DALTON PERSPECTIVE J. Chem. Soc. Dalton Trans. 1999 639–651 639 The {Pt2S2} core—a butterfly that stings S.-W. Audi Fong and T. S. Andy Hor * Department of Chemistry Faculty of Science National University of Singapore Kent Ridge 119260 Singapore. E-mail chmandyh@nus.edu.sg Received 7th August 1998 Accepted 9th October 1998 The recent developments in the syntheses structures and reactivities of sulfide-bridged aggregates with the {M2S2} core (M 5 Pd or Pt) are discussed. The nucleophilicity of the parent {Pt2S2} core and its synthetic utility in the preparations of aggregates and clusters are emphasised. Other {M2S2} systems are also described for comparison. 1 Introduction The ubiquity of transition metal sulfur compounds in nature has been augmented by the synthesis of many transition metal co-ordination complexes and clusters.While the literature is replete with examples of these sulfur compounds,1 their biological and industrial significance notwithstanding the chemistry of sulfido complexes of platinum remained largely unexplored. This is somewhat stupendous especially when one considers the richness of the individual chemistries of both platinum and sulfur. Hence there should be no compelling reasons why a diverse chemistry involving both elements cannot be developed. The tremendous versatility of sulfur as a ligand is demonstrated by its propensity to (i) extend its co-ordination from terminal (e.g. in [Mo2S10]22) 2 to an encapsulated form {e.g. in [Rh17(S)2(CO)32]32 which consists of three stacked square antiprisms sharing faces with interstitial S atoms in outer antiprisms and Rh in the inner},3 (ii) catenate giving rise to polysulfido ligands of the type Sn 22 (where n = 1–5) and (iii) stabilise a variety of clusters.More recently we have also observed a rare case of a m6-sulfide ligand in a complex containing a novel {Pd6S4} core.4 Although any venture into platinum sulfide chemistry should prove propitious our knowledge of these compounds remain tenuous,5 being hampered mainly by an inimical climate that can be ascribed to a lack of incentives in this research area. Unlike the Chevrel–Sergent compounds M9[Mo6(m3-S)8] (M9 = PbII CuII etc.),6 which exhibit superconducting properties under low temperature conditions in the presence of high magnetic fields and the [Fe4(m3-S)4] and [Fe3Mo(m3-S)4] “cubane” units that eVectively model the respective biological enzymes ferredoxins and nitrogenase,7 there did not seem to be a comparable significance in platinum sulfide complexes to warrant extensive academic interest.However there is a resurgence of interest in some of these complexes fueled by studies on the concentration-dependent nephrotoxicity associated with the use of antitumour platinum drugs. It has been suggested that this toxicity is a result of the binding of platinum to sulfurfunctionalised protein residues.8 This has thus provided the impetus for some extensive studies.9 In 1903 Hofmann and Höchlen 10 reported the isolation of the first platinum–sulfur complex [NH4]2[Pt(h2-S5)3]. This early study spawned some fervent work into these compounds of platinum and sulfur. A summary of the main historical milestones in the development of general Pt–S chemistry en route to the {Pt2S2} system is provided in Table 1.The origins of {Pt2S2} chemistry may be traced to the first report of [Pt2(PMe2Ph)4- (m-S)2] 1a by Chatt and Mingos 11 in 1970 followed almost immediately by Ugo et al.12 in a study of the reactions of zerovalent platinum phosphine complexes with H2S and elemental sulfur yielding [Pt2(PPh3)4(m-S)2] 1b. Since then interest in these complexes had centred around rudimentary structural characterisations 13 as well as their reactivities with common organic electrophiles.14–16 A simplistic theoretical model 17 was also proposed to explain the nucleophilicity of the {Pt2S2} core. More recently there has been a gradual shift of Weltanschauung towards using complex 1b as a versatile metalloligand building block to various homo-,18 hetero- 19,20 and intermetallic 21 sulfide aggregates.A facile method22 was also developed to reductively excise one of the sulfur atoms using CO under mild pressures thereby inducing the formation of a Pt–Pt bond. The synthetic value of this strategy was demonstrated when we successfully adopted it to synthesize a series of heterometallic clusters from their respective aggregates via a reductive desulfurisation procedure.23 Concurrent studies in the analogous palladium sulfido complexes have also been enunciated.4,24 In all it has taken nearly three decades for this field to mature with much of the activities occurring only within the Audi Fong was born in Singapore in 1972. He obtained his B.Sc.(Hons.) from the National University of Singapore (NUS) in 1996 where he is currently staying on for Ph.D.research under the supervision of Professor Andy Hor. He is a graduate tutor and a mentor of the Talent Development Programme in Science at NUS. His research interests include platinum sulfide complexes and heterometallic ruthenium sulfide clusters. Andy Hor (b 1956 Hong Kong) joined the National University of Singapore in 1984 after graduating from Imperial College [B.Sc.(Hons.)] Oxford (D.Phil.) (with D. M. P. Mingos) and postdoctoral work at Yale (with R. D. Adams). He has authored 120 international papers on carbonyls sulfide aggregates ferrocene-based materials and their catalysis. He was a fellow of Humboldt Commonwealth Academic Staff the Association of Southeast Asian Institutions of Higher Learning (ASAIHL) and the Japan Society for the Promotion of Science (JSPS).His major prizes include the Association of South-East Asian Nations (ASEAN) Achievement Award the National Science and Technology Board (NSTB) Young Scientist & Engineer Award (Singapore) the Japanese Chamber of Commerce & Industry Singapore (JCCI) Education Award and several teaching awards. A Singaporean and currently the vice-dean of the Faculty of Science he is married with two children. Audi Fong Andy Hor 640 J. Chem. Soc. Dalton Trans. 1999 639–651 Table 1 Highlights in the development of general Pt–S chemistry en route to the {Pt2S2} system Author a Hofmann and Höchlen (1903) 10 Molecular system [Pt(h2-S5)3]22 Structural class Synthesis (NH4)2S 1 S8 1 H2PtCl6 10 H2PtCl6 1 NH3 (aq) 1 H2S 1 S8 49c Significance First reported isolation of a h-pentasulfido complex of Pt First example of a purely inorganic chiral molecule (C3 point group).Only the L enantiomer was isolated 49a Structure of [NH4]2[Pt(S5)3] solved by Jones and Katz (1967) 49b Undergoes cyanide degradative reaction to give [Pt(S5)2]22,49c and reduction with PPh3 in ethanol to give [Pt(PPh3)2S4] 49d(i) Ref. 10 49 Baird and Wilkinson (1966) 50a [Pt2(CO)(PPh3)3- (m-S)] [Pt(PPh3)2(COS)] in refluxing CH2Cl2 or CHCl3 First reported as the dinuclear Pt2 complex [Pt2(CO)2(PPh3)2(S)] 50a Reformulated as [Pt2(CO)(PPh3)3(m-S)] based on X-ray structural data (1969):50b a novel Pt–S–Pt triangle is thus established with the first unequivocal structural evidence of a Pt–Pt bond (2.647 Å) Robustness of the Pt2S core demonstrated:50c the Pt–Pt bond resists insertion by CO CH3NC and C2H2 favouring substitution of the supporting ligands instead e.g.