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The salts [ML2]2[{M′(mnt)2}2] {M = Co, L = Cp; M = Fe or Co, L = Cp* (Cp* = η-C5Me5); M′ = Fe or Co; mnt = [S2C2(CN)2]2−} have been prepared and structurally characterised to investigate the effects of cation size, metal d-electron configuration and the packing of pseudo-spherical cations with ellipsoidal dianions. By comparing the structures of [CoCp2]2[{M′(mnt)2}2] (M′ = Fe 1 or Co 2) with those of [FeCp*2]2[{M′(mnt)2}2] (M′ = Fe 3 or Co 4) the effect of cation size and magnetic character on the crystal lattice is investigated. Two polymorphs of 3 were identified, α-3 and β-3, both of which crystallise in the space group P21/n. The structures of 1, 2, and α-3 are similar with columns of anion dimers separated by cations lying perpendicular to the anion stacks, whereas β-3 comprises mixed, face-to-face ⋯cation⋯cation⋯anion⋯ stacks. Two forms of 4 were characterised, namely the polymorph α-4, which is isostructural with α-3, and the dichloromethane-solvated pseudo-polymorph 4S2. The structure of 4S2 is analogous to that of β-3 in comprising face-to-face stacks of mixed ions. Three related structures are known, namely [FeCp*2]2[{Co(mnt)2}2] [denoted γ-4 (CSD refcode ZAXJAJ)] and the two pseudo-polymorphs [FeCp*2]2[{M′(mnt)2}2]·MeCN [M′ = Fe 3S1 (ZAXJEN) or Co 4S1 (ZAXHUB)]. These are included for comparison with the structures determined here. The packing coefficients, k, varied in the range 0.59(3) (for 4S2) to 0.73(1) (for β-3).

Treatment of [Rh2(µ-CO)(bipy)(dppe)(µ-N1,N3-RNNNR)2]2+12+[bipy = 2,2′-bipyridyl, dppe = 1,2-bis(diphenylphosphino)ethane, R =p-tolyl] with a catalytic amount of [NBun4][BH4] gave [Rh2(µ-CO)-(bipy)(dppe)(µ-N1,3,N3-RNNNR)(µ-N1,N3-RNNNR)]2+32+ X-ray studies on which show one ‘normal’ bridging triazenide ligand and one in which one of the terminal N atoms binds one Rh atom while the second bonds to both metal atoms; the triazenide ligands of 32+ are cis-bound to one metal centre and trans-bound to the second.

The reaction of [Rh(η4-cot)(η-C5H5)](cot = cyclo-octatetraene) with [Fe{P(OMe)3}(NO)2-(η3-CH2CHCH2)][PF6] gives [Rh(η2,η3-C8H9)(η-C5H4R)][PF6](R = CH2CHCH2)via allyl addition to the cyclopentadienyl ring and proton migration to the co-ordinated cot.

The redox-active cyanomanganese ligands trans-[Mn(CN)(CO)2{P(OEt)3}(dppm)](dppm = Ph2PCH2PPh2), cis-[Mn(CN)(CO)2(PEt3)(dppe)](dppe = Ph2PCH2CH2PPh2) and trans-[Mn(CN)(CO)(dppm)2] reacted with [{Fe(µ-I)(NO)2}2] in CH2Cl2 to give the heterobinuclear complexes [FeI{(µ-NC)MnLx}(NO)2]{Lx=trans-(CO)2[P(OEt)3](dppm)1, cis-(CO)2(PEt3)(dppe)2 and trans-(CO)(dppm)23}; the molecular structure of 3 is consistent with a tetrahedral iron(–I) centre bound to octahedral manganese(I) by a near linear Mn–CN–Fe bridge. The ESR spectra of complexes 1–3 are very similar to those of the tetrahedral Fe–1 complex [FeI2(NO)2]–. Complexes 1–3 reacted with PPh3 in the presence of TIPF6 to give [Fe(PPh3){(µ-NC)MnLx}(NO)2][PF6]{Lx=trans-(CO)2[P(OEt)3](dppm)5+, cis-(CO)2(PEt3)(dppe)6+ and trans-(CO)(dppm)27+} which were also prepared from [Fe(PPh3)2(NO)2][PF6] and the appropriate cyanomanganese ligand in a 1 : 1 ratio; the related cation [Fe(PPh3){(µ-NC)MnLx}(NO)2]+{Lx=trans-(CO)2[P(OPh)3](dppm)8+} was generated in solution. The ESR spectra of complexes 6+–8+ showed hyperfine coupling to N(O), P, N(C) and Mn, suggesting a structure similar to that of [Fe(PPh3)(OPPh3)(NO)2]+ with some delocalisation through the cyanide bridge to manganese. Treatment of 2 equivalents of trans-[Mn(CN)(CO)2{P(OEt)3}(dppm)] or cis-[Mn(CN)(CO)2(PEt3)(dppe)] with [Fe(PPh3)2(NO)2][PF6] gave the heterotrinuclear complexes [Fe{(µ-NC)MnLx}2(NO)2][PF6]{Lx=trans-(CO)2[P(OEt)3](dppm)9+ and cis-(CO)2(PEt3)(dppe)10+} which may also be prepared from the reaction of 1 with trans-[Mn(CN)(CO)2{P(OEt)3(dppm)] or 2 with cis-[Mn(CN)(CO)2(PEt3)(dppe)] in the presence of TIPF6. Complexes 1–10+, which contain diamagnetic MnI and paramagnetic Fe–I centres, undergo oxidation and reduction at a platinum electrode in CH2Cl2. The MnII derivatives 3+ and 72+ and the Fe–II complex 7 have been generated in solution by ferrocenium ion oxidation or cobaltocene reduction respectively.

