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Metal cluster topology—IX. Mercury vertices in transition metal clusters and alkali metal amalgams
Po~yhfrm Vol. 7, No. 19/20, pp. 181>1817, Printed in Great Britain
1988 0
METAL CLUSTER TOPOLOGY-IX.? IN TRANSITION METAL CLUSTERS AMALGAMSS
0277-5387188 S3.00+ .OO 1988 Pergamon Press
plc
MERCURY VERTICES AND ALKALI METAL
R. B. KING
Department of Chemistry, University of Georgia, Athens, GA 30602, U.S.A. Abstract-Mercury vertices in metal clusters use a seven-orbital spd’ bonding manifold with linear sp hybrids for strong primary bonding and dn + px* and/or da + pa* interactions for weaker secondary bonding. Transition metal clusters containing mercury vertices can be classified into two types: two-dimensional clusters in which the mercury atoms form a polygon such as a triangle in [HgOs,(CO),,], or a square in [HgMn(CO)2C5H4Me]4 and three-dimensional clusters in which the mercury atoms form a polyhedron such as an octahedron in Hg,[Rh(PMe,),], or a tricapped trigonal prism in Hg9[Co(C0)3]6. The structures of such clusters consist of a mercury polygon or polyhedron having secondary Hg-Hg interactions inside a transition metal macropolygon or macropolyhedron with M-Hg-M edges having primary Hg-M interactions. These clusters thus all exhibit edgelocalized chemical bonding topology. Alkali metal amalgam structures are constructed from mercury quadrilaterals of which the prototypical species is square Hgi-, found in discrete units in the structure of Na3Hg2. This homonuclear mercury cluster anion may be regarded as a globally delocalized two-dimensional planar polygonal aromatic system with an aromatic sextet like benzene but with no a-bonding between adjacent mercury atoms in the Hgz- square.
A characteristic property of metal carbonyls, which was recognized in the very early days of the development of their chemistry, is their reactivity towards mercury and its compounds to form transition metal-mercury derivatives, which in general are relatively stable towards air and water. Early discoveries of this type include the formation of cisFe(CO),(HgCl), from Fe(CO), and HgClz2 and the formation of Hg[Co(CO)& from Co2(CO)8 and elemental mercury.3 Mercury derivatives of metal carbonyls such as Hg[Mn(CO),],, Hg[Co(C0)4]2, Hg[Mo(C0)3C,H5]2 and Hg[Fe(C0)2C5H,]2 are frequent by-products from the preparation of metal carbonyl anions by the treatment of binuclear metal carbonyl derivatives with sodium amalgam.4 Structural studies on mercury metal carbonyl derivatives such as Hg[Mn(C0)5]2,5’6 Hg[Co (co)412y7
Hg[CWO)3PEt3128and Hg[Fe(W2
(NO)PEt,]29 indicate linear M-Hg-M
units con-
7 For part VIII, see ref. 1. $This paper is dedicated to Sir Geoffrey Wilkinson, F.R.S., in recognition of his many contributions organometallic and inorganic chemistry. (Receiued 18 April 1988).
to
taining two-coordinate mercury atoms. This area of transition metal chemistry thus appeared to be relatively uninteresting until the seminal discovery by Wilkinson and co-workers” of the sodium amalgam reduction of [(Me3P)4Rh]Cl or (Me3P)3RhCl to give high yields of the high nuclearity transition metal-mercury cluster Hge[Rh(PMe3),],, which was shown by X-ray diffraction to consist of an Hg6 octahedron with four tetrahedrally related faces capped by rhodium atoms. This paper surveys the currently known types of such high nuclearity transition metal-mercury clusters with the objective of relating their skeletal bonding to previously discussed”-‘4 graphtheory derived models for metal cluster bonding topology. An important conclusion from this study is that almost all of the complicated transition metal-mercury cluster structures can be derived from simple edge-localized bonding models with two-coordinate mercury vertices. The ideas developed in this study of transition metal-mercury clusters also provide some insight into the chemical bonding topology of alkali metal amalgams, which are important as reducing agents in metal carbonyl chemistry and other areas of transition metal chemistry. Thus Wilkinson” as well as Hieber16 and