[Pt2(PPh3)2(m-dppm)(m-S)] is formed with dppm 50 Chatt and Mingos (1970) 11 [Pt2(PMe2Ph)4- (m-S)2] 1a cis- [PtCl2(PMe2Ph)2] 1 Na2S?9H2O EtOH11 Isolation of doubly bridged dinuclear complex [Pt2(PMe2Ph)4(m-S)2] 1a 11 Subsequent X-ray study by Mason et al.13c con- firmed the structure and revealed that the two PtS2 planes are hinged across the S ? ? ? S bridge (q = 1218) with a non-bonding S ? ? ? S distance of 3.06 Å Reacts with a variety of alkyl halides notably PhCH2Br and MeBr to yield [Pt2(PMe2Ph)4- (m-S)(m-SCH2Ph)]Br and cis-[Pt(PMe2Ph)2Br2] respectively 11 11 13(c) [Pt3(PMe2Ph)6- (m3-S)2]Cl2 1a 1 cis- [PtCl2(PMe2Ph)2] 11 Chatt and Mingos proposed the structure of this white crystalline product based on 1H NMR and analytical data.No M–M bonding was invoked since the square-planar PtII bridged by two S atoms can readily be accommodated 11 First example of a triplatinum complex using 1a as a metalloligand towards a transition metal fragment. Structure solved by Bushnell et al. (1984) 13e 11 13(e) [Pt(PPh3)2(h2-S4)] [Pt(PPh3)4] 1 S (1 6) in benzene 11 [Pt(S5)3]22 1 PPh3 (1 3) or excess PPh3 EtOH49d(i),51a cis-[PtCl2(PPh3)2] 1 Na2Sn EtOH49d(i) First mononuclear tetrasulfido-complex reported 11 Independent work by Beck and co-workers (1972) 51a and Schmidt and HoVmann (1979) 49d(i) also advocated the same product. However limited solubility precluded NMR and X-ray investigations. Structure solved by Dudis and Fackler (1982) 51b 11 49(d ) 51 [Pt(dppe)(h2-S4)] [Pt(dppe)2] 1 S (1 6) in benzene 11 [Pt(S5)3]22 1 dppe EtOH49d(i) This complex was first proposed in 1970 and its structure only solved by Mingos and co-workers (1983) 20a confirming that the S4 22 adopts a htetrasulfido co-ordination mode and eliminated the alternative bis(h-disulfido) and cyclo-tetrasulfido structures proposed earlier 11 11 20(a) 49(d ) [Pt2(PPh3)4(m-S)] cis-[PtCl2(PPh3)2] 1 excess Na2S ammoniacal EtOH Proposed to have the Pt–S–Pt three-membered ring structure analogous to [Pt2(CO)(PPh3)3- (m-S)] 11 11 Ugo et al.(1971) 12 [Pt2(PPh3)4(m-S)2] 1b [Pt(PPh3)3] 1 S (1 1) or H2S in benzene 12 cis-[PtCl2(PPh3)2] 1 excess Na2S in benzene 12 [Pt(SH)2(PPh3)2] 1 NaOEt EtOH– benzene (3 1) 49d(ii) Tetrameric [{PtS2(PPh3)}4] and dimeric [{PtS- (PPh3)2}2] 1b are proposed12 Structure of 1b purported to be similar to 1a Complex 1b reported to be reactive to a variety of electrophiles and nucleophiles 15,16 12 15 16 49(d ) a Indicates correspondence author(s) of work when first reported.last 15 years notably from the research groups of Mingos 16,20 and Adams25 during the early 1980s and in recent years by our group.19,21–24,26,27 González-Duarte and co-workers have also synthesized a series of m-thiolato complexes with the {M2S2} core (M = Ni Pd or Pt) 28 as well as provided an ab initio MO study on the hinge distortion of the {Pt2S2} ring.29 A summary of some important developments in the {Pt2S2} system is given in Table 2. J. Chem. Soc. Dalton Trans. 1999 639–651 641 Table 2 Highlights in the development of selectively derivatised {Pt2S2} compounds with a hinged butterfly core Structural class Author a Chatt and Mingos (1970) 11 Ugo et al.(1971) 12 Morris and co-workers (1983) 14 Mingos and co-workers (1984) 16 Hor and Tan (1988) 17 Significance Complex 1a used as a nucleophile towards alkyl halides Nucleophilicity of the m-sulfido groups established The nucleophilicity of 1b is also confirmed through derivatisation using PhCH2Br to give 4 Confirmation of the earlier findings 11,12 in regard to the synthesis and nucleophilicity of 1b First fully dedicated study on the nucleophilicity of 1b towards a series of alkyl halides A simple qualitative MO model invoked to explain the nucleophilicity of the sulfur bridges of 1b in terms of p interactions Examplesb [Pt2(PMe2Ph)4(m-S)(m-SCH2- Ph)]Br 3 11 [Pt2(PPh3)4(m-S)(m-SCH2Ph)]- Br 4 12 [Pt2(PPh3)4(m-S)(m-SMe)]X (X = I 5a PF6 5b or BPh4 5c) 16 [Pt2(PPh3)4(m-S)(m-SR)]X (R = CH2Cl 614,16 or CHCl2 7;14 X = Cl PF6 or BPh4) Ref.11 12 14 16 17 Chatt and Hart (1960) 13a Chatt and Hart 13a isolated the di-m-thiolate complexes of PtII; proposed the formation of an “inorganic aromatic ring” involving dp–pp interaction within the {Pt2S2} core Single-crystal analysis of 9 by Hall et al. (1972) 13b Bird et al.13d advocated fusion in vacuo of complexes of type [Pt(SR)2(PR3)2] as a facile route to 10 cis-[Pt2(NO2)2(PPh3)2- (m-SMe)2] 8 16 [Pt2X2(PR3)2(m-SR)2] (X = Cl or SR) 13a cis-[Pt2Cl2(PPrn 3)2(m-SEt)2] 913b cis-[Pt2(SCH2Ph)2(PMePh2)2- (m-SCH2Ph)2] 10 13d 13 16 Chatt and Mingos (1970) 11 Mingos and co-workers (1983– 1987) 20b–d Hor and coworkers (1994– 1997) 19,27b,c Chin and Hor (1996) 22 Hor and co-workers (1997) 23 First example of a triplatinum complex 14 resulting from the use of 1a as a metalloligand towards a transition metal fragment Structure of 14 solved by Bushnell et al.(1984) 13e Emergence of 1b as a useful building block for heterometallic polynuclear complexes functioning as a versatile metalloligand towards a wide array of transition metal fragments; co-ordination mode of the sulfido ligand changes from m-S to m3-S The term “aggregates” was first coined by Mingos and coworkers 20b to describe polynuclear structures in which no formal M–M bonding occurs Novel use of X-ray photoelectron spectroscopy (XPS) to study heterometallic aggregates of 1b; binding energies were related to the charge distribution and nucleophilicity of the sulfur centres.Allows for parallel studies of these polynuclear aggregates and other established inorganic solid systems 19 Establishment of a synthetic path involving facile reductive desulfurisation of the {Pt2S2} core to give Pt–Pt bonded {Pt2S} triangle under mild CO pressure 22 Novel facile deheterometallation of a heterometallic aggregate 28 via carbonylative desulfurisation to yield [Pt2(CO)2(PPh3)(m-S)] 27a Induction of hetero-M–M bond formation by reductive desulfurisation using CO leading to a decapacitative transformation of a {Pt2MS2} aggregate to a {Pt2MS} terahedral cluster [{Pt(PMe2Ph)2(m3-S)}2- Pt(PMe2Ph)2]21 14 11,13e [(S–S)2M]n1 (n = 2 M = Pd 18 or Hg 24;20b n = 1 M = Cu 25a 19,27c) [{(S–S)Pd}2(m-Cl)2]21 19 20b [(S–S)ML2]n1 [ML2 = Pd(dppe) 20,20b Rh(cod) 22 20c or Rh(CO)2 23a 20d ] [(S–S)M(dppe)]21 (M = Ni 26 19 or Hg 27 20b) [(S–S)CoCl2] 28 27a [{(S–S)Cu}2(m-dppf)]21 29 19 [Pt2(PPh3)4(m-SMCl)2] (M = Ag 30a 27e or Au 3120f ) [(S–S)M(PPh3)]1 (M = Cu 33a,27c Ag 34 20f or Au 35 20f ) 11 13(e) 19 20 22 23 27 Hor and co-workers (1993– 1997) 21 Tan et al.(1997) 26 Emergence of 1b as a versatile metalloligand toward various main-group Lewis acidic fragments to give “intermetallic” aggregates First intermetallic aggregates 42a and 42b reported with unusual “Mexican-hat-like” structure; presence of stereochemically active lone pair on Tl suppresses the electrophilicity of the complex 21a Aggregates of diVering local geometry at the heterometal Y viz. bent TlI 42 tetrahedral GaIII 46 and BiIII 47 and squarepyramidal InIII 49a and BiIII 48 are synthesized 21a–d Ligand transformation of analogous [Pt2(dppf)2(m-S)2] 2a in CH2Cl2 to give [Pt(SCH2Cl)2(dppf)] 21e and reaction with TlNO3 to yield 43a21f Structural peculiarities of intermetallics 42 44 46 and 49a are investigated in a theoretical study 26 [(S–S)Tl]NO3 42a 21a [(S–S)Pb(NO3)2] 44a 21b [(S–S)Pb(NO3)]PF6 44b 21b [(S–S)GaCl2][GaCl4] 46 21c [(S–S)InCl3] 49a 21c [(S–S)BiCl3] 48 21d [(S–S)BiCl2]PF6 47 21d [(S–S)Tl]NO3 43a 21f 21 26 Mingos and co-workers (1983) 20e First assembly of two {Pt2S2} moieties on a dimetal unit in the formation of a “giant” structure [(S–S)2Ag2]21 32 20e 20(e) Yam et al.(1996) 36 First synthesis of a heterometallic cluster based on the {Pt2S2} core with extensive M–M bonding [{Pt2(dppy)4(m3-S)2}2Ag3]31 36 36 36 a Indicates correspondence author(s) of cited work.b S–S = (Ph3P)4Pt2(m-S)2. 642 J. Chem. Soc. Dalton Trans. 1999 639–651 The chemistry of platinum sulfido aggregates and clusters was reviewed recently.30 In this paper we shall broaden the scope to include other {M2S2} complexes highlight the latest developments and give a perspective view in this field. 