The complex [Rh2(CO)4(µ-RN3R)2]1 reacted with the cyanomanganese ligands cis-[Mn(CN)(CO)2(L–L)][L–L = Ph2PCH2PPh2(dppm), L = P(OEt)32 or P(OPh)33; L–L = Ph2PCH2CH2PPh2(dppe), L = PEt34] in n-hexane under reflux, or with trans-[Mn(CN)(CO)2L(L–L)][L–L = dppm, L = P(OEt)38 or P(OPh)39] or [Mn(CN)(NO)L(cp′)][L = P(OPh)312 or PPh313, cp′=η-C5H4Me], in CH2Cl2 in the presence of ONMe3, giving [Rh2{(µ-NC)MnLx}(CO)3(µ-RN3R)2]{Lx=cis-(CO)2[P(OEt)3](dppm)5, cis-(CO)2[P(OPh)3](dppm)6, cis-(CO)2(PEt3)(dppe)7, trans-(CO)2[P(OEt)3](dppm)10, trans-(CO)2[P(OPh)3](dppm)11, (NO)[P(OPh)3](cp′)14 or (NO)(PPh3)(cp′)15}. The tricarbonyls underwent one-electron oxidation at a platinum-disc electrode and reacted with [Fe(cp)2][PF6](cp =η-C5H5) in CH2Cl2 to give the corresponding paramagnetic complexes 5+–7+, 10+, 11+, 14+ and 15+ which contain [Rh2]3+ cores. The addition of compounds 2 or 8 to 5+ or 10+, or of 3 or 9 to 6+ or 11+, resulted in further carbonyl substitution to give the dicarbonyl complexes [Rh2{(µ-NC)MnLx}2(CO)2(µ-RN3R)2]+16+–21+ which reacted with [Fe(cp)2][PF6] in the presence of chloride ion to give the diamagnetic [Rh2]4+-containing cations [Rh2Cl{(µ-NC)MnLx}2(CO)2(µ-RN3R)2]+22+–24+. The ESR spectra of the paramagnetic [Rh2]3+-containing cations, and the effects of varying the cyanomanganese ligand on the order in which the redox-active sites (Rh2 and Mn) of the tri- and tetra-metallic complexes are oxidised, are discussed.

The complexes fec-[Mn(CO)3L(bipy)]+[L = CNBut or P(OMe)3, bipy = 2,2′-bipyridyl], cis,trans-and cis,cis-[Mn(CO)2L(L′)(bipy)]+[L = L′= CNBut, P(OEt)3, P(OPh)3 or PPh3; L = CNBut, L′= P(OMe)3], fac-[Mn(CO)(CNBut)3(bipy)]+ and mer-[Mn(CO)L3(bipy)]+[L = CNBut or P(OR)3, R = Me or Et] underwent one-electron oxidation at a platinum-disc electrode in CH2Cl2 and/or one-electron reduction in tetrahydrofuran (thf). The complexes mer-[Mn(CO)L3(bipy)]+[L = P(OEt)3 or CNBut] were oxidised by [N2C6H4F-p][PF6] to give dications with ESR spectra characteristic of low-spin d5 transition-metal complexes. The ESR spectroscopic studies of the sodium amalgam reduction of mer- and fac-[Mn(CO)(CNBut)3(bipy)]+, cis,trans-and cis,cis-[Mn(CO)2(CNBut)2(bipy)]+, mer-[Mn(CO){P-(OMe)3}(bipy)]+, or cis,trans-[Mn (CO)2{P(OMe)3}2(bipy)]+ in thf provide evidence for the intermediacy of neutral bipyridyl ligand-based radicals in the reductive isomerisation of the cis,cis to the cis,trans, and of the fac to the mer, cations. Extended-Huckel molecular-orbital calculations on the mer and fac isomers of [Mn(CO)L3(bipy)]+ and the cis,trans and cis,cis isomers of [Mn(CO)2L2(bipy)]+(L = CNH or PH3) provide support for the formation of metal- and ligand-based radicals on oxidation and reduction respectively.