1813
1814
R. B. KING
their co-workers were pioneers in such applications of alkali metal amalgams for the preparation of metal carbonyl anions from neutral metal carbonyl dimers. MERCURY
VERTICES
The position of mercury to the right of gold in the periodic table suggests that mercury vertices will have some similar properties to gold verticesi in metal clusters. Thus two of the outer p orbitals in both mercury and gold can be shifted to such high energies that they no longer participate in the primary chemical bonding. ‘7*‘8This manifests itself structurally in d’ ’ Au’ and Hg” as two linear sp hybrid orbitals superimposed on a spherically symmetrical filled d-orbital manifold thereby leading to a cylindrical spd’ seven-orbital primary bonding manifold, which requires 14 electrons for filling. This limitation of the primary bonding of mercury vertices to two linear sp hybrids prevents the construction of stable polyhedra containing exclusively mercury vertices. However, transition metal cluster macropolyhedra can be constructed in which the edges are nearly linear M-Hg-M units rather than simple M-M bonds. Thus the Hg6[Rh(PMe,),], clusterlO can be interpreted as an Rh4 macrotetrahedron with six Rh-Hg-Rh edges. The empty p orbitals not in the spd’ cylindrical bonding manifold of the d lo Hg” can participate in secondary dx +pn* or da + da* bonding similar to that suggested by Dedieu and Hoffmann” for isoelectronic d” Pt” complexes and by a previous paper of this seriesI to account for the peripheral Au-Au bonding distances in centred gold clusters. The mercury-mercury interactions within the mercury polygons and polyhedra in the transition metal-mercury clusters consist exclusively of such secondary bonding since the mercury sp hybrid orbitals are used for primary bonding to transition metals in the M-Hg-M edges of the transition metal macropolyhedra. The mercury-mercury distances in polynuclear mercury derivatives (Table 1) provide an excellent indication as to the nature of the mercury-mercury interactions. In Hg,2+ derivatives as well as similar derivatives of longer linear mercury chains (e.g. Hg:+, Hg$+) the mercury-mercury distances fall in the range 2.42.7 A indicative of strong primary covalent bonding arising from overlap of linear sp hybrid orbitals. 20-22 In cases where the primary mercury bonding is diverted to other atoms, such as transition metal atoms in transition metalmercury clusters, the mercury-mercury distances increase to the range 2.9-3.2 A indicating the
Table 1. Mercury-mercury Compound Hg metal Hg,Fz Hg,Cl, Hg,Br, HgA Hg0sF.A Hg,(AlCl& Hg,(AsF& Hg,.,AsF,
[HgOs,(CQ ,113
WgWCOW5H4Mel~ Hg6DW+W314 H&K+WO)316 Nd-MHd-) NaHg KHg NaHg, KHg,
bond distances Hg-Hg
(A)
3.00, 3.47 2.43 2.45 2.50 2.69 2X2(4) 2.56 2.70 (inner), 2.57 (outer) 2.64(l) 3.122(3), 3.082(3), 3.097(3) 2.888(2) 3.131(3E3.149(3) 3.094 (av., trig.-trig.), 3.151 (av., trig.-sq.) 2.99 3.05, 3.05, 3.22 3.02, 3.04, 3.36 2.90, 3.23 3.00, 3.02, 3.08
presence of only secondary mercury-mercury bonding. Transition metal clusters containing mercury vertices can be classified into two types: twodimensional clusters (Fig. 1) in which the mercury atoms form a polygon such as a triangle in or a square in [HgMn(CO), [HgOs3(CO)1 11323 C5H4Me]424 and three-dimensional clusters (Fig. 2) in which the mercury atoms form a polyhedron
Fig. 1. Transition metal mercury clusters containing mercury polygons. Mercury vertices are starred, mercury-mercury bonds and external groups (carbonyl and cyclopentadienyl) are omitted, and transition metal + mercury dative bonds are indicated by arrows.
1815
Metal cluster topology--IX.
Hg.$o (Co),] 6
Fig. 2. Transition metal mercury clusters containing mercury polyhedra. Mercury vertices are starred and mercury-mercury bonds and external groups (carbonyl and trimethylphosphine) are omitted.
