Trigonal-Prismatic Coordination - Advances in Chemistry (ACS

42 Trigonal-Prismatic Coordination H A R R Y B. G R A Y ,

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R I C H A R D E I S E N B E R G , and E D W A R D I. S T I E F E L 2

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Downloaded by CORNELL UNIV on May 30, 2017 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch042

Department of Chemistry, Columbia University, New York, N. Y.

Recently several 6-coordinate metal complexes have been shown to exhibit a trigonal-prismatic rather than an octa hedral structure. The events leading to the discovery of the new coordination geometry are placed in historical per spective. The evidence which led to assigning the trigonal-prismatic structure to many of the complexes is summarized. At present, at least 14 complexes definitely possess the unusual geometry and, furthermore, the struc ture is highly likely for many other species. The bonding in trigonal-prismatic complexes is described in molecular orbital language. Electronic structural considerations show that it is not meaningful to assign definite oxidation states to the central metal in these extremely covalent sys tems. Finally, the possible reasons for the occurrence of trigonal-prismatic coordination are discussed, and some speculation is offered as to its scope. In

a brilliant paper published i n 1893 (38), Alfred Werner established octahedral stereochemistry for the 6-coordinate transition metal ion. In his deliberations Werner also considered both the hexagonal-planar and trigonal-prismatic structures but successfully eliminated these in the cases he investigated. It is a great tribute to Werner that i n the 70-odd years since his historic work, countless structural investigations have failed to reveal a truly nonoctahedral, molecular, 6-coordinate complex. It has only been i n the past year that an authentic example of nonoctahedral stereochemistry has been found. Werner's experiments were performed i n liquid solutions, and the complexes he worked with maintain their basic octahedral structure i n the 1

2 3

Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, Calif. National Science Foundation Predoctoral Fellow, 1965-66. National Science Foundation Predoctoral Fellow, 1965-66. 641

Kauffman; Werner Centennial Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

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WERNER CENTENNIAL

solid state. Our main interest, as was Werner's, is i n molecular complexes, but it should be noted that a 6-coordinate geometry other than octahedral has been known in certain crystals for many years. For example, in 1923, Dickinson and Pauling (11) found S trigonal-prismatic coordination in molybdenite (MoS ) and tungstenite (WS ). Since then, the N i A s struc ture has been found to possess, in part, a trigonal-prismatic array of atoms (40). In these compounds the lattice structure is infinite and threedimensionally extended, and it is clear that crystal packing is a principal determinant of the stereochemistry. 6


2

2

The first molecular, 6-coordinate complexes which proved to adopt a nonoctahedral structure contain the bidentate, sulfur-donor ligands of the general structures, I and I I : R

R

s-

I

c

R

S-

/

S:

>

>

/

V \ /

I la lb Ic Id Ie

R R R R R

= = = = =

II CN(mnt) Ph(sdt) CF H CH

H a R = H(bdt) l i b R - CH,(tdt)

3

3

The first of these ligands to be used was toluene 3,4-dithiolate (tdt) ( l i b ) . Clark used this ligand (6, 7) in 1936 for determining tin. Since that time, tdt has been used by several investigators (3, 5, 23, 28) as a qualitative test for such metals as M o , W , and Re, but no attempts were made to isolate any of the complexes (much less establish their geometry). The first actual isolation of a complex was by Gilbert and Sandell who made M o (tdt) (22). However, these workers were not able to prepare an analytically pure sample. 3

In 1963, the first well-characterized tris complex i n the I series was prepared by K i n g (24) i n the reaction of M o ( C O ) with bis(trifluoromethyl)dithietene to give M o ( S C ( C F ) ) . 6

2

Mo(CO)

6

2

3

2

3

+


42.