The iodide-abstraction reaction of [{Rh2(µ-I)(CO)(bipy)(µ-RNNNR)2}2][PF6]21(bipy = 2,2′-bipyridyl, R =p-tolyl) with AgPF6 in MeCN gave [Rh2(CO)(NCMe)2(bipy)(µ-RNNNR)2][PF6]22 which slowly decarbonylated at room temperature to [Rh2(NCMe)3(bipy)(µ-RNNNR)2][PF6]23. The crystal structure of 3 shows a Rh–Rh single bond [2.534(2)A] and one of the three terminal nitrile ligands axially attached to the bipy-bound rhodium atom. Complex 3 reacts with Na[S2CNMe2]·2H2O to give [Rh2(NCMe)(S2CNMe2)(bipy)(µ-RNNNR)2][PF6]4. In CH2Cl2, the iodide-abstraction reaction of 1 affords a green solution containing a carbonyl complex [Rh2(CO)(solv)2(bipy)(µ-RNNNR)2]2+A(solv = CH2Cl2), an analogue of 2. Complex A(solv = CH2Cl2) reacted with neutral chelating ligands to give the carbonyl-bridged complexes [Rh2(µ-CO)(L–L)(bipy)(µ-RNNNR)2][PF6]2[L–L = bipy 5, 4,4′-dimethyi-2,2′-bipyridyl (dmbipy)6, 1,10-phenanthroline (phen)7, di-2-pyridylamine (dpa)8, or Ph2PCH2CH2PPh2(dppe)9] the last of which undergoes reduction with NaBH4 to give paramagnetic [Rh2(CO)(dppe-P)(bipy)(µ-RNNNR)2][PF6]11 having a face-to-face structure with a monodentate dppe ligand. With Ph2PCH2PPh2(dppm), A(Solv = CH2Cl2) afforded [Rh2(µ-CO)(dppm)(bipy)(µ-RNNNR)2][PF6]210a X-ray studies on which, as a CH2Cl2 solvate, reveal a carbonyl-bridged open-book structure with a Rh–Rh distance of 3.179(2)A and a large Rh–C(O)–Rh angle of 108.3°. Complex 10a equilibrates with the face-to-face, terminal carbonyl isomer [Rh2(CO)(dppm)(bipy)(µ-RNNNR)2][PF6]210b in solution. The electronic structures of the two isomers have been probed by extended-Huckel molecular-orbital calculations on the model compound [Rh2H4(CO)5]. These show (i) the absence of metal–metal bonding in 10a while 10b has a Rh–Rh σ bond, and (ii) that the CO in 10a is best viewed as more ketonic than a typical bridging carbonyl. The reaction of complex A(solv = CH2Cl2) with neutral chelating ligands also gives low yields of [{Rh2(CO)(O2PF2)(µ-O2PF2)(bipy)(µ-RNNNR)2}2]12 the crystal structure of which shows two dirhodium fragments [Rh–Rh 2.505(4)A] linked by two O2PF2 groups bridging across axial and equatorial sites in different [Rh2]4+ moieties. The reaction of A(solv = CH2Cl2) with N–SH ligands yields [Rh2(CO)(N–S)(bipy)(µ-RNNNR)2][PF6](N–S = 1-methyl-2-sulfanylimidazolate 13, 2-sulfanylpyrimidinate 14, 2-sulfanylthiazolinate 15, or 2-sulfanylbenzimidazolate 16), and NaX gives [Rh2(CO)X2(bipy)(µ-RNNNR)2](X = Cl or NO2).

The reactions of trans-[Mn(CN)(CO)(dppm)2](dppm = Ph2PCH2PPh2) and cis- or trans-[Mn(CN)(CO)2(PR3)(L–L)][R = OEt or OPh, L–L = dppm; R = Et, L–L = dppe (Ph2PCH2CH2PPh2)] with [{Rh(µ-Cl)(CO)2}2] give the heterobinuclear complexes [trans-(dppm)2(OC)Mn(µ-CN)Rh(CO)2Cl] and [(L–L)(R3P)(OC)2Mn(µ-CN)Rh(CO)2Cl] respectively. Cyclic voltammetry shows that each complex undergoes oxidation at the manganese centre; chemical oxidation of [trans-(dppm)2(OC)Mn(µ-CN)Rh(CO)2Cl] gives [trans-(dppm)2(OC)Mn(µ-CN)Rh(CO)2Cl]+ which may also be prepared from trans-[Mn(CN)(CO)(dppm)2]+ and [{Rh(µ-Cl)(CO)2}2]. The crystal structures of the redox pair [trans-(dppm)2(OC)Mn(µ-CN)Rh(CO)2Cl]z(Z= 0 or +1) show that substantial changes in geometry resulting from oxidation are limited to the vicinity of the manganese atoms (e.g. mean Mn–P increases from 2.284 to 2.352 A). These changes are similar to those observed for the free cyanomanganese complexes trans-[Mn(CN)(CO)(dppm)2]z(Z= 0 or +1) and indicate that the singly occupied molecular orbital in the radical cation is largely composed of the Mn dπ orbital in the MnP4 plane. Changes in geometry in the Mn(µ-CN)Rh(CO)2Cl unit are very small. Treatment of [cis-(L–L)(R3P)(OC)2Mn(µ-CN)Rh(CO)2Cl] or [trans-(L–L)(R3P)(OC)2Mn(µ-CN)Rh(CO)2Cl] with TIPF6 in the presence of [Mn(CN)(CO)2(PR3)(L–L)] gives the heterotrinuclear cations [{(L–L)(R3P)(OC)2Mn(µ-CN)}2Rh(CO)2]+; the crystal structure of one, [{trans-(dppm)[(EtO)3P](OC)2Mn(µ-CN)}2Rh(CO)2], shows the two cyanomanganese ligands cis-co-ordinated to the rhodium centre. Cyclic, differential-pulse, and square-wave voltammetry of [{trans-(dppm)(R3P)(OC)2Mn(µ-CN)}2Rh(CO)2]+(R = OEt or OPh) show two closely spaced oxidation waves; the small separation (ca. 80–90 mV) suggests weak interaction between the two cyanomanganese ligands in the mixed-valence dication [{trans-(dppm)(R3P)(OC)2Mn(µ-CN)}2Rh(CO)2]2+(R = OEt or OPh). The results provide evidence for the dependence of spin delocalisation from MnII to RhI on the geometry of the ancillary ligands bound to manganese.