in Hg,[Rh(PMe,),],” or a tricapped trigonal prism in Hg,[Co(CO) J 6.25Transition metal clusters containing mercury polygons arise from bifunctional transition metal fragments such as Fe(CO), and CpMn(CO)* leading to degree two vertices in the large transition metal macropolygon. Transition metal clusters containing mercury polyhedra arise from trifunctional transition metal fragments such as COG and Rh(PR& leading to degree three vertices in the large transition metal macropolyhedron. The graph of the polygon or polyhedron formed by the mercury vertices is thus the line graph26 of the graph of the polygon or polyhedron formed by the transition metal vertices. such as an octahedron
MERCURY
POLYGONS
Combinations of difunctional transition metal vertices with mercury vertices leads to clusters with equal numbers of transition metal and mercury vertices consisting of a mercury polygon within a transition metal macropolygon (Fig. 1). The bond angles at the transition metal vertices determine the sizes of the polygons with squares apparently being preferred. A structurally characterized cluster of this type is mgMn(C0)2CSH.,Me]4.24 The difunctional Mn(C0)2C5H4Me vertices have the favoured 18-electron rare gas configuration for the manganese atom and are derived from the corresponding CpMn(CO), system through a formal oxidative addition reaction. The Mn-Hg-Mn edges of the Mn, macrosquare in [HgMn(C0)2C5H4Me]4 are bent inwards towards the centre of the square in accord with secondary mercury-mercury bonding
leading to 2.888(2) Hg-Hg bond distances. A similar structure (Fig. 1) can be postulated for the longknown2927but insoluble [HgFe(CO),], on the basis of its similar IR spectrum to the structurally characterized” [CdFe(CO),],* 2Me,CO. A structure containing a mercury triangle (Fig. 1) is found in the “heteronuclear raft cluster”23 [HgOs,(CO), I13. In this case the difunctional transition metal vertex is the Os(CO), vertex of an 0s3(CO),, triangle. The Os-Hg bond lengths in the Os-Hg-Os edges of the 0~ macrotriangle fall in the range 2.71-2.76 A. In addition there is one 2.98-3.05 A dative bond from an Os(CO), vertex of each 0s3(CO)i 1 triangle to a mercury atom as indicated by arrows in Fig. 1. Such dative bonds from osmium atoms to other types of acceptor atoms have been found in structurally characterized complexes.29-3’ The Hg-Hg distances of 3.08-3.13 A in the mercury triangle of [HgOs3(CO)i,]3 are significantly longer than those in the mercury square of [HgMn(C0)2C5H4Me]4 suggestive of weaker mercury-mercury secondary bonding possibly because of the tram influence of the OS + Hg dative bond. The OS + Hg dative bond as well as the OS--OS bonds in the Os,(CO), , triangles perturb the external bond angles at the Os(CO), vertices so that a mercury triangle is found in [HgOs, (CO), ,I3 in contrast to the mercury square in lI-IgMn(CO)2C,H4Me]4 discussed above. In [HgOs, (CO), ,I3 all of the osmium atoms have the favoured 18-electron rare-gas configuration and the mercury atoms have the 1Celectron configuration corresponding to a filled cylindrical spd5 sevenorbital manifold. MERCURY
POLYHEDRA
Combination of trifunctional transition metal vertices with mercury vertices leads to clusters having a mercury polyhedron within a transition metal macropolyhedron (Fig. 2). The known examples of such clusters have trifunctional transition metal vertices of the type ML3 (M = Co, Rh, Ir ; L = CO, R3P). The number of mercury atoms in such clusters is equal to the number of edges in the transition metal macropolyhedron. If this macropolyhedron has m vertices all of degree three implied by the trifunctionality of the transition metal units at the vertices, then there are (3/2)m edges and such clusters have the overall stoichiometry [Hg3(ML3)2]x where x is even. The properties of the first three clusters of this homologous series, for which both the mercury polyhedron and the transition metal macropolyhedron are readily recognizable polyhedra, are listed in Table 2. In addition to the hypothetical cluster [Hg3(ML3)2]4 having Oh skel-
1816
R. B. KING
Table 2. Smallest possible skeletons for clusters of the type [Hg,(ML,)J, consisting of a mercury polyhedron inside a transition metal macropolyhedron Transition metal macropolyhedron
Mercury polyhedron x
Skeletal symmetry
2 3 4
Td Dv, Oh
Type
V
Octahedron Tricapped trigonal prism Cuboctahedron
e
6 12 9 21 12 24
f
Type
V
e
f
Example
8 14 14
Tetrahedron Trigonal prism cube
4 6 8
64 95 12 6
HgACo(CW6
eta1 symmetry consisting of a mercury cuboctahedron inside a transition metal macrocube, a less symmetrical isomeric cluster [Hgs(MLs)& having Cti skeletal symmetry is also possible in which the macropolyhedron is the eight-vertex cuneane polyhedron, namely the dua13* of the bicapped tetrahedron. I* The Wilkinson cluster lo Hg, [Rh(PMe,)J, with a mercury octahedron inside a rhodium macrotetrahedron and the Burlitch cluster2’ Hg,[Co(CO),], with a mercury tricapped trigonal prism inside a cobalt trigonal macroprism (Fig. 2) represent the first two members of this homologous series. In both of these clusters all of the transition metal atoms have the favoured 18-electron rare-gas configuration and all of the mercury atoms have the 1Celectron configuration corresponding to a full cylindrical spd’ seven-orbital manifold. The linear sp primary mercury bonding is used for the M-Hg-M edges of the transition metal macropolyhedra leaving only secondary bonding for the edges of the mercury polyhedra in accord with the 3.06-3.15 8, Hg-Hg distances in these polyhedra.
ALKALI
METAL
AMALGAMS
Several alkali metal amalgams have been structurally characterized including the sodium amalgams33 NaHg,, NaHg and Na,Hg,, and the potassium amalgams 34 KHg, and KHg. The most highly reduced of these species, namely Na,Hg,, has been shown33*35to contain discrete 2.99 8, square planar Hgi- clusters which are not isoelectronic with other square planar post-transition element clusters3c38 such as Bi:-,3g Se:+ 4o and Te:+.4’ In gold coloured NaHg33 slightly distorted 3.05 x 3.22 A Hg, rectangles are fused into a zigzag ribbon whereas in the likewise gold coloured KHg,34 slightly distorted (93.6 rather than 90°) 3.03 A Hg, squares are linked by 3.36 8, Hg-Hg bonds. More complicated networks of mercury rectangles and parallelograms are present in NaHg, and KHg,.
