i t s as the e l e c t r o n count i s v a r i e d across the permitted range? What i s the d e t a i l e d bond s t r u c t u r e ? How many d i s c r e t e bands separated by forbidden gaps w i l l occur? Answers to these and r e l a t e d questions are very important as guides to f u r t h e r s y n t h e t i c work, i . e . what compounds are p o s s i b l e , and as a base to understand p h y s i c a l p r o p e r t i e s . At the present time s e v e r a l compounds are known having chain s t r u c t u r e s of the same type discussed here. These are found among the rare e a r t h and group I I I t r a n s i t i o n metal subhalides, v i z . T b B r (20) with 6 MCE; and S c C l ( 2 8 ) , G d B r ( 2 9 ) , T b B r (29) and Eri+Is Ç30) with 7 MCE. Thus a wide range of MCE counts per repeat u n i t i s indeed p o s s i b l e . The compounds at the low end of the range are composed of both metal and nonmetal atoms having r e l a t i v e l y l a r g e r a d i i . Because of the chain c o n s t r u c t i o n , which r e q u i r e s that the average M-M separation must equal the nonmetal-nonmetal separation along the chain d i r e c t i o n , the l a r g e nonmetal atoms d i c t a t e long M-M d i s t a n c e s . This c o n d i t i o n i s met by metal atoms having l a r g e r a d i i forming bonds of low net bond order, hence metals with low valence e l e c t r o n counts. At the upper end of the range are the molybdenum oxide compounds with the much smaller nonmetal atoms, comparatively short metal-metal bonds, and higher net bond orders. I t w i l l be i n t e r e s t i n g to see i f the gaps between these two extremes can be f i l l e d - i n by s u i t a b l e synthesis and r e q u i s i t e matching of MCE with metal and nonmetal r a d i i , and e l e c t r o n i c band s t r u c t u r e . 2

5

8

5

3

5

8

8

Concluding Remarks As a f i n a l comment we should note that no ternary molybdenum oxide phase has yet been found having d i s c r e t e o c t a h e d r a l c l u s t e r u n i t s l i k e those found i n the famous Chevrel phases M Mo S and M Mo Se (31,32). Only i n the case of Mg3Nb 0 (33) has a d i s c r e t e octahedral c l u s t e r u n i t been found i n an oxide phase. The existence of the l a t t e r however s i g n a l s opportunity f o r f u r t h e r research, and taken together with the r e s u l t s reported here and those known f o r a few other metal oxides, extensive metal-metal bonded, metal oxide chemistry can be a n t i c i p a t e d f o r Nb, Ta, W, Tc and Re. The great s t a b i l i t y and unusual s t r u c t u r e types observed f o r the known metal oxide c l u s t e r compounds should make them i n t e r e s t i n g c a n d i dates f o r the study and development of u s e f u l p h y s i c a l and chemical p r o p e r t i e s . x

8

6

8

x

8

8

11

Acknowledgments The author g r a t e f u l l y acknowledges the s e v e r a l coworkers upon whose work t h i s a r t i c l e i s based and whose c o n t r i b u t i o n s are i n d i c a t e d i n the l i t e r a t u r e c i t a t i o n s . The continuing support of the U.S. Department of Energy, D i v i s i o n of Basic Energy Sciences, f o r t h i s research program i s a l s o g r a t e f u l l y acknowledged.


18.

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Metal Oxide Systems

289

Literature Cited

1. 2. 3. 4. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 8, 2017 | http://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