The complexes [Fe(CCR)(L–L)(η-C5R′5)][R = But, CO2Me, CO2Et, SiMe3, Ph or CH2OMe, L–L = Ph2PCH2PPh2(dppm), R′= H; R = But or Ph, L–L = Ph2PCH2CH2PPh2(dppe), R′= Me] undergo reversible one-electron oxidation at a platinum electrode in CH2Cl2. Chemical oxidation with [Fe(η-C5H5)2][PF6] gave the isolable salts [Fe(CCR)(dppe)(η-C5Me5)][PF6](R = But or Ph) which Mossbauer spectroscopy suggests to be complexes of FeIII. The ESR spectra of these salts, and of the cations [Fe(CCR)(dppm)(η-C5H5)]+(R = But, CO2Me, CO2Et or Ph), generated by in situ oxidation with [Fe(η-C5H5)2][PF6], are similar to those of low-spin d5 complexes of CrI, MnII and FeIII.

X-Ray structural studies on the redox pair trans-[Mn(CN)(CO)(dppm)2](dppm = Ph2PCH2PPh2) and trans-[Mn(CN)(CO)(dppm)2][PF6]·CH2Cl2 showed that one-electron oxidation results in changes consistent with depopulation of an orbital involved in Mn–P π-back bonding. The ESR spectra of trans-[Mn(CN)(CO)(dppm)2]+, [Mn(CO)(CNCH2CHCH2)(dppm)2]2+, trans-[Mn(CN)(CO)2(PEt3)(dppe)]+(dppe = Ph2CH2CH2PPh2) and trans-[MnBr(CO)2(PEt3)(dppe)]+ in frozen dichloromethane–dichloroethane (1:1) solutions at 90 K, and extended-Huckel molecular-orbital calculations on the model compounds [Mn(CN)(CO)(H2PCH2PH2)2]+, [Mn(CO)(CNMe)(H2PCH2PH2)2]2+, cis- and trans-[Mn(CN)(CO)2(PH3)3]+ and trans-[MnBr(CO)2(PH3)3]+, showed that the semi-occupied molecular orbital of these low-spin octahedral cyanomanganese(II) carbonyls is always primarily manganese dπ in character and in the plane of the phosphorus ligands, either aligned along the Mn(CN) axis as in trans-[Mn(CO)2(PEt3)(dppe)]+ or perpendicular to this axis as in trans-[Mn(CN)(CO)(dppm)2]. The relative arrangement of the cyanide and carbonyl ligands is shown to control the extent of spin delocalisation onto the cyanide ligand.

The complexes [Co2(CO)2(µ-RC2R)(µ-dppm)2]1(R = Me, Ph or CO2Me; dppm = Ph2PCH2PPh2) undergo reversible one-electron oxidation at a platinum-bead electrode in CH2Cl2; chemical oxidation of 1 with the ferrocenium ion or with iodine gives stable salts of the monocations 1+. The X-ray structures of 1(R = Me and Ph) show the alkyne bridge to be orthogonal to a cobalt–cobalt bond which is also bridged by the two dppm ligands. The overall structure of 1+(R = Me), as its [PF6]– salt, is similar but with the alkyne bridge rotated by 12° about an axis perpendicular to the shortened metal–metal bond. The structural changes observed provide direct experimental support for previous bonding studies of the structural preferences of hydrocarbon-bridged M2(CO)6 complexes.

Cyclopropenylium ions [C3R′3]+(R′= Ph or But) react with [Co(η4-cot)(η-C5R5)](R = H or Me, cot = cyclooctatetraene) to give [M{η5-C8H8(C3R′3)}(η-C5R5)]+3(M = Co; R = H, R′= Ph; R = Me, R′= Ph or But); X-ray structural studies on 3(M = Co, R = H, R′= Ph) show an exo-cyclopropene substituent on the cyclooctatrienyl ring. The reaction of [C3Ph3]+ with [Rh(η4-cot)(η-C5Me5)] gives a similar product 3(M = Rh, R = Me, R′= Ph) which rearranges to an unknown isomer 5 whereas [Rh(η4-cot)(η-C5H5)] undergoes electrophilic substitution at the cyclopentadienyl ring to give [Rh(η2,η3-C8H9){η-C5H4(C3Ph3)}]+4. The addition of [C7H7]+ to [Co(η4-cot)(η-C5R5)](R = H or Me) gives [Co{η5-C8H8(C7H7)}(η-C5R5)]+6(R = H or Me) which, in the case of 6(R = Me), rearranges to the 1,2,3,3a-tetrahydropentalenyl complex [Co{η5-C8H8(C7H7)}(η-C5Me5)]+7. Finally, the reactions between [Rh(η4-cot)(η-C5R5)](R = H or Me) and [C7H7]+ give [Rh(η2,η3-C8H9){η-C5H4(C7H7)}]+8 and [Rh{η2,η3-C8H8(C7H7)}(η-C5Me5)]+9 respectively.