Hg6BW+W314
Thus the prototypical building blocks for alkali metal amalgams are derived from the Hg:- squares found in Na3Hg2. The mercury atoms in Hgz- can be considered to have seven-orbital spd5 cylindrical bonding manifolds. Six of these seven orbitals, namely the s orbital and the five d orbitals, are external orbitals, whereas the single p orbital is an internal orbital. A neutral mercury atom with six external orbitals is a zero skeletal electron donor42-44 so that the Hgianion has six skeletal electrons. Since the overlap of the external p orbitals in Hgz- has the topology of the square, the Hgi- anion has a n-bonding network similar to a planar aromatic hydrocarbon such as the cyclobutadiene dianion or benzene. Furthermore, the six skeletal electrons of Hgi- can all be rc-electrons so that Hgz- is an 4k+ 2 electron aromatic system with a n-electron network analogous to that of benzene. However, Hgi- hasneither the electrons nor the orbitals for any Hg-Hg abonding. Thus the Hgz- anion is a unique example of a two-dimensional aromatic system without any a-bonding. Thus the eight-electron difference in the skeletal electron counts of the square clusters Bi:and Hgi- relate to the presence of a-bonding in the Bij- square but the absence of a-bonding in the Hgi- square. In a number of areas of chemistry, including metal carbonyl chemistry, amalgamation of alkali metals is often used to facilitate their use as strong reducing agents. The presence of mercury anions, such as Hgi-, in alkali metal amalgams indicates that mercury in the form of clusters can be an electron sink so that the reducing action of alkali metal amalgams can be viewed as analogous to that of alkali metal naphthalenides or alkali metal graphite intercalation compounds. 45Recent electrochemical studies46*47 indicate that similar mercury cluster anions are present in “quaternary ammonium amalgams” although difficulties in obtaining crystals from these air- and moisture-sensitive systems have precluded their definitive structural characterization.
Metal cluster topology-IX. Acknowledgement-I am indebted to the U.S. Office of Naval Research for partial support of this work.
REFERENCES 1.R. B. King, J. Mof. Struct. Theochem, in press. 2. H. Hock and H. Stuhlmann, Chem. Ber. 1928,61, 2097. 3. W. Hieber and U. Teller, 2. Anorq. A&em. Chem. 1942,249,43. 4. R. B. King, J. Znorg. Nucl. Chem. 1963,25, 1296. 5. W. Clegg and P. J. Wheatley, J. Chem. Sot. A 1971, 3572. 6. M. L. Katcher and G. L. Simon, Znorq. Chem. 1972, 11, 1651. 7. G. M. Sheldrick and R. N. F. Simpson, J. Chem. Sot. A, 1968, 1005. 8. R. F. Bryan and A. R. Manning, J. C/rem. Sot., Chem. Commun. 1968, 1316. 9. F. S. Stephens, J. Chem. Sot., Dalton Trans. 1972, 2257. 10. R. A. Jones, F. M. Real, G. Wilkinson, A. M. R. Galas and M. B. Hursthou~, J. Chem. Sot., D&on Trans. 1981, 126. 11. R. B. King and D. H. Rouvray, J. Am. Chem. Sot. 1977,99,7834. 12. R. B. King, in Chemical Applications of Topology and Graph Theory (Edited by R. B. King), pp. 99123. Elsevier, Amsterdam (1983). 13. R. B. King, in Molecular Structure and Enerqetics (Edited by J. F. Liebman and A. Greenberg), Vol. 1, pp. 123-148. VCH Publishers, Deerfield Beach, Florida (1986). 14. R. B. King, Znorg. Chim. Actu 1986,116, 109. 15. T. S. Piper and G. Wilkinson, J. Znorq. Nucl. Chem. 1956,3,104. 16. W. Hieber and G. Wagner, 2. Nuturforsch. 1958, 13b, 339. 17. R. S. Nyhohn, Proc. Chem. Sot. 1961,273. 18. P. Pyykko and J.-P. Desclaux, Act. Chem. Res. 1979, 12,276. 19. A. Dedieu and R. Hoffmann, J. Am. Chem. Sot. 1978,100,2074. 20. D. GrdeniC, Quart. Revs. 196519,303. 21. H. L. Roberts, Adv. Inorq. Chem. Radiochem. 1968, 11,309. 22. R. J. Gillespie and J. Passmore, Adv. Znorq. Chem. Radiochem. 1975,17,49.