2 Complexes with a hinged {Pt2S2} core The birth of a butterfly—synthesis of {Pt2S2} precursors Complexes with a hinged {Pt2S2} core are structured like a butterfly with the metal atoms at the wing-tip and sulfur at the hinge positions. The first of such complexes of platinum are [Pt2(L)4(m-S)2] (L = PMe2Ph 1a11,13c or PPh3 1b12). The synthesis of 1a is achieved through metathesis of cis-[PtCl2(PMe2Ph)2] with Na2S?9H2O in ethanol 11 while 1b from oxidation of [Pt- (PPh3)n] (n = 2 or 3) with H2S or S8 12 or via metathesis of cis- [PtCl2(PPh3)2] with Na2S?9H2O in benzene.15 The use of an inert solvent in these preparations is essential in view of the high nucleophilicity of the doubly bridging sulfido ligands.Complex 1b is an orange solid insoluble in all common nonchlorinated organic solvents thereby hampering elucidation of its solid state structure. Little has been developed using 1a as a precursor compared with 1b presumably because of the ease in handling PPh3 and the high crystallinity of its complexes. The analogous [Pt2(dppf)2(m-S)2] 2a was similarly synthesized from either [PtCl2(dppf)] or [Pt(MeCN)2(dppf)][BF4]2 with Na2S?9H2O in benzene.21e The recent structural elucidation of [Pt2(dppe)2(m-S)2] 2b by Capdevila et al.27d also revealed the expected hinged {Pt2S2} butterfly core.Another approach is demonstrated by Mitchell et al.31 who used a base-assisted condensation reaction of [PtCl2(2,29-bipy)] with a model protein Cys residue e.g. N-acetyl-L-cysteine (L-Haccys) to give [Pt2(2,29-bipy)2(m-L-accys-S)2]. Yam et al.32 have recently synthesized [Pt2(dppy)4(m-S)2] 1c (dppy = 2-diphenylphosphinopyridine) from [PtCl2(dppy)2] and NaSH and successfully used it as a synthetic precursor to some novel platinum sulfide clusters. Scheme 1 gives a summary of some known routes to the {Pt2S2} core. The use of desulfurisation of polysulfide complexes by phosphines and oxidative sulfurisation of low-valent platinum complexes are strategies we have used but only with limited success.33 Disfiguring a butterfly—reactions with organic electrophiles Chatt and Mingos 11 as well as Ugo et al.12 independently demonstrated the nucleophilicity of the sulfur sites in complexes 1a and 1b.Complex 1a reacts with a variety of organic electro- Scheme 1 Some established and projected pathways for the preparation of [Pt2(P)4(m-S)2] (P = phosphine). Pt S Pt S P P P P P S P S Pt S S S S P P P S Pt S S P P Pt P P Pt SH SH P P Pt Cl Cl P P Pt Cl Cl P P S Pt P P S S P S Pt Pt S Na2S NaCl benzene PtP4 H2S or S8 PR3 PR3 [Pt(S5)2 or 3]4- or 2– EtOH + NaSH NaSH NaCl + H2 NaOEt PtP4 Yam et al. (ref. 32) Chatt and Mingos (ref. 11) Ugo et al. (ref. 12) Schmidt and Hoffmann [ref. 49(d )(ii)] philes e.g. MeI and PhCH2X (X = Cl or Br) to give the thiolate-bridged complexes [Pt2(P)4(m-S)(m-SR)]X (P = PMe2- Ph R = CH2Ph X = Br 3;11 P = PPh3 R = CH2Ph X = Br 4; P = PPh3 R = Me X = I 512).The sulfide ligand in 1b is so nucleophilic that upon exposure of 1b to weak alkylating agents and chlorinated solvents like CH3Cl and CH2Cl2 alkylation of the sulfide bridge rapidly occurs giving rise to [Pt2- (PPh3)4(m-S)(m-SR)]X (R = CH2Cl 6 or CHCl2 7; X = Cl).14,16 Further alkylation would result in bridge cleavage to give mononuclear thiolato complexes. Other examples of thiolatebridged complexes supported by mixed ligands include cis- [Pt2(L)2(X)2(m-SR)2] (L = PPh3 X = NO2 R = Me 8;16 L = PPr3 X = Cl R = Et 9;13b L = PMePh2 X = SCH2Ph R = CH2Ph 10 13d ). The first example of mononuclear complexes with the crystallographically proven {PtS2CH2} ring is [Pt(S2CH2)- (PMe2Ph)2] 11a prepared by Shaver et al.34 and the second is [Pt(S2CH2)(dppy)2] 11b by Yam et al.32 Both complexes are formed from the respective nucleophilic attack of the sulfide centres in 1a and 1c by the solvent molecules viz.CH2Cl2. A recent structurally established example is provided by [Pt2- Cl(PPh3)4(m-S2CH2)]PF6 12 which is an unexpected product of [(Ph3P)4Pt2(m3-S)2Zn][PF6]2 with CH2Cl2.35 Similarly [Pt2- (dppf)2(m-S)2] 2a in CH2Cl2 gives mononuclear [Pt(SCH2Cl)2- (dppf)] 13,21e thereby showing a facile pathway to the formation of a terminal dithiolato complex from a di-m-thio complex. Our characterisations of 12 by single-crystal X-ray diVractometry and 13 through NMR experiments provide some additional information as to the overall mechanism of disintegration of dim- thio complexes 1a 1b 1c and 2a in chlorinated solvents like CH2Cl2.This process involves alkylation at the highly nucleophilic sulfur to yield the substituted thiolato complexes. Monoalkylation would preserve the dinuclear core whilst dialkylation would lead to the collapse to mononuclear thiolato complexes. Further metallation could explain the formation of 12 (Scheme 2). Spiking a butterfly—addition of d-block fragments and conversion of aggregates into clusters Although derivatisation of complex 1a to give the homometallic aggregate [Pt3(PMe2Ph)6(m3-S)2]Cl2 1411,13e had been known for some time the synthetic usefulness of 1b remained unexploited until the early 1980s when Mingos and co-workers synthesized a series of heterometallic complexes of general formula [{Pt2(PPh3)4(m3-S)2}xMLy]n1 based on their Lewis acid– base reactions with a variety of transition metal fragments.The Pt S Pt S P P P P 1a P = PMe2Ph 1b P = PPh3 2a 2P = dppf 2b 2P = dppe 1c P = dppy Pt S Pt S P P P P Pt S Pt S P P P P R Pt S Pt S L X L X R R X 3 P = PMe2Ph R = CH2Ph X=Br 4 P = PPh3 R = CH2Ph X = Br 5a P = PPh3 R = Me X = I 5b P = PPh3 R = Me X = PF6 5c P = PPh3 R = Me X = BPh4 6 P = PPh3 R = CH2Cl X = Cl 7 P = PPh3 R = CHCl2 X = Cl 8 L = PPh3 X = NO2 R = Me 9 L = PPr3 X = Cl R = Et 10 L = PMePh2 X = SCH2Ph R = CH2P + J. Chem. Soc. Dalton Trans. 1999 639–651 643 Scheme 2 A possible mechanism for the formation of various thiolato complexes from the disintegration of the {Pt2S2} core in CH2Cl2. Pt S P P Pt S P P CH2Cl2 Pt S P P Pt S P P CH2Cl Pt S P P Pt S P P CH2 Pt S P P CH2 S 2+ + Pt S P P CH2 S Cl Pt P P e.g. 11a P = PMe2Ph 11b P = dppy e.g.12 P = PPh3 + Pt S P P Pt S P P CH2Cl 2+ CH2Cl Pt SCH2Cl P P SCH2Cl e.g. 13 2P = dppf P = PMe2Ph 1a P = PPh3 1b P = dppy 1c 2Cl Cl Cl P Pt Cl P Cl P Pt Cl P Cl 2Cl CH2Cl2 2P = dppf CH2Cl2 P Pt Cl P Cl ? Scheme 3 Expansion of the {Pt2S2} core in complex 1b to a series of interconvertible homotriplatinum aggregates. S Pt S Pt Pt P P P P P P 14 P = PMe2Ph 15 P = PPh3 S Pt S Pt Pt P P P P Cl Cl 17 P = PPh3 S Pt S Pt Pt P P P P P Cl 16 P = PPh3 2+ + Cl- P Cl- P Cl- P Cl- P [PtCl2(CH3CN)2] (ref. 19) [Pt2Cl2(P)2(m-Cl)2] (0.5 equivalent) [(ref. 27(b )] [Pt(P)2(CH3CN)2]2+ (ref. 19) S Pt S Pt Pt P P Cl P Cl P 17' P = PPh3 Pt S Pt S P P P P 1b lack of active metal–metal bonds in these polynuclear structures prompted Mingos20b to describe them as sulfido “aggregates” (as opposed to “clusters”).The true value of sulfide ligands stems from their innate ability to form relatively strong bonds with nearly all the transition elements. They can also serve as bridging ligands with variable degrees of electron donation. Furthermore in the case of the dinuclear complex 1b the flexibility of the hinge angle between the two PtS2 planes the variability of the S ? ? ? S non-bonding separation together with the high nucleophilicity of the m-sulfido ligands all serve to promote the stabilisation of a variety of these heterometallic aggregates. The nuclearity of these resultant aggregate complexes and the co-ordination modes of their sulfur centres are determined largely by the local geometry of the heterometal M square planar e.g. [(S–S)2Pd]21 18,20b [(S–S)Rh(cod)]1 22,20c [(S–S)Rh(CO)2]1[Rh(CO)2Cl2]2 23a or [(S–S)Rh(C2H4)2]1 23b;20d tetrahedral e.g.[(S–S)2Hg]21 24,20b [(S–S)2Cu]1 25a,27c [(S–S)Ni(dppe)]21 26 19 or [(S–S)CoCl2] 28;27a linear e.g. [{(S– S)Cu}2(m-dppf)]21 29,19 [(S–S)(AuCl)2] 31,20f [(S–S)2Ag2]21 3220e or [(S–S)Au(PPh3)]1 35 20f [where (S–S) = (Ph3P)4Pt2(m-S)2 1b]. A related copper(II) analogue of 25a viz. [{Pt2(dppe)2(m3-S)2}2- Cu]21 25b was also reported recently.27d Complex 1b can also be expanded to the homotriplatinum complexes (i.e. Pt2 æÆ Pt3) [Pt3(PPh3)6(m3-S)2]21 15 and [Pt3Cl2(PPh3)4(m3-S)2] 17 through nucleophilic attack of [PtCl2(MeCN)2] and [Pt(MeCN)2- (PPh3)2]21 respectively19 (Scheme 3). The intermediate aggregate [Pt3Cl(PPh3)5(m3-S)2]1 16 has also been isolated.27b The related {PdPt2} aggregate has been assembled recently in a nonstoichiometric complex [M3Cl(PPh3)5(m3-S)2]1 (M3 = Pd1.1Pt1.9 or Pt3).27b The introduction of a palladium(II) unit to 1b gives rise to a series of isomers which are structurally isomorphous but diVer only by the metal distribution on the triangular structure.Other d8 fragments such as [M(CO)(PPh3)]1 (M = Rh or Ir) have also been successfully planted into the {MPt2} triangle. 