The X-ray crystal structures of the salts [Pt(CNMe)4][Pd(mnt)2], [Pt(CNMe)4][Pd(mnt)2]2·2MeCN and [Pt(CNMe)4][Au(mnt)2]2·MeCN [mnt = 1,2-S2C2(CN)2] show a variety of stacking and layer motifs in the solid state; inter-complex interactions lead to low-dimensional molecular aggregates.

Planar complexes [M(mnt)(CNMe)2] of the nickel triad metals show two different stacking motifs in the solid state, characterised by X-ray crystal structure analysis and EXAFS spectroscopy, in which inter-complex interactions lead to pseudo-one-dimensional molecular aggregates.

The reaction of [Mn(CN)(CO)2L(L–L)] with [MnBr(CO)2L′(L′–L′)] in the presence of TlPF6 gives [(L–L)L(OC)2Mn(µ-CN)Mn(CO)2L′(L′–L′)][PF6] which may have trans,trans; cis,trans; trans,cis or cis,cis geometries depending on the structures of the mononuclear precursors. The trans,trans complexes undergo two reversible one-electron oxidations, the first at the N-bonded Mn(CO)2L′(L′–L′) site to give dications which IR carbonyl and ESR spectroscopy and cyclic voltammetry suggest to be localised mixed-valence species. The oxidation of the cis,trans and trans,cis monocations occurs first at the trans site and then at the cis site. Where the difference in potential, Δ, between the two redox processes (which depend on the ligands L, L–L, L′ and L′–L′) is large the first oxidation occurs with retention of geometry; subsequent oxidation at the cis site results in isomerisation and the formation of the trans,trans trications. Where Δ is small, oxidation of the trans site is accompanied by isomerisation at the cis centre implying some delocalisation in the mixed-valence dication. The oxidative-isomerisation process therefore provides a probe of the intermetallic interactions in the Mn2(µ-CN) core.

The cyanomanganese complexes trans-[Mn(CN)(CO)(dppm)2]z(z= 0, 1+)(dppm = Ph2PCH2PPh2), cis-[Mn(CN)(CO)2L(L–L)], and trans-[Mn(CN)(CO)2L(L–L)]z(z= 0, 1+) have been used as ligands in the synthesis of redox-active heterobinuclear complexes such as [L(L–L)(CO)2Mn(µ-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)]z(z= 0, 1+) and [(dppm)2(CO)Mn(µ-CN)RhCl(CO)2]z(z= 0, 1+).

Cyclic-voltammetric studies in CH2Cl2 and MeCN have shown that [V(η-C5H5)(CO)3L]{1; L = PPh3, PMePh2, PEt3, P(NMe2)3, P[(OCH2)3CMe], and P(OPh)3} and [V(η-C5H5)(CO)2(dppe)](2; dppe = Ph2PCH2CH3PPh2) are oxidised at a platinum electrode to [V(η-C5H5)(CO)3L]+(3) and [V(η-C5H5)(CO)2(dppe)]+(4) respectively. For L = PPh3,PMePh2,PEt3,and P(NMe2)3, andfor(2), this processis reversible, but for L = P(OPh)3 or P[(OCH2)2-CMe] it is irreversible at all the scan rates used. Complexes (3; L = PPh3 or PMePh2) and (4) may be generated in CH2CL2 by treating (1) or (2) with [p-FC6H4N2][BF4]. and may be isolated, as their [PF6]– salts, by treating (1; L = PPh3) or (2) with [NO][PF6] in mixtures of methanol and toluene. The e.s.r. spectra of the paramagnetic cations are reported.