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23. M. Fajardo, H. D. Holden, B. F. G. Johnson, J. Lewis and P. R. Raithby, J. Chem. Sot., Chem. Commun. 1984,24. 24. W. Glide and E. Weiss, Anqew. Chem. Znt. Ed. Enql. 1981,20, 803. 25. J. M. Ragosta and J. M. B&itch, J. Chem. Sot., Chem. Commun. 1985,1187. 26. N. L. Biggs, Algebraic Graph Theory, p. 17. Cambridge University Press, London (1974). 27. H. Hock and H. Stuhlmann, Chem. Ber. 1929, 62, 431. 28. R. D. Ernst, T. J. Marks and J. A. Ibers, J. Am. Chem. Sot. 1977,99,2090. 29. F. W. B. Einstein, R. K. Pomeroy, P. Rushman and A. C. Willis, J. Chem. Sot., Chem. Commun. 1983, 854. 30. F. W. B. Einstein, T. Jones and R. K. Pomeroy, J. Am. Chem. Sot. 1984,106,2707. 31. L. R. Martin, F. W. B. Einstein and R. K. Pomeroy, Orqanometallics 1988,7,294. 32. B. Grtinbaum, Convex Polytopes, pp. 46-48. Interscience, New York (1967). 33. J. W. Nielsen and N. C. Baenziger, Acta. Cryst. 1954, 7,277. 34. E. J. Duwell and N. C. Baenziger, Acta. Cryst. 1955, 8,705. 35. J. D. Corbett, Znorq. Nuct. Chem. Lett. 1969, 5, 81. 36. J. D. Corbett, Proq. Znorq. Chem. 1976,21, 129. 37. J. D. Corbett, Chem. Rev. 1985,85,383. 38. C. Glidewell, Inorq. NucZ. Chem. Lett. 1975, 11, 761. 39. A. Cisar and J. D. Corbett, Znorq. Chem. 1977, 16, 2482. 40. I. D. Brown, D. B. Cmp and R. J. Gillespie, Znorq. Chem. 1971,10,2319. 41. T. W. Couch, D. A. Lokken and J. D. Corbett, Znorq. Chem. 1972,11,357. 42. R. B. King, Znorg. Chim. Actu 1982,57, 79. 43. R. B. King, in The Physics and Chem~try of Small Clusters (Edited by P. Jena, B. K. Rao and S. N. Khanna), pp. 79-82. Plenum Press, New York (1987). 44. R. B. King, J. Phys. Chem., in press. 45. R. Csuk, B. I. GHnzer and A. Fiirstner, Ado. Orqanometal. Chem. 1988,28, 85. 46. E. Garcia, A. H. Cowley and A. J. Bard, J. Am. Chem. Sue. 1986,108,6082. 47. EL Kariv-Miller and V. SvetliGE, J. Electroanal. Chem. 1986,20!$319.
1988 0
METAL CLUSTER TOPOLOGY-IX.? IN TRANSITION METAL CLUSTERS AMALGAMSS
0277-5387188 S3.00+ .OO 1988 Pergamon Press
plc
MERCURY VERTICES AND ALKALI METAL
R. B. KING
Department of Chemistry, University of Georgia, Athens, GA 30602, U.S.A. Abstract-Mercury vertices in metal clusters use a seven-orbital spd’ bonding manifold with linear sp hybrids for strong primary bonding and dn + px* and/or da + pa* interactions for weaker secondary bonding. Transition metal clusters containing mercury vertices can be classified into two types: two-dimensional clusters in which the mercury atoms form a polygon such as a triangle in [HgOs,(CO),,], or a square in [HgMn(CO)2C5H4Me]4 and three-dimensional clusters in which the mercury atoms form a polyhedron such as an octahedron in Hg,[Rh(PMe,),], or a tricapped trigonal prism in Hg9[Co(C0)3]6. The structures of such clusters consist of a mercury polygon or polyhedron having secondary Hg-Hg interactions inside a transition metal macropolygon or macropolyhedron with M-Hg-M edges having primary Hg-M interactions. These clusters thus all exhibit edgelocalized chemical bonding topology. Alkali metal amalgam structures are constructed from mercury quadrilaterals of which the prototypical species is square Hgi-, found in discrete units in the structure of Na3Hg2. This homonuclear mercury cluster anion may be regarded as a globally delocalized two-dimensional planar polygonal aromatic system with an aromatic sextet like benzene but with no a-bonding between adjacent mercury atoms in the Hgz- square.