Shortly before this time a substantial interest had developed in these ligands for a very different reason (10, 31, 39). W i t h the metals i n the Fe through C u families i n the transition series, these ligands form bis com plexes with two very unusual properties. Firstly, the square planar geom etry is stabilized over a large range of metals including Fe, Co, R h , N i , P d , P t , C u , A g , and A u . Secondly, for any given metal and ligand the complex is capable of stable existence in more than one oxidation state. The various oxidation states are simply related by one-electron transfer reactions. For example, N i ( s d t ) ~ has been isolated as n = 0, 1, and 2. Because of the unusual nature of these complexes, several workers sought to extend the square planar geometry by producing bis complexes of metals which lie farther to the left in the periodic table. A t one point (83) it was reported that bis planar complexes of sdt with V , C r , M o , W , Re, R u , and Os had been synthesized. On closer examination, however, i t turned out that these were not unusual 4-coordinate complexes but were instead 6coordinate systems. The 6-coordinate nature of these complexes was firmly established in several different studies (8, 30, 37), and many other 6-coordinate complexes of the bidentate sulfur-donor ligands were synthesized and characterized (9, 26). 2


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Trigonal Prisms

GRAY, EISENBERG, AND STIEFEL

n

In one of these studies (26), it was observed that Co(mnt) ~~ reversibly adds one mole of ligand to form the complex Co(mnt) "" . The tendency of the square planar Co(mnt) ~ anion to form readily 6-coor dinate adducts with bidentate ligands and only 5-coordinate species with monodentate ligands led to the suggestion that the tris complexes might possess a trigonal-prismatic structure. 2

3

3

2

Langford (25), in an attempt to confirm or disprove this possibility, tried unsuccessfully to resolve the Co(mnt) ~ complex into optical isomers. Octahedral complexes containing bidentate ligands are reduced to D symmetry and should be resolvable because of the absence of mirror planes or symmetry centers. A t the same time, Archer (2) tried to resolve the neutral M o ( S C ( C F ) ) complex, but he too was unsuccessful. The inability to resolve the tris complexes, C o ( m n t ) " and M o ( S C ( C F ) ) , raised some doubts concerning the conventional octahedral formulation of these systems. 3

3

3

2

2

3

2

3

3

3

2

2

3

2

3

In an x-ray structural investigation (15, 16) of the 6-coordinate Re (sdt) complex, rhenium was found to be surrounded by the six sulfur atoms i n a nearly perfect, trigonal-prismatic coordination. The sides of the prism are square with no significant difference between the average intra- and closest interligand S-S distances. A perspective drawing of the coordination geometry is shown i n Figure 1. Only the twisting of the phenyl rings out of the planes of the five-membered, metal-chelate rings prevents this molecular complex from having rigorous D symmetry. Unlike the hexagonal lattices of M o S and W S , the crystal structure of 3

zk

2

2



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WERNER CENTENNIAL

Figure 1.

Coordination geometry of the Re(sdt)z complex

Re(sdt) consists of the packing of discrete, well-separated, molecules, the closest Re-Re approach being approximately 9.5 A. With this structure determination, it became important to establish the generality of trigonal-prismatic coordination in the tris complexes of the bidentate sulfur ligands. First, however, it had to be shown that the molecular geometry found for Re(sdt) in the crystal was maintained in solution. This problem was easily solved by a comparison of the solid state and solution properties of Re(sdt) . It was found (36) that the electronic and electron spin resonance spectra of the complex remain essentially unchanged on dissolution in nonpolar solvents such as C H C 1 and C C U . Thus, the trigonal-prismatic coordination remains in solution. The main feature of the electronic spectrum consists of an intense, two-band pattern in the visible region which gives both the solution and the crystals their dark green color. Now the probe began as to whether the trigonal-prismatic coordination was limited to this particular ligand, or to rhenium, or perhaps to just this particular complex. In checking for other possible ligands, it was realized from previous experience that a neutral complex would most likely be produced using tdt as a ligand. Thus, toluene 3,4-dithiol reacted with ReCl in CC1 and Re(tdt) was prepared (36). A comparison of the electronic spectra, E S R spectra in solution, frozen glass ESR spectra, and polarographic behavior revealed the Re (tdt) complex to be amazingly 3

3

3

3

5

4

3

3


42.