The complexes [Fe(CO)3 –nLn(η4-cot)][1: cot = cyclo-octatetraene; n= 0–3, L = P(OMe)3; n= 1, L = P(NMe2)3, PPh3, P(OCH2)3CMe, or P(OPh)3] undergo chemically irreversible one-electron oxidation in CH2Cl2 at a platinum electrode. Chemical generation of the highly reactive radical cation [Fe(CO)2L(C8H8)]+[2; L = CO or P(OPh)3], by oxidation of (1) with silver(I) salts or [N(C6H4Br-p)3][PF6], is followed by isomerisation and dimerisation, via C–C bond formation, to give [Fe2(CO)6 –nLn(η5:η′5-C16H16)]2+[3; n= 0 or 2, L = P(OPh)3]. The crystal structure of [3; n= 2, L = P(OPh)3], as the [PF6]– salt, reveals the presence of a dimeric C16H16 unit comprising two fused ring systems bonded to one another across a two-fold crystallographic axis of symmetry. The five unbridged carbon atoms of the C7 ring are coplanar and η5-bonded to the iron atom, which in turn is orthogonally co-ordinated to the two CO ligands and the P(OPh)3 group. The C7 ring folds away from the metal atom, and there are further folds at the junction with the C3 ring and again at the apex of the C3 ring, all in the same sense. The dimeric ligand thus has an overall S shape when viewed down the two-fold axis. Along the Fe ⋯ Fe vector, by contrast, the C3 rings are seen edgewise and the two C7 rings are almost eclipsed. Complex (3; n= 0) reacts with Na[BH4] to give [Fe2(CO)6(η4:η′4-C16H18)](4). With halide ion, however, (3; n= 0) affords [Fe(CO)3(η4-C16H16)](5)via reductive C–C coupling. X-Ray analysis shows that in complex (5) a further link between the two halves of the dimer has been formed, in that the fifth C atom of the C7 ring which in (3) was bonded to the metal atom is now bonded to its counterpart in the other half of the dimer, forming an additional central C6 ring. As might be expected, the C7 ring which is not co-ordinated to iron is more nearly planar and the C–C bonds are more localised. With [Fe2(CO)9], complex (5) yields [Fe2(CO)6(η4:η′4-C16H16)](6), but with NMe3O·2H2O in refluxing benzene, ring detachment results in the isolation of the free polycyclic hydrocarbon, C16H16(7).

Les complexes pentaphenylcyclopentadienyl («η 5 »-C 5 Ph 5 ) M(CO) 2 avec M=Co ou Rh, sont reduits par un electron en leurs radicaux anioniques correspondants. Les potentiels E° (vs ecs), mesures par voltammetrie cyclique, sont de −1,57 V pour M=Co et de −1,70 V pour M=Rh, dans des solutions de THF. Les etudes de spectroscopie RPE montrent que l'anion avec Co est moins stable que celui avec Rh

(C6Me6−nHn)Cr(CO)2 (acetylene), n = 0 and 1; acetylene = PhCCPh and p-MeOC6H4CCC6H4OMe, are reversibly oxidised to the corresponding monocations by NO+, Ag+ or I2. Electrochemical data for the oxidation process, and the ESR spectra of the resulting cations, are reported.

The coupling of [Ru(CO) 2 L(η 4 -cot)] (L = CO or PPh 3 , cot = cyclooctatetraene) with [Fe(CO) 3 (η 5 -cyclohexadienyl)] + or [Fe{P(OMe) 3 }(NO) 2 (η 3 -allyl)] + yields respectively the dimetallic species [Ru(CO) 2 L(η 2 ,η 3 -C 8 H 8 {Fe(CO) 3 (η 4 -C 6 H 7 )}] ( 3 ) and the allyl-substituted derivative [Ru(CO) 2 L(η 5 -C 8 H 8 CH 2 C(Me)CH 2 )][PF 6 ] ( 5 ) whose X-ray structure is reported; paramagnetic [Co(η-C 5 H 5 ) 2 ] and [Ru(CO) 3 (η 5 -cyclohexadienyl)] + give diamagnetic [Ru(CO) 3 (η 4 -C 6 H 7 C 5 H 6 ( o -C 5 H 5 )] ( 8 ) via CC bond formation and one-electron reduction.

The tetraphenylcyclobutadiene complexes [M(CO)3–n Ln(η4-C4Ph4)][1: M = Fe, n= 1, L = PPh3 or P(OMe)3; M = Fe, n= 2, L = P(OMe)3; M = Ru, n= 2 or 3, L = P (OMe)3] undergo reversible one-electron oxidation, at the platinum electrode in CH2Cl2, to give the paramagnetic radical cations [M(CO)3–nLn(C4Ph4)]+(2). Chemical oxidation of (1) with Ag[BF4] or [N(C6H4Br-p)3][PF6] in CH2 Cl2 affords salts of [2: M = Fe, n= 1, L = PPh3; M = Fe, n= 2, L = P(OMe)3]. Complexes (2) are implicated as intermediates in the formation of [Fe(CO)(NO){P(OMe)3}(η4-C4Ph4)]+ from [Fe (CO){P(OMe)3}2(η4-C4Ph4)] and Ag[NO3], of [Fe(CO){P(OMe)3}(η2-tcne)(η4-C4Ph4)](3) from tetracyanoethylene (tcne) and [Fe(CO){P(OMe)3}2(η4-C4Ph4)], and of [MX(CO)3–nLn(η4–C4Ph4)]+[4; X = Cl, Br, or I] from (1) and halogens, X2. The reaction of polyhalide salts of [RuX(CO){P(OMe)3}2(η4-C4Ph4)]+(4; X = Cl or Br) with PPh3 leads to cleavage of the metal–ring bond and formation of halogenotetraphenylcyclobutenyl radicals, C4Ph4X.