A characteristic property of metal carbonyls, which was recognized in the very early days of the development of their chemistry, is their reactivity towards mercury and its compounds to form transition metal-mercury derivatives, which in general are relatively stable towards air and water. Early discoveries of this type include the formation of cisFe(CO),(HgCl), from Fe(CO), and HgClz2 and the formation of Hg[Co(CO)& from Co2(CO)8 and elemental mercury.3 Mercury derivatives of metal carbonyls such as Hg[Mn(CO),],, Hg[Co(C0)4]2, Hg[Mo(C0)3C,H5]2 and Hg[Fe(C0)2C5H,]2 are frequent by-products from the preparation of metal carbonyl anions by the treatment of binuclear metal carbonyl derivatives with sodium amalgam.4 Structural studies on mercury metal carbonyl derivatives such as Hg[Mn(C0)5]2,5’6 Hg[Co (co)412y7
Hg[CWO)3PEt3128and Hg[Fe(W2
(NO)PEt,]29 indicate linear M-Hg-M
units con-
7 For part VIII, see ref. 1. $This paper is dedicated to Sir Geoffrey Wilkinson, F.R.S., in recognition of his many contributions organometallic and inorganic chemistry. (Receiued 18 April 1988).
to
taining two-coordinate mercury atoms. This area of transition metal chemistry thus appeared to be relatively uninteresting until the seminal discovery by Wilkinson and co-workers” of the sodium amalgam reduction of [(Me3P)4Rh]Cl or (Me3P)3RhCl to give high yields of the high nuclearity transition metal-mercury cluster Hge[Rh(PMe3),],, which was shown by X-ray diffraction to consist of an Hg6 octahedron with four tetrahedrally related faces capped by rhodium atoms. This paper surveys the currently known types of such high nuclearity transition metal-mercury clusters with the objective of relating their skeletal bonding to previously discussed”-‘4 graphtheory derived models for metal cluster bonding topology. An important conclusion from this study is that almost all of the complicated transition metal-mercury cluster structures can be derived from simple edge-localized bonding models with two-coordinate mercury vertices. The ideas developed in this study of transition metal-mercury clusters also provide some insight into the chemical bonding topology of alkali metal amalgams, which are important as reducing agents in metal carbonyl chemistry and other areas of transition metal chemistry. Thus Wilkinson” as well as Hieber16 and
1813
1814
R. B. KING
their co-workers were pioneers in such applications of alkali metal amalgams for the preparation of metal carbonyl anions from neutral metal carbonyl dimers. MERCURY
VERTICES
The position of mercury to the right of gold in the periodic table suggests that mercury vertices will have some similar properties to gold verticesi in metal clusters. Thus two of the outer p orbitals in both mercury and gold can be shifted to such high energies that they no longer participate in the primary chemical bonding. ‘7*‘8This manifests itself structurally in d’ ’ Au’ and Hg” as two linear sp hybrid orbitals superimposed on a spherically symmetrical filled d-orbital manifold thereby leading to a cylindrical spd’ seven-orbital primary bonding manifold, which requires 14 electrons for filling. This limitation of the primary bonding of mercury vertices to two linear sp hybrids prevents the construction of stable polyhedra containing exclusively mercury vertices. However, transition metal cluster macropolyhedra can be constructed in which the edges are nearly linear M-Hg-M units rather than simple M-M bonds. Thus the Hg6[Rh(PMe,),], clusterlO can be interpreted as an Rh4 macrotetrahedron with six Rh-Hg-Rh edges. The empty p orbitals not in the spd’ cylindrical bonding manifold of the d lo Hg” can participate in secondary dx +pn* or da + da* bonding similar to that suggested by Dedieu and Hoffmann” for isoelectronic d” Pt” complexes and by a previous paper of this seriesI to account for the peripheral Au-Au bonding distances in centred gold clusters. The mercury-mercury interactions within the mercury polygons and polyhedra in the transition metal-mercury clusters consist exclusively of such secondary bonding since the mercury sp hybrid orbitals are used for primary bonding to transition metals in the M-Hg-M edges of the transition metal macropolyhedra. The mercury-mercury distances in polynuclear mercury derivatives (Table 1) provide an excellent indication as to the nature of the mercury-mercury interactions. In Hg,2+ derivatives as well as similar derivatives of longer linear mercury chains (e.g. Hg:+, Hg$+) the mercury-mercury distances fall in the range 2.42.7 A indicative of strong primary covalent bonding arising from overlap of linear sp hybrid orbitals. 20-22 In cases where the primary mercury bonding is diverted to other atoms, such as transition metal atoms in transition metalmercury clusters, the mercury-mercury distances increase to the range 2.9-3.2 A indicating the
Table 1. Mercury-mercury Compound Hg metal Hg,Fz Hg,Cl, Hg,Br, HgA Hg0sF.A Hg,(AlCl& Hg,(AsF& Hg,.,AsF,
[HgOs,(CQ ,113
WgWCOW5H4Mel~ Hg6DW+W314 H&K+WO)316 Nd-MHd-) NaHg KHg NaHg, KHg,
bond distances Hg-Hg
(A)
3.00, 3.47 2.43 2.45 2.50 2.69 2X2(4) 2.56 2.70 (inner), 2.57 (outer) 2.64(l) 3.122(3), 3.082(3), 3.097(3) 2.888(2) 3.131(3E3.149(3) 3.094 (av., trig.-trig.), 3.151 (av., trig.-sq.) 2.99 3.05, 3.05, 3.22 3.02, 3.04, 3.36 2.90, 3.23 3.00, 3.02, 3.08
presence of only secondary mercury-mercury bonding. Transition metal clusters containing mercury vertices can be classified into two types: twodimensional clusters (Fig. 1) in which the mercury atoms form a polygon such as a triangle in or a square in [HgMn(CO), [HgOs3(CO)1 11323 C5H4Me]424 and three-dimensional clusters (Fig. 2) in which the mercury atoms form a polyhedron
Fig. 1. Transition metal mercury clusters containing mercury polygons. Mercury vertices are starred, mercury-mercury bonds and external groups (carbonyl and cyclopentadienyl) are omitted, and transition metal + mercury dative bonds are indicated by arrows.