645

Trigonal Prisms


similar to the Re (sdt) complex. The new geometry was thus strongly confirmed for the tdt complex, and we see that the "non-classical" structure is not limited *to the sdt ligand. The next step was to show that this geometry is not limited to Re alone, for it is well known that Re forms a multitude of quite unusual com plexes, not the least of which is R e H ~ which has the geometry of a facecentered trigonal-prism (21). It was first found that W(sdt) and Re (sdt) are isomorphous (IS, 16, 86), as shown by their x-ray powder patterns, indicating the probability of trigonal-prismatic coordination for W(sdt) . The spectrum of W(sdt) is the same in solid and solution (85) and, signif icantly, the dominant feature of the spectrum is again an intense, two-band pattern i n the visible region much like that found in the Re complexes. T h i s pattern has now been found (85) in all of the neutral complexes of R e , W , and M o with the bidentate sulfur-donor ligands. The first band always occurs at ~14,000 c m . " with e ~20,000, and the second always occurs at ~24,000 cm."" with € ~15,000. We consider these bands as characteristic of the trigonal-prismatic MSe chromophore for second- and third-row transition elements. It is this pattern which gives all of the complexes their characteristic green to blue-green color. Further evi dence for the occurrence of the trigonal-prismatic coordination i n M o com plexes was supplied by an x-ray study of Mo(S2C2H ) by Smith and co workers (84), which showed this complex to have an MSe framework almost identical to that found i n Re (sdt) . Thus, there is good evidence that all the neutral complexes of M o , W , and Re with these sulfur-donor ligands are trigonal-prismatic. 3

9

2

3

3

3


3

1

1

2

3

3

The coordination geometry of the tris complexes containing first-row transition metals still had to be established. While isomorphism studies indicated the possibility of trigonal-prismatic coordination for V(sdt) and Cr(sdt) , direct proof of this unusual geometry was clearly needed. T o this end, the molecular structure of V ( S C P h ) was determined (18, 17). Once again, the metal is found at the center of a trigonal-prismatic array of sulfur atoms. The average intra- and interligand S-S distances are approximately equal (3.058 and 3.064 A . , respectively), but there is a slight distortion of the prism about the three-fold axis. The dimensions of the prism are strikingly similar to those found for Re (sdt) and M o ( S C H ) . It is quite remarkable that i n going from the first-row complex V ( S C P h ) to analogous second- and third-row systems the closest interligand sulfur-sulfur distance does not change significantly (3.050 A . i n Re(sdt) and 3.11 A . in M o ( S C P h ) ) , and that the metalsulfur distance is effectively constant (V-S, 2.337 A . ; M o - S , 2.33 A . ; Re-S, 2.325 A . ) . The implications of the near constancy of the S-S distances i n these complexes are discussed below. 3

3

2

2

2

3

3

2

2

2

3

2

2

2

3

3

2

2

2

3

On the basis of its isomorphism with V(sdt) , the chromium complex is assumed to be trigonal-prismatic. A comparison of the spectra of the 3


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WERNER

CENTENNIAL

isoelectronic species Cr(sdt) and V(sdt) ~ shows them to be quite similar and indicates that the trigonal-prismatic coordination extends to V(sdt) ~. It is now of interest to look at the polarographic behavior of all of these complexes. A s i n the bis series, it is found that reversible one-elec tron transfers occur (8, 9, 85, 86), corresponding to the reductions M L —» M L r , M L r -> M L r , and (often) M L r -> M L r . In addition, there is a reversible, one-electron oxidation for the R e L complexes. These indi cate at least the transient existence of reduced (and oxidized) species which probably also possess the trigonal-prismatic geometry. However, there is considerable uncertainty as to the structure these complexes will adopt when they are isolated as salts. Certain of the reduced forms have already been isolated with the ligands mnt, l a , and tdt ( l i b ) , and it is possible that one or more of them will exhibit a conventional octahedral structure (18, 17). More will be said on this point later. 3