The reaction of [Co2(CO)8] or [{RhCl(CO)2}2] with C5Ph5Br and zinc dust in tetrahydrofuran (thf) gives [M(CO)2(η-C5Ph5)](1, M = Co or Rh); in the case of rhodium, the intermediate [RhBr(CO)2(η-C5Ph5)][ZnCl3] is isolable if the reaction is carried out in benzene. In the presence of [Fe(η-C5H5)2]+, (1, M = Rh) undergoes oxidatively-induced carbonyl substitution with ligands, L; for L = P(OPh)3, the formation of [M(CO)L(η-C5Ph5)][2, M = Rh, L = P(OPh)3] is catalytic in the oxidant. The complexes (2) are also prepared via(a) the substitution of [RhBr(CO)2(η-C5Ph5)][ZnCl3] with PPh3 or AsPh3 and subsequent reduction by [N(PPh3)2][Mn(CO)5], (b)[Co(η-C5H5)2] reduction of [MI2(CO)(η-C5Ph5)](3, M = Co) in the presence of P(OMe)3 or P(OPh)3, or (c)[Co(η-C5H5)2] reduction of [RhBr2{P(OMe)3}(η-C5Ph5)](5) in the presence of CO. Complexes (3, M = Co or Rh) are prepared from (1) and iodine in toluene–hexane at 0 °C; the rhodium complex (3, M = Rh) is reduced by [Co(η-C5H5)2] to give [{Rh(µ-CO)(η-C5Ph5)}2]. Compound (5) results from the addition of P(OMe)3 to [{RhBr2(η-C5Ph5)}2](4), which is the product of the reaction between (1, M = Rh) and bromine in CH2Cl2. The compounds (2) undergo two one-electron oxidations at a platinum electrode in CH2Cl2; the first is reversible and the second is irreversible. The monocations [2+, M = Rh; L = PPh3, P(OPh)3, or AsPh3] have been characterised in solution by i.r. and e.s.r. spectroscopy. Low-temperature e.s.r spectroscopy has shown that the radical cation [Rh(CO)(PPh3)(η-C5Me5)]+ is stable at –196 °C.

The reaction of [{Rh(CO)2(µ-RNNNR)}2](1; R =p-tolyl) with 2,2′-bipyridyl (bipy) in boiling n-heptane gives [Rh2(CO)2(bipy)(µ-RNNNR)2](2) which undergoes two one-electron oxidations at a platinum-bead electrode in CH2Cl2. Chemical oxidation of (2) with [Fe(η-C5H5)2]+ gives [Rh2(CO)2(bipy)(µ-RNNNR)2]+(2+) the X-ray structure of which, as a dichloromethane solvate of the [BF4]– salt, shows a Rh–Rh separation of 2.646(1)A consistent with a formal metal–metal bond order of 0.5. The cation (2+) undergoes carbonyl substitution reactions with PPh3 or P(OPh)3 to give [Rh2(CO)L(bipy)(µ-RNNNR)2][PF6](3+), and with iodide ion to yield [Rh2l(CO)(bipy)(µ-RNNNR)2](4) which may also be prepared directly from (2) and iodine at –78 °C. The cyclic voltammogram of (4) in CH2Cl2 shows one reduction and three oxidation waves; chemical oxidation of (4) with [Fe(η-C5H5)2][PF6] gives the tetranuclear complex [{Rh2l(CO)-(bipy)(µ-RNNNR)2}2][PF6]2(5). X-ray structural studies on the CH2Cl2 solvate of (5) show that the dication is centrosymmetric, having two binuclear (4+) units [Rh–Rh distance within each, 2.544(1)A] linked by asymmetric iodide bridges, Rh–laxial, 2.760(1)A and Rh–lequatorial 2.670(1)A. Complex (5) reacts with I– to give [Rh2l2(CO)(bipy)(µ-RNNNR)2](6), which may also be made directly from (2) and iodine, and with Na[S2CNMe2] to yield [Rh2(CO)(S2CNMe2)(bipy)-(µ-RNNNR)2][PF6](7). The complexes [3+; L = PPh3 or P(OPh)3] react with chloride ion in the presence of [Fe(η-C5H5)2][PF6] giving [Rh2Cl(CO)L(bipy)(µ-RNNNR)2][PF6][8; L = PPh3 or P(OPh)3]. Both (7) and (8) undergo one-electron oxidation and reduction at a platinum electrode. The mechanism of the oxidative addition of iodine to (2) to give (5) and the low-temperature e.s.r. spectra of the paramagnetic [Rh2]3+ complexes (2+), (3+), and (4) are discussed.

The reaction of [Rh(η-C5H5)(CO)2] with R1SSR1 affords [{Rh(µ-SR1)(η-C5H5)}2](R1= Ph or C6H4Me-p) as a mixture of syn and anti isomers. Each isomer is not fluxional but syn–anti isomerization, via a bridge-opening mechanism, occurs at room temperature. The isomerization process has been studied by 1H n.m.r. spectroscopy and by cyclic voltammetry which also reveals that both isomers undergo irreversible one-electron oxidation at the platinum electrode in CH2Cl2. The complex syn-[{Rh(µ-SC6H4Me-p)(η-C5H5)}2] reacts with di-p-tolyl disulphide, in the presence of [NO][PF6], to give [Rh2(µ-SC6H4Me-p)3(η-C5H5)2][PF6], and with [N(C6H4Br-p)3]-[SbCl6] to give [Rh2(µ-Cl)(η-SC6H4Me-p)2(η-C5H5)2][SbCl6], via one-electron oxidation followed by insertion into the metal-metal bond of the radical cation [{Rh(µ-SC6H4Me-p)(η-C5H5)}2]+.