1815
Metal cluster topology--IX.
Hg.$o (Co),] 6
Fig. 2. Transition metal mercury clusters containing mercury polyhedra. Mercury vertices are starred and mercury-mercury bonds and external groups (carbonyl and trimethylphosphine) are omitted.
in Hg,[Rh(PMe,),],” or a tricapped trigonal prism in Hg,[Co(CO) J 6.25Transition metal clusters containing mercury polygons arise from bifunctional transition metal fragments such as Fe(CO), and CpMn(CO)* leading to degree two vertices in the large transition metal macropolygon. Transition metal clusters containing mercury polyhedra arise from trifunctional transition metal fragments such as COG and Rh(PR& leading to degree three vertices in the large transition metal macropolyhedron. The graph of the polygon or polyhedron formed by the mercury vertices is thus the line graph26 of the graph of the polygon or polyhedron formed by the transition metal vertices. such as an octahedron
MERCURY
POLYGONS
Combinations of difunctional transition metal vertices with mercury vertices leads to clusters with equal numbers of transition metal and mercury vertices consisting of a mercury polygon within a transition metal macropolygon (Fig. 1). The bond angles at the transition metal vertices determine the sizes of the polygons with squares apparently being preferred. A structurally characterized cluster of this type is mgMn(C0)2CSH.,Me]4.24 The difunctional Mn(C0)2C5H4Me vertices have the favoured 18-electron rare gas configuration for the manganese atom and are derived from the corresponding CpMn(CO), system through a formal oxidative addition reaction. The Mn-Hg-Mn edges of the Mn, macrosquare in [HgMn(C0)2C5H4Me]4 are bent inwards towards the centre of the square in accord with secondary mercury-mercury bonding
leading to 2.888(2) Hg-Hg bond distances. A similar structure (Fig. 1) can be postulated for the longknown2927but insoluble [HgFe(CO),], on the basis of its similar IR spectrum to the structurally characterized” [CdFe(CO),],* 2Me,CO. A structure containing a mercury triangle (Fig. 1) is found in the “heteronuclear raft cluster”23 [HgOs,(CO), I13. In this case the difunctional transition metal vertex is the Os(CO), vertex of an 0s3(CO),, triangle. The Os-Hg bond lengths in the Os-Hg-Os edges of the 0~ macrotriangle fall in the range 2.71-2.76 A. In addition there is one 2.98-3.05 A dative bond from an Os(CO), vertex of each 0s3(CO)i 1 triangle to a mercury atom as indicated by arrows in Fig. 1. Such dative bonds from osmium atoms to other types of acceptor atoms have been found in structurally characterized complexes.29-3’ The Hg-Hg distances of 3.08-3.13 A in the mercury triangle of [HgOs3(CO)i,]3 are significantly longer than those in the mercury square of [HgMn(C0)2C5H4Me]4 suggestive of weaker mercury-mercury secondary bonding possibly because of the tram influence of the OS + Hg dative bond. The OS + Hg dative bond as well as the OS--OS bonds in the Os,(CO), , triangles perturb the external bond angles at the Os(CO), vertices so that a mercury triangle is found in [HgOs, (CO), ,I3 in contrast to the mercury square in lI-IgMn(CO)2C,H4Me]4 discussed above. In [HgOs, (CO), ,I3 all of the osmium atoms have the favoured 18-electron rare-gas configuration and the mercury atoms have the 1Celectron configuration corresponding to a filled cylindrical spd5 sevenorbital manifold. MERCURY
POLYHEDRA
Combination of trifunctional transition metal vertices with mercury vertices leads to clusters having a mercury polyhedron within a transition metal macropolyhedron (Fig. 2). The known examples of such clusters have trifunctional transition metal vertices of the type ML3 (M = Co, Rh, Ir ; L = CO, R3P). The number of mercury atoms in such clusters is equal to the number of edges in the transition metal macropolyhedron. If this macropolyhedron has m vertices all of degree three implied by the trifunctionality of the transition metal units at the vertices, then there are (3/2)m edges and such clusters have the overall stoichiometry [Hg3(ML3)2]x where x is even. The properties of the first three clusters of this homologous series, for which both the mercury polyhedron and the transition metal macropolyhedron are readily recognizable polyhedra, are listed in Table 2. In addition to the hypothetical cluster [Hg3(ML3)2]4 having Oh skel-
1816
R. B. KING