3

3

3

2

2

3


3

We would now like to reach some kind of an understanding as to the bonding i n these complexes. Based on past experience, it is obvious that a molecular orbital treatment is the best way to describe the bonding be cause of the thoroughly delocalized nature of the electronic systems and because of the significant role the ligand plays i n the electronic structure. Molecular orbital calculations were performed (35) on Re(sdt) i n D symmetry which corresponds to the coordination geometry as found i n the original structure determination. The principal electronic energy levels are shown i n Figure 2. The levels labeled 2a ' 3e', and 3e" are mainly combinations of in-plane, w functions on the ligands. These levels are filled with 10 electrons i n the complexes under consideration. The level 4e' is a delocalized combination of the 3x„ ligand functions with the d i- i and d metal functions. This level is filled with four electrons i n the complexes. The next higher molec ular orbital is 2a ', which is nonbonding in the complex and localized on the ligands. It is strictly composed of ligand ST functions. The next three levels are of d symmetry, and may be considered the ligand field levels i n trigonal-prismatic complexes. I n order of increasing energy they are 3a/(z ) < 5e'(x - y , xy) < 4e"(xz, yz). According to the energy levels shown in Figure 2, the ground state electronic structure for R e ( S C P h ) is (4e ) (2a ) (3ai ) = T h i s ground state is compatible with the gross features of the E S R and magnetic susceptibility results for Re(sdt) . Another recent molecular orbital calculation (82) for these systems predicts a E' ground state for the neutral R e complex, but this state is inconsistent with the observed magnetic properties. 3

2

zh

;

x

v

xy

2

V

2

2

2

2

2

2

/

3

4

2

,

2

/

1

3

2

The oxidation state assignment problem stems from the nature of the filled 4e' level. The classical case of Re (VI) and a ligand unit of L r corresponds to 4e' being designated a ligand level. If 4e' is more properly identified with the metal, the four electrons should be included with the 6


42.

/

r

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Trigonal Prisms


4e"

d

/ / / / /

/ Downloaded by CORNELL UNIV on May 30, 2017 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch042

/ 6s

/

VA ,

'

5e' 1_

d2 2 x -y ,xy \ / 3^v

\

u

2

2

\

\

V

\

\ —frit (» ) h

Figure 2.

11 11 11 11

3e"

\

h

n

3e'

It

Suggested molecular orbital energy levels for Re(sdt)z

metal and the oxidation state assignment is [Re(II)][L " ]. However, i t is not possible to state unequivocally the "ownership" of 4e', owing to its highly delocalized nature. The various calculations on these systems and the optical spectral and polarographic experiments tend to confirm this uncertain state of affairs. Therefore, we are left with the general molecular orbital model as the only realistic means of describing the ground state electronic structures of the MS6-type, trigonal-prismatic systems. The neutral W and M o complexes contain one less valence electron, and we expect the highest filled orbitals to be (4e') (2a ') or a 3

2

4

American Chemical Society Library

1155 Werner 16th St., N.W. Kauffman; Centennial Advances in Chemistry; American Chemical Washington, DC, 1967. Wasbiftftofi OSociety: X . 20036

2

2

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WERNER

CENTENNIAL

Ai ground state. We do not expect the molecular orbitals to change greatly in going from R e to W or M o , and thus the limiting oxidation-state formulations are still M ( I I ) [ L ] and M ( V I ) [ L ~ ] , with neither isolated configuration providing a true picture of electronic structure. l