Marinder, B.; Magneli, A. Acta Chem. Scand. 1957, 11, 1635. Goodenough, J.B. "Magnetism and the Chemical Bond," Interscience Monographs in Chemistry, Inorganic Section, Vol. 1, F. A. Cotton, Ed., Interscience Division, John Wiley and Sons, Inc., New York, Ν. Y. 1963. Goodenough, J.B. Bull. Soc. Chim. France 1965, 4, 1200. Rogers, D.B.; Shannon, R.D.; Sleight, A.W.; Gillson, J.L. Inorg. Chem. 1969, 8, 841. Sleight, T.P.; Hare, C.R.; Sleight, A.W. Mat. Res. Bull. 1968, 3, 437. Skaik, S.; Hoffman, R. J. Am. Chem. Soc. 1980, 102, 1194. Abraham, F.; Trehoux, J.; Thomas, D. J. Solid State Chem. 1979, 29, 73. Waltersson, K. Acta Crystallogr. 1976, B32, 1485. Schäfer, H.; von Schnering, H.G. Angew. Chem. 1964, 76, 833. Imoto, H.; Corbett, J.D.; Cisar, A. Inorg. Chem. 1981, 20, 145. McCarroll, W.H.; Katz, L.; Ward, R. J. Am. Chem. Soc. 1957, 79, 5410. Ansell, G.B.; Katz, L. Acta Crystallogr. 1966, 21 482. McCarroll, W.H. Inorg. Chem. 1977, 16, 3351. Donohue, P.C.; Katz, L. Nature (London) 1964, 201, 180. Kerner-Czeskleba, H.; Tourne, G. Bull. Soc. Chim. Fr. 1976, 729. Cotton, F.A. Inorg. Chem. 1964, 3, 1217. Torardi, C.C. and McCarley, R.E. J. Solid State Chem. 1981, 37, 393. Torardi, C.C. and McCarley, R.E. to be published. Torardi, C.C. and McCarley, R.E. J. Am. Chem. Soc. 1979, 101, 3963. Simon, A. Angew. Chem. Int. Ed. Engl. 1981, 20, 1. Corbett, J.D. Acc. Chem. Res. 1981, 14, 239. McCarley, R.E.; Ryan, T.R.; Torardi, C.C. ACS Symp Ser. 1981, 155, 41. Pauling, L. The Nature of the Chemical Bond, 3rd Ed. Cornell University Press, 1960, p.400. Chisholm, M.H.; Huffman, J.C.; Leonelli, J. J. Chem. Soc., Chem. Commun. 1981, 270. Chisholm, M.H.; Huffman, J.C.; Kirkpatrick, C.C.; Leonelli, J.; Folting, K. J. Am. Chem. Soc. 1981, 103, 6093. Cotton, F.A.; Fang, A. J. Am. Chem. Soc. 1982, 104, 113. Brough, L.F.; Carlin, R.T.; McCarley, R.E. Results to be published. Poeppelmeier, K.R.; Corbett, J.D. J. Am. Chem. Soc. 1978, 100, 5039. Mattausch, H.J.; Simon, Α.; Eger, R. Rev. Chim. Miner. 1980, 17, 516. Berroth, K.; Simon, A. J. Less-Common Met., to be published.


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31 32 33.

INORGANIC

TOWARD

THE

21ST

CENTURY

Chevrel, R.; Sergent, M.; Prigent, J. J. Solid State Chem. 1971, 3, 515. Yvon, K. Current Topics Mat. Sci. 1978, 3, 55. Marinder, B.-O. Chem. Scripta 1977, 11, 97.

RECEIVED


CHEMISTRY:

September 13,

1982

Discussion J.P. Fackler, J r . , Case Western Reserve University: Bob, I f i n d y o u r s t u d y t o be most i n t e r e s t i n g . Has i t been p o s s i b l e f o r y o u t o i s o l a t e m i x e d 0,S p h a s e s o f t h e C h e v r e l t y p e by o x y g e n s u b s t i t u t i o n ?

R. Ε. McCarley: No, we have done very l i t t l e work along t h i s l i n e , although I t h i n k important r e s u l t s w i l l be obtained from s t u d i e s of mixed 0, S phases. So f a r we have found no d i r e c t s t r u c t u r a l analogies between the reduced ternary molybdenum oxide phases and the v a r i o u s ternary s u l f i d e or s e l e n i d e phases of the Chevrel type.

A.R. S i e d l e , 3M C e n t r a l R e s e a r c h L a b o r a t o r y : I f one o f t h e e x t e n d e d s t r u c t u r e s d e s c r i b e d by P r o f e s s o r M c C a r l e y w e r e t r u n c a t e d t h r o u g h a low M i l l e r i n d e x p l a n e , c a n one, f o l l o w i n g t h e a p p p r o a c h of Solomon, p r e d i c t what m e t a l o r b i t a l s w o u l d p r o t r u d e f r o m t h e s u r f a c e so g e n e r a t e d ? Have u l t r a v i o l e t p h o t o e l e c t r o n s p e c t r a been o b t a i n e d on s i n g l e c r y s t a l s of any o f t h e s e materials?

R. Ε. McCarley: In response to the f i r s t question, i t should be easy to p r e d i c t those metal o r b i t a l s which would pro trude from the 001 and 002 faces of N a M o ^ ô , i . e . those faces normal to the d i r e c t i o n of chain growth. However, up to the present time we have not considered such questions because of our g e n e r a l l y poor understanding of bonding d e t a i l s i n these extended systems. In reference to the l a t t e r question, we have obtained UPS data on p o l y c r y s t a l l i n e samples of NaMo406, but not on s i n g l e c r y s t a l s . These measurements i n d i c a t e a r e l a t i v e l y high d e n s i t y of s t a t e s at the Fermi l e v e l , which i s i n agreement with the low r e s i s t i v i t y and m e t a l l i c character of the m a t e r i a l .


Metal Clusters and Extended Metal-Metal Bonding in Metal Oxide

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