644 J. Chem. Soc. Dalton Trans. 1999 639–651 Table 3 Selected structural parameters a (distances in Å angles in 8) of the heterometallic adducts of complex 1b [S–S = Pt2(PPh3)4(m-S)2] with d-block transition metal fragments Complex 18 [(S–S)2Pd]21 22 [(S–S)Rh(cod)]1 23a [(S–S)Rh(CO)2]1 23c [(S–S)Rh(Me2C6H3NC)2]1 25b [{Pt2(dppe)2(m3-S)2}2Cu]21 28 [(S–S)CoCl2] 30a [(S–S)(AgCl)2] 31 [(S–S)(AuCl)2] 32 [(S–S)2Ag2]21 (Ag–Ag) 33a [(S–S)Cu(PPh3)]1 33b [Pd2Cu(PPh3)5(m3-S)2]1 34 h [(S–S)Ag(PPh3)]1 34 h [(S–S)Ag(PPh3)]1 35 [(S–S)Au(PPh3)]1 d-Block metal ion M Pd21 Rh1 Rh1 Rh1 Cu21 Co21 Ag1 Au1 Ag1 Cu1 Cu1 Ag1 Ag1 Au1 Co-ordination geometry of M Square planar Square planar Square planar Square planar Square planar Tetrahedral Linear Linear Linear “Y-shaped” “Y-shaped” “Y-shaped” “Y-shaped” Distorted linear Pt ? ? ? Pt c 3.255(1) 3.269(1) 3.2907(6) c 3.197(4) 3.569(2) c 3.350(2) 3.278(2) 3.279 3.276(2) f 3.351(2) 3.375(2) 3.279 S ? ? ?S c 3.004(4) 3.024(3) 3.008(4) 3.1 c c c 3.06(1) 3.14(1) c c c c c Dihedral angle q b/8 c 131.9 132.0 112.1 118.4 121.1 127.9(6) 180 180 c 137.5(5) 136.5 c c 135 M? ? ? Pt c 3.063(1) 3.040(1) 3.048(1) 3.034(1) 3.0543(10) c 3.066(1) 3.130(2) 3.060(2) 3.111(1) 3.218(1) c 2.875(2) 2.864(2) 2.789(3) g 2.889(2) g 3.061(1) 3.066(1) 3.240(1) 2.962(1) 3.314(1) 3.231(1) M–S 2.348(2) 2.347(3) 2.356(3) 2.365(2) 2.314(3) 2.359(4) 2.389(3) 2.692(7) 2.389(9) 2.303(5) 2.269(5) 2.251(4) 2.266(5) 2.479(1) 2.585(1) 2.607(2) 2.502(2) 2.345(2) 2.959(2) i S–M–S 76.4(1) 79.6(1) 79.9(1) 79.0(1) 83.60(9) 80.0(1) 173.4(2) d 174.2(1) d 176.2(3) e 174.1(3) 84.6(2) 85.7(2) 76.04(3) 74.33(7) 70.5(1) Ref.20(b) 20(c) 20(d ) 20(d ) 27(d ) 27(a) 27(e) 20( f ) 20(e) 27(c) 45(e) 23 23 20( f ) a Mean values are cited for chemically equivalent bonds and angles from previously published structures. b Dihedral angle between two {PtS2} (or {PdS2}) planes. c Value not reported. d Angle S–Au–Cl given. e Angle reported for S atoms of opposing S–S since Ag is linear.f Pd ? ? ? Pd distance. g M? ? ? Pd distance. h Exists as two polymorphic modifications. i Au–S contact distance. A common structural feature among these compounds is the short non-bonding metal–metal separations. The metals are locked in close proximity chiefly by the capping sulfide ligands. Catalytic co-operation (e.g. in 23b) 20d and electrochemical communication (e.g. in 43a) 21f among the metals are subjects of our interest. The ability of 1b to capture a variety of metal entities is partly attributed to the adjustable Pt ? ? ? Pt and S ? ? ? S non-bonding distances as well as the dihedral angle q between the PtS2 planes. Some representative parameters are collated in Table 3. We have recently provided a methodology for the sequential growth of heteropolymetallic Pt–Ag sulfide aggregates.27e Based on the early report of the Ag2Pt4 aggregate 32,20e our recent Pt S Pt S Pd Pt S Pt S Pt S Pt S Rh Pt S Pt S Rh L L Pt S Pt S Pd Pt S Pt S Pd Cl Cl 18 22 20 P2 = dppe 19 2+ 2+ Pt S Pt S Pd P P 2+ 23a L = CO X = [Rh(CO)2Cl2] 23b L = C2H4 X = Cl 23c L = 2,6-Me2C6H3NC X = PF6 Pt S Pt S Pd Cl PPh3 + 21 + + X isolation of the intermediate aggregates [(Ph3P)4Pt2(m3-S)2- Ag2Cl2] 30a and [(Ph3P)4Pt2(m3-S)2Ag(PPh3)]1 34 has provided clear evidence of the stepwise build-up of higher nuclearity aggregates viz.from Pt2 1b æÆ AgPt2 34 æÆ Ag2Pt2 30a Pt S Pt S M P P M S S Pt S Pt S Pt Pt 26 M = Ni P2 = dppe 27 M = Hg P2 = dppe 2+ n+ Pt S Pt S Co Cl Cl 28 24 M = Hg n = 2 25a M = Cu n = 1 25b M = Cu n = 2 M S M S M¢ PPh3 Pt S Pt S Pt S Pt S Ag Ag M S M S Cl Cl M¢ M¢ 2+ 32 Pt S Pt S Pt S Pt S 29 P2 = dppf Cu Cu P P 33a M = Pt M¢ = Cu 33b M = Pd M¢ = Cu 34 M = Pt M¢ = Ag 35 M = Pt M¢ = Au 30a M = Pt M¢ = Ag 30b M = Pd M¢ = Ag 31 M = Pt M¢ = Au + 2+ J.Chem. Soc. Dalton Trans. 1999 639–651 645 æÆ Ag2Pt4 32. The methodology developed was based on the reactivity of 1b towards simple Ag1 compounds. A similar approach by Yam et al.36 using AgI on the analogous complex 1c which bears a difunctional phosphine/pyridine ligand yielded some delightful polynuclear complexes with Pt–Ag bonds. Formation of [{(dppy)4Pt2(m3-S)2}2Ag3]31 36 and [(dppy)4Pt2(m3-S)2Ag2(m-dppm)]21 37 from the reaction of 1c with [Ag(MeCN)4]PF6 and [Ag2(dppm)2(MeCN)2][BF4]2 respectively demonstrates that a pendant pyridine can help the sulfide in bringing the metals to close proximity.Complex 36 shows how an {Ag3} triangle can be sandwiched by two {Pt2S2} moieties whereas in 32 the sandwich filling is made up of an Ag–Ag bond.20e In order to sustain the barrel-shaped structure in 36 the planar {Pt2S2} core in 1c has to fold to 132.68. While the m3-thio ligand is well established in both heteroand inter-metallic aggregates the m4 co-ordination mode is less common but can be found in clusters such as [Os6(CO)16(m3-S)- (m4-S)],37a [Os6(CO)17(m4-S)],37b,c etc. The possibility of using the {M3S2} as a synthon to make the {M4S2} aggregate is being explored. Recent eVort in our laboratory is directed at the synthesis of a complex like [RhPt3Cl(CO)(PPh3)6(m4-S)2]21 38.27b Recent results suggested that complex 1b picks up binary metal halides like MCl2 (M = Zn Cd or Hg) to give simple adducts [Pt2(PPh3)4(m-S)2]?MCl2.27b The prospect of using the {Pt2S2} moiety as a “carrier” to introduce insoluble MCl2 into an organic medium is enticing.This oVers a possibility for a MCl2-catalysed metal exchange reaction or {Pt2S2} as a “phasetransfer catalyst” for metal halide reactions (Scheme 4). The use of complex 1b as a precursor to heterometallic sul- fide complexes oVers a simple yet powerful strategy for the design of mixed-metal materials. Since almost any metal compound with some Lewis acidic character can be incorporated into the {Pt2S2} core the generality simplicity and versatility of S Pt S Pt P P P P 36 P N = dppy Ag Ag Ag Pt S Pt S P P N P P N 3+ N N N N N N Ag Ag Pt S Pt S P P P N Ph2 P P Ph2 P N 2+ 37 P N = dppy N N Pt S Pt S Pt PPh3 Ph3P PPh3 Ph3P Rh CO PPh3 Ph3P Cl 38 2+ this method remains unrivaled by most other established routes in heteromultimetallic syntheses.25c,30,38 A related but diVerent approach is demonstrated by Hidai and co-workers 38c in the use of {Ru2S2} as a core for heterometallic synthesis.Insertion of a [Pt(PPh3)2] fragment into an S–S bond of a disulfide complex [Ru2(h5-C5Me5)2(m-S2)(m-SPri)2] yields [PtRu2(h5-C5Me5)2- (PPh3)2(m-S)2(m-SPri)2] 39. The absence of M–M bonds in this PtII/RuIII aggregate is understood in a reaction best described as a reductive cleavage of a disulfide ligand to two m-monosulfides. If one can direct the reduction to the metal instead of the sul- fide sites a clusterification process can be envisaged. This approach is illustrated in our recent eVort to introduce lowvalent platinum(0) fragments e.g.[Pt(P)2] (P2 = 2PPh3 or dppf),39 to the {Pt2S2} core. Given the nucleophilicity of the m-thio ligands the incoming metal group is expected to anchor onto the sulfur site or insert into the Pt–S bond thereby creating a PtII–Pt0–PtII triangle which should ease its electronic imbalance through Pt–Pt interactions. Recently we showed that complex 1b undergoes a facile reductive desulfurisation reaction under mild CO pressure (60 psi) to give [Pt2(CO)n(PPh3)4 2 n(m-S)] (n = 1 or 2) with a strong Pt–Pt bond (Scheme 5).22 The insertion of CO into the Pt–S bond and the formation of an intermediate bridging COS complex are exciting ideas which need substantiation. This is Scheme 4 Possible use of {Pt2S2} complexes as a “carrier” for the MCl2 species (a) metal exchange reaction and (b) “phase transfer catalysis”.MCl2 + {Pt2S2} {Pt2S2}MCl2 (a) M'Ln (b) L in organic medium MCl2 + {Pt2S2}M'Ln MCl2Ln + {Pt2S2} 39 P = PPh3 Ru S Ru S Cp* Cp* S S Pt P P Pri Pri Scheme 5 Possible reaction mechanism of reductive desulfurisation of complex 1b (P = PPh3) under mild CO pressure.22 Pt S P P Pt S P P Pt S P P Pt S P CO Pt S P P Pt S P CO C Pt S P P Pt S P CO C O Pt S OC P Pt P CO Pt S OC P Pt P P 1b CO P CO O CO COS + P P CO 646 J. Chem. Soc. Dalton Trans. 