3

-2

6

3


We are now i n a position to speculate on factors which contribute to the stabilization of the trigonal-prismatic geometry. It is important while doing this to keep in mind that x-ray and other evidence have been presented that establishes square planar coordination for 4-coordinate complexes containing structurally-related sulfur-donor ligands. From the nine x-ray structural determinations (12, 13, H, 15, 16, 17, 18,19, 20, 29, 84) on various complexes containing these bidentate sulfurdonor ligands, it is an interesting and significant result that, independent of the coordination geometry or the central metal, the S-S distance always takes a value close to 3.05 A . We take this relatively short, nominally "nonbonded" S-S distance to indicate that there are interligand bonding forces present i n these complexes which are considerably stronger than i n classical octahedral, tetrahedral, or planar complexes. It is the com promise between these S-S bonding interactions and the S-S nonbonded repulsions which leads to the ubiquitous 3.05 A . separation and to the stability of the nonclassical structures. Assuming this argument is sound, we extrapolate it and suggest that 8-coordinate complexes containing these sulfur-donor ligands in a state of oxidation comparable to Re(sdt) should exhibit cubic (or approximately cubic) coordination, with each edge of the S cube being 3.0-3.1 A . This requires an M - S distance of 2.6 A . , which may possibly be attained using actinide central metals. 3

8

A contributing factor to the stability of trigonal-prismatic coordina tion may be the effective use of the three valence d orbitals not involved i n strong a bonding. For example, involvement of the sulfur TH orbitals and the metal d leads to a stable bonding orbital (2ai), which is occupied i n all the complexes. Another possible stabilizing influence for trigonalprismatic coordination is the large interaction of the d , d 2- 2 orbitals with the thoroughly delocalized ligand ST level. This leads to a particu larly stable 4e' level, which is occupied i n a l l of these complexes. A stabilizing a interaction i n the e' molecular orbitals has also been put for ward (21) as an explanation of trigonal-prismatic stability. I n this respect, it is pertinent to note that R e H ~ has the structure (1) of a facecentered trigonal prism, with the three face-centered hydrogens strongly a bonded to the metal, presumably through extensive interaction with the d , d -y2 orbitals. I n the Re(sdt) trigonal prism, the ligand T orbit als may play a stabilizing role similar to that of three face-centered hydrogens i n ReH$r . Further investigations should help elucidate whether or not irh-d»t bonding, strong T "d , d - 2 bonding, and S-S bond ing are essential features of trigonal-prismatic coordination. z2

xy

X

V

V

9

xy

2

3

x2

V

2

V

xy

x 2

y


42.


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Trigonal Prisms

The next step in determining the overall structural patterns in the tris-bidentate-sulfur complexes is to investigate by x-ray methods one or more of the species reduced to at least the dianionic stage. It is a fact that the three available structures are all on the highly oxidized, neutral M L complexes and, aside from some physical evidence on certain of the M L ~ complexes, nothing is known about the structures of the anionic systems. If S-S bonding is a strong contributing factor to the stability of the trigonal-prismatic complexes, the reduced species M L ~ and M L ~ may prefer to adopt an octahedral (Z>) configuration. Presumably, this is because the reduced species must accommodate electrons in antibonding molecular orbital levels (such as 5e') which possess a high degree of sulfurorbital character. Placing electrons in these levels will tend to decrease the S-S bonding and therefore favor an octahedral (staggered) arrangement of donor atoms. Recent work (4,21) has afforded a number of new complexes, the struc tures of which should be of great interest. These complexes and their mag netic ground states are Mn(mnt) " (S = 3/2), Fe(mnt) ~ (S = 1), Mo(mnt)r (S - 0), W(mnt)r (S - 0), and Re(mnt)r (S = 1/2). Structural studies on several of these systems are now in progress (27). 3

3

2

8

3

3


3

2

2

2

2

2

3

2

Acknowledgment This research was supported by the National Science Foundation. Literature