Table 2. Smallest possible skeletons for clusters of the type [Hg,(ML,)J, consisting of a mercury polyhedron inside a transition metal macropolyhedron Transition metal macropolyhedron
Mercury polyhedron x
Skeletal symmetry
2 3 4
Td Dv, Oh
Type
V
Octahedron Tricapped trigonal prism Cuboctahedron
e
6 12 9 21 12 24
f
Type
V
e
f
Example
8 14 14
Tetrahedron Trigonal prism cube
4 6 8
64 95 12 6
HgACo(CW6
eta1 symmetry consisting of a mercury cuboctahedron inside a transition metal macrocube, a less symmetrical isomeric cluster [Hgs(MLs)& having Cti skeletal symmetry is also possible in which the macropolyhedron is the eight-vertex cuneane polyhedron, namely the dua13* of the bicapped tetrahedron. I* The Wilkinson cluster lo Hg, [Rh(PMe,)J, with a mercury octahedron inside a rhodium macrotetrahedron and the Burlitch cluster2’ Hg,[Co(CO),], with a mercury tricapped trigonal prism inside a cobalt trigonal macroprism (Fig. 2) represent the first two members of this homologous series. In both of these clusters all of the transition metal atoms have the favoured 18-electron rare-gas configuration and all of the mercury atoms have the 1Celectron configuration corresponding to a full cylindrical spd’ seven-orbital manifold. The linear sp primary mercury bonding is used for the M-Hg-M edges of the transition metal macropolyhedra leaving only secondary bonding for the edges of the mercury polyhedra in accord with the 3.06-3.15 8, Hg-Hg distances in these polyhedra.
ALKALI
METAL
AMALGAMS
Several alkali metal amalgams have been structurally characterized including the sodium amalgams33 NaHg,, NaHg and Na,Hg,, and the potassium amalgams 34 KHg, and KHg. The most highly reduced of these species, namely Na,Hg,, has been shown33*35to contain discrete 2.99 8, square planar Hgi- clusters which are not isoelectronic with other square planar post-transition element clusters3c38 such as Bi:-,3g Se:+ 4o and Te:+.4’ In gold coloured NaHg33 slightly distorted 3.05 x 3.22 A Hg, rectangles are fused into a zigzag ribbon whereas in the likewise gold coloured KHg,34 slightly distorted (93.6 rather than 90°) 3.03 A Hg, squares are linked by 3.36 8, Hg-Hg bonds. More complicated networks of mercury rectangles and parallelograms are present in NaHg, and KHg,.
Hg6BW+W314
Thus the prototypical building blocks for alkali metal amalgams are derived from the Hg:- squares found in Na3Hg2. The mercury atoms in Hgz- can be considered to have seven-orbital spd5 cylindrical bonding manifolds. Six of these seven orbitals, namely the s orbital and the five d orbitals, are external orbitals, whereas the single p orbital is an internal orbital. A neutral mercury atom with six external orbitals is a zero skeletal electron donor42-44 so that the Hgianion has six skeletal electrons. Since the overlap of the external p orbitals in Hgz- has the topology of the square, the Hgi- anion has a n-bonding network similar to a planar aromatic hydrocarbon such as the cyclobutadiene dianion or benzene. Furthermore, the six skeletal electrons of Hgi- can all be rc-electrons so that Hgz- is an 4k+ 2 electron aromatic system with a n-electron network analogous to that of benzene. However, Hgi- hasneither the electrons nor the orbitals for any Hg-Hg abonding. Thus the Hgz- anion is a unique example of a two-dimensional aromatic system without any a-bonding. Thus the eight-electron difference in the skeletal electron counts of the square clusters Bi:and Hgi- relate to the presence of a-bonding in the Bij- square but the absence of a-bonding in the Hgi- square. In a number of areas of chemistry, including metal carbonyl chemistry, amalgamation of alkali metals is often used to facilitate their use as strong reducing agents. The presence of mercury anions, such as Hgi-, in alkali metal amalgams indicates that mercury in the form of clusters can be an electron sink so that the reducing action of alkali metal amalgams can be viewed as analogous to that of alkali metal naphthalenides or alkali metal graphite intercalation compounds. 45Recent electrochemical studies46*47 indicate that similar mercury cluster anions are present in “quaternary ammonium amalgams” although difficulties in obtaining crystals from these air- and moisture-sensitive systems have precluded their definitive structural characterization.
Metal cluster topology-IX. Acknowledgement-I am indebted to the U.S. Office of Naval Research for partial support of this work.
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