1999 639–651 hitherto the only known route linking the {Pt2S2} and {Pt2S} series. As already established in cluster synthesis reduction and ligand removal are powerful means to induce metal–metal bond formation. Furthermore it is significant that after the removal of one sulfide bridge the remaining sulfide is inert towards carbonyl attack even under higher CO pressure.This illustrates the selectivity of CO towards sulfide excision as well as its ability to reduce PtII to PtI without driving to the unstable platinum(0) state. On this basis a simple clean yet powerful method was recently developed for the conversion of heterometallic aggregates into clusters.23 It involves an unprecedented series of concomitant processes heterometallation carbonylation reductive desulfurisation and metal–metal bond formation (Scheme 6). These ideal qualities of CO serve very well to preserve any cluster framework formed. Indeed no other common reagent can fulfill such a multifaceted role with such eY- ciency. An added advantage is the easy removal of the gaseous COS by-product.It remains an interesting possibility whether a similar strategy can be developed to convert other sulfur-rich complexes into their sulfur-poor counterparts. The isoelectronic {AuIII 2S2} analogue has emerged recently with the syntheses of [Au2Cl4(m-SAuCl)2],40 [({[C6H3(CH2- NMe2)-2-(OMe)-5]Au(m-S)}2)3Ag3(m3-Cl)2]1 40 and [({[C6H4- (CH2NMe2)-2]Au(m-S)}2)3Ag3(m3-Br)2]1 41 from an unexpected thiourea desulfurisation reaction.41 These novel Au–Ag aggregates were studied by electrospray mass spectrometry (ESMS). Although there are indications that parallel chemistry can be developed on these {Au2S2} complexes their metallacyclic chemistry appears to be diVerent and unpredictable. From a butterfly to a hat—formation of intermetallics with p-block Lewis acids The nucleophilicity of the di-m-thio groups in complex 1b is also exploited when it functions as a bidentate ligand towards various main-group Lewis-acids.21 The intermetallic complexes so obtained exhibit some unusual co-ordination geometries on the heterometal.For example in [Pt2Tl(P–P)2(m3-S)2]X (P–P = Scheme 6 Direct conversion of heterometallic aggregates to clusters involving a series of concomitant processes namely heterometallation carbonylation reductive desulfurisation and metal–metal bond formation; MClLx = CuCl AgCl(PPh3) or RuCl2(PPh3)3.23 Pt S Pt S P P P P 1b MClLx CO COS heat 60 psi S Pt S Pt LxM P P P P + Cl S Pt Pt ClM P P P CO Ag X 40 X = Cl (N C) = 2-dimethylaminomethyl-5-methoxyphenyl 41 X = Br (N C) = 2-dimethylaminomethylphenyl X Ag Ag S S Au Au C N S S Au Au N C N C C N S S Au Au C N N C 2PPh3 X = NO3 42a or PF6 42b;21a P–P = dppf X = NO3 43a or PF6 43b 21f ) the Tl1 is two co-ordinate bent and exposes a non-participative (inert) lone pair.This results in a “Mexican hat-like” structure with the {Pt2TlS2} core resembling the “Mercedes-Benz” insignia when viewed along the S ? ? ? S axis. The sustainability of such structures by altering the peripheral phosphine (from PPh3 to dppf for example) oVers an advantage in the design of electroactive multimetallic materials. The diverse chemistry of dppf 42 and its role in cluster chemistry 43 were recently reviewed. The presence of a ferrocenyl moiety in dppf is known to confer electroactivity to its complexes.44 Although some cyclic voltammetric studies of 43a and 43b and the related complexes [(dppf)2Pt2(m3-S)2Pb(NO3)]NO3 45 and [(dppf)2Pt2(m3-S)2InCl3] 49b are not conclusive,21f the possibility for charge-transfer communication among the diVerent metals and the ferrocenyl moieties serves as an incentive for further work.The ability to design a multimetallic architecture supported by electroactive ligands and connected by ligands that are known for charge distribution is an appetising prospect. Other p-block metals also display some interesting properties when anchored on the {Pt2S2} core. For example Pb21 shows a “vacant site” occupied by NO3 2 or PF6 2 in [(Ph3P)4Pt2(m3-S)2- Pb(NO3)]X (X = NO3 44a or PF6 44b21b) whereas GaIII gives an ion-pair [(Ph3P)4Pt2(m3-S)2GaCl2]1[GaCl4]2 46 but InIII an almost perfect square pyramid in [(Ph3P)4Pt2(m3-S)2InCl3] 49a. The co-ordinatively exposed thallium(I) structures in 42 and 43 appear to be stabilised by some unusual TlI ? ? ? PtII interactions as suggested by the short Tl ? ? ? Pt distances and strong Pt–Tl coupling (195Pt NMR).Some notable ligand dissociation and solution dynamics can also be found in the bismuth(III) adducts [(Ph3P)4Pt2(m3-S)2BiCl2]PF6 47 and [(Ph3P)4Pt2(m3-S)2BiCl3] 48. Both high and low co-ordination are found for the heterometal atom on the {Pt2S2} core. All these data suggest that these intermetallics have rich and often surprising chemistry.26,29 A Pt S Pt S P P P P Pb O O N O O O N O Pt S Pt S P P P P 44b Pb O O N O F P F F F F F Pt S Pt S P P P P ClCl Cl Pt S Pt S P P P P 48 Bi Cl Cl Cl X + Pt S Pt S P P P P M Cl Cl Pt S Pt S P P P P 42a P2 = 2PPh3 X = NO3 42b P2 = 2PPh3 X = PF6 43a P2 = dppf X = NO3 43b P2 = dppf X = PF6 Tl X + 44a P2 = 2PPh3 45 P2 = dppf 49a P2 = 2PPh3 49b P2 = dppf 46 M = Ga X = GaCl4 47 M = Bi X = PF6 J.Chem. Soc. Dalton Trans. 1999 639–651 647 Table 4 Selected structural parameters a (distances in Å angles in 8) of the intermetallic adducts of complex 1b [S–S = Pt2(PPh3)4(m-S)2] with p-block Lewis acidic fragments Complex 42b [(S–S)Tl]PF6 44a [(S–S)Pb(NO3)]NO3 44b [(S–S)Pb(NO3)]PF6 46 [(S–S)GaCl2][GaCl4] 47 [(S–S)BiCl2]PF6 48 [(S–S)BiCl3] 49a [(S–S)InCl3] p-Block metal ion Y Tl1 Pb21 Pb21 Ga31 Bi31 Bi31 In31 Co-ordination geometry of Y Bent Distorted trigonal prismatic Distorted trigonal prismatic Tetrahedral Distorted tetrahedral Distorted square-based pyramidal Square-based pyramidal Pt ? ? ? Pt 3.293(2) 3.266(2) 3.290(2) 3.220(1) c c 3.253(2) S ? ? ?S 3.126(5) 3.060(4) 3.072(3) 3.066(4) c c 3.030(2) Dihedral angle q b/8 135.7(1) 132.2(1) 133.5(1) 123.1(3) 130.7(2) 128.8(3) 128.3(2) Y–S 2.764(3) 2.718(4) 2.704(3) 2.294(2) 2.692(7) 2.762(3) 2.614(2) S–Y–S 68.9(1) 68.5(1) 69.2(1) 83.9(1) 70.1(2) 66.6(1) 70.8(1) Ref.21(a) 21(b) 21(b) 21(c) 21(d ) 21(d ) 21(c) a Mean values are cited for chemically equivalent bonds and angles from previously published structures. b Dihedral angle between two PtS2 planes. c Value not reported. summary of selected structural data for these adducts is given in Table 4. 3 Complexes containing a tri-palladium core The palladium analogue of complex 1b [Pd2(PPh3)4(m-S)2] is poorly characterised 45 and its chemistry virtually unknown. Although its synthesis is plagued by the lability of the phosphine ligand we circumvented the problem by replacing PPh3 with the chelating diphosphine dppf yielding [Pd2(dppf)2- (m-S)2] 50.24 The aggregate obtained on reaction between 50 and AgCl viz.