Cited

(1) Abrahams, S. C., Ginsberg, A. P., Knox, K., Inorg. Chem. 4, 559 (1965). (2) Archer, R., private communication. (3) Bagshawe, B., Truman, R. J., Analyst, 72, 189 (1947). (4) Bennett, L., Simo, C., Crawford, T. H., Stiefel, E . I., Gray, H. B., unpublished work. (5) Bickford, C. F., Jones, W. S., Keen, J. S., J. Am. Pharm. Assoc. Sci. Ed. 37, 255 (1948). (6) Clark, R. E. D., Analyst 61, 242 (1936). (7) Ibid. 62, 661 (1937). (8) Davison, A., Edelstein, N., Holm, R. H., Maki, A. H., Inorg. Chem. 4, 55 (1965). (8) Ibid., J. Am. Chem. Soc. 86, 2799 (1964). (10) Ibid., Inorg. Chem. 3, 814 (1964). (11) Dickenson, R., Pauling, L., J. Am. Chem. Soc. 45, 1466 (1923). (12) Enemark, J. H., Lipscomb, W. N., Inorg. Chem. 4, 1729 (1965). (13) Eisenberg, R., Gray, H. B., Inorg. Chem., in press. (14) Eisenberg, R., Ibers, J. A., Inorg. Chem. 4, 605 (1965). (15) Ibid. 5, 411 (1966). (16) Ibid., J. Am. Chem. Soc. 87, 3776 (1965). (17) Eisenberg, R., Stiefel, E. I., Rosenberg, R. C., Gray, H. B., J. Am. Chem. Soc. 88, 2874 (1966). (18) Forrester, J. D., Zalkin, A., Templeton, D. H., Inorg. Chem. 3, 1500 (1964). (19) Ibid. 3, 1507 (1964). (20) Fritchie, C. J., Jr., Acta Cryst. 20, 107 (1966).



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(21) Gerloch, M . , Kettle, S. F. A., Locke, J., McCleverty, J . A., Chem. Commun. 1966, 29. (22) Gilbert, T. W., Sandell, E . B., J. Am. Chem. Soc. 82, 3221 (1960). (23) Hammence, J . H . , Analyst 65, 152 (1940). (24) King, R. B., Inorg. Chem. 2, 641 (1963). (25) Langford, C. H . , unpublished results. (26) Langford, C. H . , Billig, E., Shupack, S. I., Gray, H . B., J. Am. Chem. Soc. 86, 2958 (1964). (27) McCleverty, J . A., private communication. (28) Miller, C. C., J. Chem. Soc. 1941, 792. (29) Sartain, D., Truter, M. R., Chem. Commun. 1966, 382. (30) Schrauzer, G. N., Finck, H . W., Mayweg, V., Angew. Chem. 76, 715 (1964). (31) Schrauzer, G. N., Mayweg, V . P., J. Am. Chem. Soc. 87, 3585 (1965). (32) Ibid. 88, 3235 (1966). (33) Schrauzer, G. N., Mayweg, V., Finck, H. W., Müller-Westerhoff, U . , Heinrich, W., Angew. Chem. 76, 345 (1964). (34) Smith, A . E., Schrauzer, Mayweg, V . P., Heinrich, W., J. Am. Chem. Soc. 87, 5798 (1965). (35) Stiefel, E . I., Eisenberg, R., Rosenberg, R. C., Gray, H. B., J. Am. Chem. Soc. 88, 2956 (1966). (36) Stiefel, E . I., Gray, H. B., J. Am. Chem. Soc. 87, 4012 (1965). (37) Waters, J. H . , Williams, R., Gray, H . B., Schrauzer, G. N., Finck, H . W., J. Am. Chem. Soc. 86, 4198 (1964). (38) Werner, A., Z. Anorg. Chem. 3, 267 (1893). (39) Williams, R., Billig, E., Waters, J. H . , Gray, H . B., J. Am. Chem. Soc. 88, 43 (1966). (40) Wyckoff, W. G., "Crystal Structures," Vol. I., Chap III, p. 28. RECEIVED July 5, 1966.


Trigonal-Prismatic Coordination - Advances in Chemistry (ACS

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