[(dppf)2Pd2(m3-S)2Ag2Cl2] 30b is entirely analogous to 30a possessing a planar {Pd2S2} core which openly bridges two AgCl moieties by the nucleophilic sulfur atoms. Its isolation has demonstrated the usefulness of 50 as a precursor to the heterometallic aggregates of Pd. It is also a clear indication that the rich heteropolymetallic chemistry based on the established {Pt2S2} core aforementioned can be extended to the palladium system. More recently complex 50 was used as a metalloligand in the assembly of three triangular palladium sulfide aggregates with a {Pd3S2} core,24b namely [Pd3Cl(h2-dppf)2(PPh3)(m3-S)2]X (X = Cl 51a or NO3 51b) and [Pd3Cl2(h2-dppf)(m-dppf)(m3-S)2] 52 (Scheme 7).The introduction of two diphosphine ligands on a Pd3 core opens a range of possibilities for positional isomerism and fluxional mechanisms. The latter can involve a “creepand- crawl” migration (Scheme 8 or Path a in Scheme 9) or a “wiper-screen” style of movement (Paths b and c of Scheme 9) of the phosphines. The former involves an interchange of chelating and bridging modes whilst the latter involves swinging of the phosphines around the metal atoms with the phosphines staying at the bridging mode. Both mechanisms are helped by Scheme 7 Conversion of a {Pd2S2} (50) to a {Pd3S2} (52) core showing positional isomerism arising from ligand migrations [dppf = 1,19-bis- (diphenylphosphino)ferrocene].24b S Pd S Pd Pd PPh3 Cl P P + Pd S Pd S P P P P 50 P2 = dppf P P PdCl2(PPh3)2 PPh3 51 S Pd S Pd Pd PPh3 P P Cl + P P S Pd S Pd Pd Cl P P Cl P P PPh3 52 Cl Cl the migratory movement of the chloride around the ring.The NMR evidence suggests that 52 shows the former type of fluxionality with an alternation of phosphine–phosphine and phosphine–chloride exchanges i.e. Scheme 8 whereas 51 is stereochemically rigid because one of the chloride sites is replaced and blocked by a PPh3 ligand. These migratory processes demonstrate the flexibility of dppf in the interchange of co-ordination modes and very significantly the stabilising role of the sulfide ligands in keeping the Pd3 triangle intact while allowing the dissociation and migration of the peripheral ligands. Other earlier work in {Pd2S2} chemistry includes the syntheses of [Pd2(PPh3)4(m-S)2],45a [Pd3Cl2(PPh3)4(m3-S)2] 45b and [Pd3(PR3)6(m3-S)2]21 (R = Me45c or Et 45d ).The recent isolation of [(Ph3P)4Pd2(m3-S)2Cu(PPh3)]1 33b through a somewhat serendipitous approach from a mixture of [Pd(PPh3)4] and [Cu2- (PPh3)3(m-OS3)] strongly suggests the thermodynamic stability of the {Pd2MS2} core. It would not be surprising that based on the isolation of 30b and 33b other heterometallic complexes of Pd can be developed. The short Cu ? ? ? Pd distances in 33b (see Table 3) are not unusual when compared to those of other similar complexes.45e We have recently isolated by accident [Pd6(NO3)3(PPh3)6- (m3-S)3(m6-S)]NO3 53 from a mixture containing [PdCl2(PPh3)2] Na2S?9H2O and AgNO3.4 The complex has a {Pd6S4} core which can be considered as an assembly of two {Pd2S2} cores and two Pd(NO3)2 units or a fusion of two {Pd3S2} cores with concomitant phosphine liberation and nitrate addition.The Scheme 8 Possible migratory processes of two diphosphines on a {Pd3S2} core supplemented by chloride and phosphine exchanges (P–P = dppf).24b Pd S Clb Cla Pb Pa Pc Pd Pa Cla Clb Pc Pb Pd Clb Pc Pa Cla Pd Pb S Pd Pd Cla Clb Pc Pa Pd Pb Clb Pa Pb Cla Pc Pd Clb Cla Pa Pc Pb Pd Cla Clb Pb Pa Pc Pd = ... 648 J. Chem. Soc. Dalton Trans. 1999 639–651 Scheme 9 Possible migratory processes of two diphosphines in diVerent co-ordination modes (either chelating or bridging) on a {Pd3S2} core accompanied by Cl exchange (P–P = dppf).24b Pd S Pc Pb Clb Cla Pa Pd Clb Cla Pc Pd Pa Pb Clb Pd Pa Pc Cla Pb Clb Pb Pa Cla Pc Pd Cla Pd Pb Pa Pc Clb Cla Clb Pc Pd Pa Pb Clb Pb Pa Pd Pc Cla Pb Pd Cla Clb Pc Pa S Pd Pd Path a ...Path b ... Path c ... = structure as a result consists of a prismatic Pd6 core with an encapsulated sulfide and four m3-S each of which caps a face comprising two Pd atoms on the bottom planes and one Pd atom on the top plane. The nitrate ligands on the top Pd3 plane are possible active sites for further metallation and hence cluster growth. 4 A flat or hinged butterfly—dihedral angle and some theoretical aspects of the {Pt2S2} ring Throughout the development of the {Pt2S2} system there had been numerous studies on the possible determinants of ring folding for example the role of the sulfido bridges and how their alkylation (or metallation) could aVect the ring geometry. Initially Chatt and Hart 13a had ascribed the observed hinging to the formation of an “inorganic aromatic ring” involving dp– pp interaction within the {Pt2S2} core.Based on this hypothesis a simplistic MO model was used to explain how alkylation of one of the sulfur bridges would initiate such interactions and hence discourage further alkylation at the second sulfur atom.17 There are suYcient examples among the platinum(II) sulfide and thiolato complexes which are known to adopt either a flat 31,46 (q = 1808) or hinged 13,16,46a,47 (q < 1808) conformation on the {Pt2S2} ring (Scheme 10). This coupled with the geometrical isomerism arising from cis and trans disposition of the ligands and the syn and anti conformation exhibited by the bridgehead R group [R = alkyl (or aryl) or metal residues] render the number of possible structures many and varied.Subtle changes in the ligands could aVect the preferred conformations just as a strong trans-influencing ligand could weaken the P Pd S S Pd Pd S 53 P = PPh3 Pd Pd Pd S P NO3 NO3 P O3N P P P opposite Pt–S bond. Indeed complexes based on a {Pt2S2} ring characterised by single-crystal X-ray diVractometry show considerable structural diversity (see Tables 3 4 and 5 for selected structural parameters). Generally when both bridging groups are S22 as in complexes 1a 1b and 2a or when either or both sulfide atoms are alkylated or metallated there is pronounced hinging (e.g. q = 1218 in 1a). A rare exception is the unsubstituted sulfide complex 1c with a {Pt2S2} core which is strictly flat.32 However when both sulfides are metallated to different groups a hinged or flattened butterfly would result depending on the relative orientation of the R group (at anti or syn position) and cis or trans arrangement of the terminal ligand.When diVerent supporting ligands such as 1,2-bis(diphenylphosphino) ethane (dppe = Ph2PCH2CH2PPh2) (e.g. in 54 and 55) and 1,2-diaminoethane (en = H2NCH2CH2NH2) (e.g. in 56 57 and 58) are used in place of unidentate phosphines or when the core metal is changed (e.g. Ni 54a Pd 54b and Pt 54c) a series of doubly substituted di-m-sulfido complexes containing a globally planar central {M2S2} ring is obtained.28 Other examples of planar trans-{M2S2} complexes are 60 to 63. When compared with the hinged cis-{Pt2S2} complexes 8 to 10 it would seem that the trans complexes favour a planar {M2S2} core while the cis favour the hinged form; subtle interplay between the trans influences of the various ligands appears to determine the most thermodynamically favoured ring conformation.Table 5 lists the available structural data for these variant {M2S2} complexes. A rigorous study into the hinge distortion in platinum(II) dimers with a {Pt2S2} ring was recently provided by Capdevila et al.29 After taking into account all possible determinants of the dihedral (hinge) angle in some representative complexes it was concluded that electronic eVects played a major role in governing the geometry about the central {Pt2S2} ring; the amount of through-ring antibonding interaction between the in-plane sulfur p orbitals (which decreases upon ring folding) Scheme 10 Two possible geometrical forms adopted by the {M2S2} ring.J. Chem. Soc. Dalton Trans. 1999 639–651 649 Table 5 Selected structural parameters a (distances in Å angles in 8) of some dinuclear m-thiolato complexes possessing a central {M2S2} (M = Ni Pd or Pt) core Complex 1a [Pt2(PMe2Ph)4(m-S)2] 1c [Pt2(dppy)4(m-S)2] 2b [Pt2(dppe)4(m-S)2] 5b [Pt2(PPh3)4(m-S)(m-SMe)]PF6 8 cis-[Pt2(NO2)2(PPh3)2(m-SMe)2] 9 cis-[Pt2Cl2(PPrn 3)2(m-SEt)2] 10 cis-[Pt2(SCH2Ph)2(PMePh2)2(m-SCH2Ph)2] 54a [Ni2(dppe)2{m-S(CH2)3NMe2}2][BPh4]2 54b [Pd2(dppe)2{m-S(CH2)3NMe2}2]21 54c [Pt2(dppe)2{m-S(CH2)3NMe2}2]21 55a [Pd2(dppe)2(m-SC5H9NMe)2][BPh4]2 55b [Pt2(dppe)2(m-SC5H9NMe)2][BPh4]2 56 [Pt2(en)e{m-S(CH2)3NMe2}2]Br2 57 [Pt2(en)2(m-SC5H9NMe)2]Br2?6H2O 58 [Pt2(en)2(m-SC5H9NHMe)2]Br4?2H2O 59 [Pt2(bpy)2(m-accys-S)2] 60 trans-[Pt2Ph2(PMe2Ph)2(m-SPh)2] 61 trans-[Pt2I2(PPh3)2{m-S(CH2)2C(Me)]] CH2}2] 62 trans-[Pd2(SC6F5)2(PPh3)2(m-SC6F5)2] k 63 trans-[Pd2Cl2(PEt3)2(m-SPh)2] M–Sb b 2.340(8) 2.333(1) 2.3496(12) 2.320(8) 2.363(11) 2.294(3) h 2.354(2) i 2.274(7) h 2.371(7) i 2.322(4) h 2.380(4) i 2.237(2) 2.386(5) 2.377(5) 2.427(2) 2.385(2) 2.393(2) 2.289(3) 2.298(2) 2.301(2) 2.308(5) 2.386(4) h 2.371(4) i 2.367(3) 2.37 2.39 2.289(2) 2.382(2) M–Lc 2.265(7) 2.277(2) g 2.269(5) 2.292(5) 2.266(1) 2.262(5) 2.248(4) 2.188(2) 2.263(5) 2.275(6) 2.242(5) 2.259(7) 2.271(2) 2.278(2) 2.263(2) 2.111(8) 2.066(8) 2.075(6) 2.066(6) 2.074(5) 2.071(5) 2.01(1) 2.232(4) 2.267(3) 2.28 2.25 2.266(2) M? ? ?M 3.175(2) 3.555(1) g 3.306(1) 3.342(1) 3.206(1) g 3.310(1) 3.504 3.556 3.497 3.550 g g g g g 3.539(1) g g 3.463 Sb ? ? ?Sb 3.06(2) 3.005(2) g 3.06(1) 3.04(1) 2.99(1) g 3.009(2) g g g 3.210 g g g g g g g g g M–Sb–M 85.5(4) 99.58(6) 88.94(4) 88.9(2) 90.9(2) 92.0(15) 85.1 89.7 90.8(1) 95.5(1) 94.4(3) 96.7(3) 93.1(1) 95.7(1) 96.9(1) 97.3(1) 97.0(1) g 97.2(1) g g g 95.7(1) Sb–M–Ld 89.5(3) 86.75(5) g 90.0(2) 85.9(2) 91.7(1) 96.1(2) 94.2(2) 95.4(1) g g 96.1(1) 95.0(1) 96.5(1) 97.1(2) 96.7(2) 96.4(1) g 98.5(1) 178.0(1) j g g g Sb–M–Sb 81.6(4) 80.42(6) 83.67(4) 81.6(2) 81.7(1) 80.2(3) 80.3(1) 84.6(1) 85.6(3) 83.3(3) 85.7(1) 88.2(1) 84.3(1) 83.1(1) 82.7(1) 83.0(1) g 82.8(1) g 87.6 80.6 84.3(1) L–M–Le g 102.98(6) g 102.3(2) 99.0(2) 85.8(1) 84.7(2) 85.2(2) 83.5(1) 84.2(1) 84.7(1) 83.3(3) 82.8(2) 82.2(2) g q f/8 121 180 140.2 138 141 130 138.8 180 180 180 180 180 180 180 180 180 180 180 180 180 180 Ref.11 13(c) 32 27(d ) 16 16 13(b) 13(d ) 28(a) 28(b) 28(b) 28(b) 28(b) 28(c) 28(c) 28(c) 31 46(b) 46(a) 52(a) 52(b) 53 a Mean values are cited for chemically equivalent bonds and angles from previously published structures.b Sb denotes bridging sulfur atom. c L denotes terminal ligand; M–L bond distances are only cited if the L donor atom is nitrogen or phosphorus. d Refers to angle between Sb and L donor atom cis to each other. e Angle is only cited for chelating or equal unidentate ligands L about a square planar M. f Dihedral angle between two SbMSb planes. g Value not reported. h cis to phosphine. i trans to phosphine. j Only trans Sb–M–L is given. k Exists as two crystalline modifications. 650 J. Chem. Soc. Dalton Trans. 1999 639–651 determines the extent of hinging. Related work48 on various binuclear d8 transition metal complexes arrived at a similar conclusion.The observed hinging of the {M2X2} core can be explained in terms of the attractive donor–acceptor interactions between the dz2 and the pz orbitals of the two metal atoms modulated by the nature of the metal atom the terminal ligands and the bridging atoms. The sulfide complexes appear to be more stable in the hinged form and their stability increases in the order NiII < PdII < PtII < RhI < IrI for diVerent metal atoms. Our experience with the heterometallic aggregates of 1b suggests that quite generally as the dihedral angle decreases the heterometal–sulfur interactions strengthen at the expense of the Pt–S overlap.26 In general binding of a heterometal M to 1b causes a weakening (lengthening) of the Pt–S bonds while coordination of additional ligands to M weakens the M–S bonds with a concomitant strengthening (shortening) of the Pt–S bonds.In particular any ligand trans to an M–S bond will render it weakened more than the other M–S bond. This may be rationalised by considering a fundamental competition for interactions (i.e. M–L vs. M–S and M–S vs. Pt–S). Furthermore ab initio calculations also indicate that M–S bonding is an important factor for M to accept additional ligands. This accounts for the diVerences in co-ordination behaviour between the isoelectronic species Tl1 and Pb21. 5 Conclusion The potential of using an {M2S2} core in the development of multimetallic structures is appetising. We have formulated the key strategies in the ring contraction (M2S2 æÆ M2S) metallation (M2S2 æÆ M3S2 æÆ M4S2) and clusterification (M3S2 æÆ M3S) reactions.They form the basis of molecular design of polynuclear frameworks based on a stepwise introduction (or removal) of metal atoms to the {M2S2} butterfly core with or without concomitant desulfurisation and/or metal–metal bond formation. The facile and controlled sulfide extrusion from {M2S2} serves as a molecular model for (hydro)desulfurisation on precious metals. It also provides a fresh impetus for the insertion of other small molecules into the notoriously strong Pt–S bonds. The catalytic potential of these complexes has not been exploited but deserves more attention in the coming years. The conventional thinking is that sulfur sulfides and other sulfur-containing substances pose a major threat to catalytic processes because of the poisoning eVect of sulfur on almost all known catalysts.It would be a significant achievement if on the basis of the work described we can develop a catalyst R M S M S L L L L R R M S M S L L' L' L R 54a M = Ni L2 = dppe R = (CH2)3NMe2 n = 2 54b M = Pd L2 = dppe R = (CH2)3NMe2 n = 2 54c M = Pt L2 = dppe R = (CH2)3NMe2 n = 2 55a M = Pd L2 = dppe R = C5H9NMe2 n = 2 55b M = Pt L2 = dppe R = C5H9NMe2 n = 2 56 M = Pt L2 = en R = (CH2)3NMe2 n = 2 57 M = Pt L2 = en R = C5H9NMe2 n = 2 58 M = Pt L2 = en R = C5H9NHMe n = 4 59 M = Pt L2 = bpy R = accys n = 0 n+ 60 M = Pt L = PMe2Ph L' = Ph R = Ph 61 M = Pt L = PPh3 L' = I R = (CH2)2C(Me)=CH2 62 M = Pd L = PPh3 L' = SC6F5 R = C6F5 63 M = Pd L = PEt3 L' = Cl R = Ph which not only has a catalytically active M–S bond but is also inherently immune to sulfur poisoning because of its sulfur-rich nature.6 Acknowledgements The authors wish to thank the National University of Singapore (NUS) (RP960664/A) for financial support. S.-W. A. F. is grateful to NUS for a research scholarship as well as to the National Science and Technology Board of Singapore for a top-up supplement. Contributions from the many able students whose names appear in the cited references are gratefully acknowledged. The analogy of the “Mexican-hat” structure of the {Pt2Tl} complex to the “Mercedes-Benz” insignia was first mooted by Professor P. Braunstein (Strasbourg). 7 References 1 E. I. Stiefel ACS Symp. Ser. 1996 653 2. 2 W. Clegg G. Christou C. D. Garner and G. M. Sheldrick Inorg. Chem. 1981 20 1562. 3 J. L. Vidal R. A. Faito L. A. Cosby and R.J. Pruett Inorg. Chem. 1978 17 2574. 4 G. Li M.Sc. Thesis National University of Singapore 1998; G. Li T. C. W. Mak and T. S. A. Hor unpublished work. 5 F. R. Hartley The Chemistry of Platinum and Palladium Applied Science Publishers London 1973. 6 R. Chevrel P. Gougeon M. Potel and M. J. Sergent J. Solid State Chem. 1985 57 25. 7 E. I. Stiefel and G. N. George in Bioinorganic Chemistry eds. I. Bertini H. B. Gray S. J. Lippard and J. S. Valentine University Science Books Mill Valley CA 1994 ch. 7 pp. 365–454; Y. Okuno K. Uoto O. Yonemitsu and T. Tomohiro J. Chem. Soc. Chem. Commun. 1987 1018; F. Bottomley and F. Grein Inorg. Chem. 1982 21 4170; R. H. Holm Acc. Chem. Res. 1977 10 427; R. H. Holm and J. A. Ibers Science 1980 209 223; R. H. Holm Chem. Soc. Rev. 1981 10 455; G.Christou C. D. Garner R. M. Miller C. E. Johnson and J. D. Rush J. Chem. Soc. Dalton Trans. 1980 2363; G. Christou and C. D. Garner J. Chem. Soc. Dalton Trans. 1980 2354. 8 R. F. Borch and M. E. Pleasants Proc. Natl. Acad. Sci. U.S.A. 1979 76 6611. 9 V. K. Jain Inorg. Chim. Acta 1987 133 261; W. Weigand R. Wünsch and S. Dick Z. Naturforsch. Teil B 1996 51 1511; B. Odenheimer and W. Wolf Inorg. Chim. Acta 1982 66 L41; E. L. M. Lempers K. Inagaki and J. Reedijk Inorg. Chim. Acta 1988 152 201; M. I. Djuran E. L. M. Lempers and J. Reedijk Inorg. Chem. 1991 30 2648; S. J. Berners-Price and P. W. Kuchel Inorg. Biochem. 1990 38 305. 10 K. A. Hofmann and F. Höchlen Chem. Ber. 1903 36 3090. 11 J. Chatt and D. M. P. Mingos J. Chem. Soc. A 1970 1243. 12 R. Ugo G. La Monica S. Cenini A. Segre and F.Conti J. Chem. Soc. A 1971 522. 13 (a) J. Chatt and F. A. Hart J. Chem. Soc. 1960 2807; (b) M. C. Hall J. A. J. Jarvis B. T. Kilbourn and P. G. Owston J. Chem. Soc. Dalton Trans. 1972 1544; (c) R. Mason D. Law and D. M. P. Mingos unpublished work; (d ) P. H. Bird U. Siriwardane R. D. Lai and A. Shaver Can. J. Chem. 1982 60 2075; (e) G. W. Bushnell K. R. Dixon R. Ono and A. Pidcock Can. J. Chem. 1984 62 696. 14 R. R. Gukathasan R. H. Morris and A. Walker Can. J. Chem. 1983 61 2490. 15 T. S. A. Hor D.Phil. Thesis University of Oxford 1983. 16 C. E. Briant C. J. Gardner T. S. A. Hor N. D. Howells and D. M. P. Mingos J. Chem. Soc. Dalton Trans. 1984 2645. 17 T. S. A. Hor and A. L. C. Tan Inorg. Chim. Acta 1988 142 173. 18 M. J. Pilkington A. M. Z. Slawin D. J. Williams and J.D. Woolins J. Chem. Soc. Dalton Trans. 1992 2425; K. Matsumoto N. Saiga S. Tanaka and S. Ooi J. Chem. Soc. Dalton Trans. 1991 1265. 19 B. H. Aw K. K. Looh H. S. O. Chan K. L. Tan and T. S. A. Hor J. Chem. Soc. Dalton Trans. 1994 3177. 20 (a) C. E. Briant M. J. Calhorda T. S. A. Hor N. D. Howells and D. M. P. Mingos J. Chem. Soc. Dalton Trans. 1983 1325; (b) C. E. Briant T. S. A. Hor N. D. Howells and D. M. P. Mingos J. Chem. Soc. Chem. Commun. 1983 1118; (c) C. E. Briant D. I. Gilmour M. A. Luke and D. M. P. Mingos J. Chem. Soc. Dalton Trans. 1985 851; (d) D. I. Gilmour M. A. Luke and D. M. P. Mingos J. Chem. Soc. Dalton Trans. 1987 335; (e) C. E. Briant T. S. A. Hor N. D. Howells and D. M. P. Mingos J. Organomet. Chem. 1983 256 C15; ( f ) W. Bos J. J. Bour P. P. J. Schlebos P. Hageman, J.Chem. Soc. Dalton Trans. 1999 639–651 651 W. P. Bosman J. M. M. Smits J. A. C. van Wietmarschen and P. T. Beurskens Inorg. Chim. Acta 1986 119 141. 21 (a) M. Zhou Y. Xu L.-L. Koh K. F. Mok P.-H. Leung and T. S. A. Hor Inorg. Chem. 1993 32 1875; (b) M. Zhou Y. Xu C.-F. Lam L.-L. Koh K. F. Mok P.-H. Leung and T. S. A. Hor Inorg. Chem. 1993 32 4660; (c) M. Zhou Y. Xu C.-F. Lam P.-H. Leung L.-L. Koh K. F. Mok and T. S. A. Hor Inorg. Chem. 1994 33 1572; (d ) M. S. Zhou A. L. Tan Y. Xu C.-F. Lam P.-H. Leung K. F. Mok L.-L. Koh and T. S. A. Hor Polyhedron 1997 16 2381; (e) M. Zhou C. F. Lam K. F. Mok P.-H. Leung and T. S. A. Hor J. Organomet. Chem. 1994 476 C32; ( f ) M. Zhou Y. Xu A.-M. Tan P.-H. Leung K. F. Mok L.-L. Koh and T. S. A. Hor Inorg. Chem. 1995 34 6425. 22 C. H.Chin and T. S. A. Hor J. Organomet. Chem. 1996 509 101. 23 H. Liu A. L. Tan K. F. Mok T. C. W. Mak A. S. Bastanov J. A. K. Howard and T. S. A. Hor J. Am. Chem. Soc. 1997 119 11006. 24 (a) G. Li S. Li A. L. Tan W.-H. Yip T. C. W. Mak and T. S. A. Hor J. Chem. Soc. Dalton Trans. 1996 4315; (b) J. S. L. Yeo G. Li W.-H. Yip W. Henderson T. C. W. Mak and T. S. A. Hor J. Chem. Soc. Dalton Trans. in press. 25 (a) R. D. Adams T. S. A. Hor and P. Mathur Organometallics 1984 3 634; (b) R. D. Adams and T. S. A. Hor Organometallics 1984 3 1915; (c) R. D. Adams and T. S. A. Hor Inorg. Chem. 1984 23 4723; (d) R. D. Adams T. S. A. Hor and I. Horváth Inorg. Chem. 1984 23 4733; (e) R. D. Adams and S. Wang Inorg. Chem. 1985 24 4447; ( f ) R. D. Adams J. E. Babin R. Mahtab and S. Wang Inorg. Chem. 1986 25 4; ( g) R.D. Adams I. Horváth and S. Wang Inorg. Chem. 1986 25 1617. 26 A. L. Tan M. L. Chiew and T. S. A. Hor J. Mol. Struct. (THEOCHEM) 1997 393 189. 27 (a) H. Liu A. L. Tan K. F. Mok and T. S. A. Hor J. Chem. Soc. Dalton Trans. 1996 4023; (b) Z. Li H. Liu K. F. Mok and T. S. A. Hor J. Organomet. Chem. 1998 in press; (c) H. Liu A. L. Tan Y. Xu K. F. Mok and T. S. A. Hor Polyhedron 1997 16 377; (d ) M. Capdevila Y. Carrasco W. Clegg R. A. Coxall P. González- Duarte A. Lledós J. Sola and G. Ujaque Chem. Commun. 1998 597; (e) H. Liu A. L. Tan C. R. Cheng K. F. Mok and T. S. A. Hor Inorg. Chem. 1997 36 2916. 28 (a) M. Capdevila P. González-Duarte C. Foces-Foces F. Hernández-Cano and M. Martínez-Ripoll J. Chem. Soc. Dalton Trans. 1990 143; (b) M. Capdevila W. Clegg P. González-Duarte B.Harris I. Mira J. Sola and I. C. Taylor J. Chem. Soc. Dalton Trans. 1992 2817; (c) M. Capdevila W. Clegg P. González-Duarte and I. Mira J. Chem. Soc. Dalton Trans. 1992 173. 29 M. Capdevila W. Clegg P. González-Duarte A. Jarid and A. Lledós Inorg. Chem. 1996 35 490. 30 T. S. A. Hor J. Cluster Sci. 1996 7 263. 31 K. A. Mitchell K. C. Streveler and C. M. Jensen Inorg. Chem. 1993 32 2608. 32 V. W.-W. Yam P. K.-Y. Yeung and K.-K. Cheung J. Chem. Soc. Chem. Commun. 1995 267. 33 T. S. A. Hor unpublished work. 34 A. Shaver R. D. Lai P. H. Bird and W. Wickramasinghe Can. J. Chem. 1985 63 2555. 35 W. Zheng H. Liu K. F. Mok and T. S. A. Hor unpublished work; W. Zheng B.Sc.(Hons) Thesis National University of Singapore 1997. 36 V. W.-W. Yam P. K.-Y. Yeung and K.-K. Cheung Angew. Chem.Int. Ed. Engl. 1996 35 739. 37 (a) R. D. Adams I. Horváth and L. W. Yang J. Am. Chem. Soc. 1983 105 1533; (b) R. D. Adams I. Horváth and K. Natarajan Organometallics 1984 3 623; (c) H. Vahrenkamp and E. J. Wucherer Angew. Chem. Int. Ed. Engl. 1981 20 680. 38 (a) F. Richter and H. Vahrenkamp Angew. Chem. Int. Ed. Engl. 1978 17 444; (b) R. D. Adams M. P. Pompeo and W. Wu Inorg. Chem. 1991 30 2899; (c) S. Kuwata Y. Mizobe and M. Hidai J. Am. Chem. Soc. 1993 115 8499. 39 S.-W. A. Fong and T. S. A. Hor unpublished work. 40 D. B. Dell’Amico F. Calderazzo N. Pasqualetti R. Hübener C. Maichle-Mössmer and J. Strähle J. Chem. Soc. Dalton Trans. 1995 3917. 41 M. Dinger Ph.D. Thesis University of Waikato 1998. 42 K.-S. Gan and T. S. A. Hor in Ferrocenes–Homogeneous Catalysis Organic Synthesis Materials Science eds.A. Togni and T. Hayashi VCH Weinheim 1995 ch. 1 p. 3. 43 S.-W. A. Fong and T. S. A. Hor J. Cluster Sci. 1989 9 351. 44 P. Zanello in Ferrocenes–Homogeneous Catalysis Organic Synthesis Materials Science eds. A. Togni and T. Hayashi VCH Weinheim 1995 ch. 7 p. 317; S. Onaka M.-A. Haga S. Takagi M. Otsuka and K. Mizuno Bull. Chem. Soc. Jpn. 1994 67 2440; W. H. Watson A. Nagl S. Hwang and S. Richmond J. Organomet. Chem. 1993 445 163; G. Pilloni and B. Longato Inorg. Chim. Acta 1993 208 17; F. Estevan P. Lahuerta J. Latorre E. Peris S. Garcia- Granda F. Gomez-Beltran A. Aguirre and M. A. Salvado J. Chem. Soc. Dalton Trans. 1993 1681. 45 (a) S. Datta and U. C. Agarwala Indian J. Chem. Sect. A 1981 20 1190; (b) D. Fenske in Clusters and colloids from theory to applications ed.G. Schmid VCH Weinheim 1994 ch. 3.4 p. 212; (c) H. Werner W. BertleV and U. Schubert Inorg. Chim. Acta 1980 43 199; (d ) C. A. Ghilardi S. Midollini F. Nuzzi and A. Orlini Transition Met. Chem. 1983 8 73; (e) B. Wu W.-J. Zhang S.-Y. Yu T.-L. Sheng and X.-T. Wu J. Organomet. Chem. 1997 546 587. 46 (a) E. W. Abel D. G. Evans J. R. Koe M. B. Hursthouse M. Mazid M. F. Mahon and K. C. Molloy J. Chem. Soc. Dalton Trans. 1990 1697; (b) V. K. Jain S. Kannan R. J. Butcher and J. P. Jasinski J. Organomet. Chem. 1994 468 285; (c) T. G. Appleton J. W. Connor J. R. Hall and P. D. Prenzler Inorg. Chem. 1989 28 2030. 47 G. Raper and W. S. McDonald J. Chem. Soc. Dalton Trans. 1972 265. 48 G. Aullón G. Ujaque A. Lledós S. Alvarez and P. Alemany Inorg. Chem. 1998 37 804. 49 (a) R. D. Gillard and F.L. Wimmer J. Chem. Soc. Chem. Commun. 1978 936; (b) P. E. Jones and L. Katz (i) Chem. Commun. 1967 842; (ii) Acta Crystallogr. Sect. B 1972 28 3438; (c) R. A. Krause A. Wickenden-Kozlowski and J. L. Cronin Inorg. Synth. 1982 21 12; (d ) M. Schmidt and G. G. HoVmann (i) Z. Naturforsch. Teil B 1979 34 451; (ii) Z. Anorg. Allg. Chem. 1980 464 209. 50 (a) M. C. Baird and G. Wilkinson (i) Chem. Commun. 1966 514 (ii) J. Chem. Soc. A 1967 865; (b) A. C. Skapski and P. G. H. Troughton (i) J. Chem. Soc. A 1969 2772 (ii) Chem. Commun. 1969 170; (c) C. T. Hunt G. B. Matson and A. L. Balch Inorg. Chem. 1981 20 2270; (d) M. F. Hallam M. A. Luke D. M. P. Mingos and I. D. Williams J. Organomet. Chem. 1987 325 271; (e) M. P. Brown J. R. Fisher R. J. Puddephatt and K. R. Seddon Inorg. Chem. 1979 18 2808. 51 (a) B. Kreutzer P. Kreutzer and W. Beck Z. Naturforsch. Teil B 1972 27 461; (b) D. Dudis and J. P. Fackler Inorg. Chem. 1982 21 3577. 52 R. H. Fenn and G. R. Segrott (a) J. Chem. Soc. A 1970 3197; (b) J. Chem. Soc. Dalton Trans. 1972 330. 53 E. M. Pakilla J. A. Golen P. N. Richmann and C. M. Jensen Polyhedron 1991 10 1343. Paper 8/06246C