The complexes trans-[MnL(CO)(dppm)2]z(1; z= 0, L = Br, CN, or NCS; z=+1, L = NCMe, CNMe, CNBut, or CO; dppm = Ph2PCH2PPh2) undergo diffusion-controlled, one-electron oxidation at a platinum electrode in CH2Cl2. The observed linear correlation between oxidation potential, E°, and ligand constant, PL, has allowed (i) a comparison of the electron-richness and polarisability of the binding site [Mn(CO)(dppm)2]+ with those of related square-pyramidal species, and (ii) comments to be made concerning the linkage isomerism of the thiocyanate ligand. All of the manganese(II) complexes trans-[MnL(CO)(dppm)2]z+1(2) except for the bromide (2; z= 0, L = Br) have been generated by chemical oxidation (with [NO][PF6] or [N2C6H4F-p][PF6]) or by controlled potential electrolysis. The paramagnetic compounds have been characterised by i.r. and e.s.r. spectroscopy, and (2; z=+1, L = CNMe or CNBut) have been isolated as crystalline salts of the [PF6]– anion. The irreversible oxidation of (1; z= 0, L = Br), either electrolytically or by [Fe(η-C5H5)2][PF6] in CH2Cl2, leads to the formation of (1; z=+1, L = CO), but in the presence of acetonitrile or CNBut the reaction gives (1; z=+1, L = NCMe or CNBut).

Cyclic voltammetric studies show that the complexes [Co(CO)L(η-C5H5)][1; L = P(C6H11)3 or PPh3] undergo one-electron oxidation to the radical cations [Co(CO)L(η-C5H5)]+. Chemical oxidation of (1) with [Fe(η-C5H5)2][PF6] gives [Co(CO){P(C6H11)3}(η-C5H5)][PF6](2) and the bis(phosphine) salt [Co(PPh3)2(η-C5H5)][PF6](3). The latter is also prepared directly from [Co(CO)2(η-C5H5)] and PPh3 in the presence of [Fe(η-C5H5)2][PF6], but [Co(CO)2(η-C5Me5)] gives [Co(CO)(PPh3)(η-C5Me5)][PF6](4). Complex (2) undergoes substitution reactions to give paramagnetic [CoL{P(C6H11)3}(η-C5H5)][PF6](L = PPh3 or pyridine) and diamagnetic [Co{P(OMe)3}3(η-C5H5)][PF6]2. With halogens, X2, with Me2NC(S)SSC(S)NMe2, and with NO gas, (2)–(4) undergo radical-coupling reactions to give [CoX(CO){P(C6H11)3}(η-C5H5)][PF6](X = Br or I), [CoL(S2CNMe2)(η-C5H5)][PF6][L = P(C6H11)3 or PPh3], and [Co(NO)(PPh3)(η-C5R5)][PF6](R = H or Me) respectively. With ortho-quinones, (2) gives [Co(O–O){P(C6H11)3}(η-C5H5)][PF6](5; O–O =o-chloranil, 1,2-naphthoquinone, or phenanthrenequinone) the e.s.r. spectra of which suggest the unpaired electron to be localised mainly on the quinone ligand.

The cyclohexadienyl complexes [Mn(CO)3–nLn(η5-C6H6Ph)][la, n= 1, L = PPh3; 1b, n= 2, L2= Ph2PCH2CH2PPh2(dppe)], prepared by U.V. photolysis of [Mn(CO)3(ηC6H6Ph)] and L, undergo reversible one-electron oxidation at a platinum electrode in CH2Cl2. The paramagnetic salt [Mn(CO)(dppe)(η-5C6H6Ph)][PF6](2) is isolable from the reaction between (1b) and [Fe(η-C5H5)2][PF6]. X-Ray structural studies on (1b) and (2)·0.5CH2Cl2 show the most pronounced effects of oxidation to be on the mean Mn–P distances which change from 2.221 A in (1b) to 2.338 A in (2). The effect on the conformation of the Mn(CO)(dppe) unit relative to the cyclohexadienyl moiety is small; in both complexes the CO ligand lies beneath the saturated ring carbon, giving approximate Cs symmetry to (1b) and (2). The structural consequences of one-electron oxidation on the geometry about the metal and within the ligands are discussed with regard to the nature of the highest occupied molecular orbital of (1b) and its depopulation in (2).

The complex [{Ir(µ-Cl)(η4-cod)}2](cod = cyclo-octa-1,5-diene) reacts with the triazene RNNNHR (R =p-tolyl) to give [IrCl(RNNNHR)(η4-cod)](1) which yields [IrCl(CO)2(RNNNHR)](2) on treatment with CO gas. Deprotonation of (2) with NEt3 in CH2Cl2 at –10 °C gives the dimer [{Ir(CO)2(µ-RNNNR)}2](3), which undergoes thermal substitution with PPh3 to yield [Ir2(CO)3(PPh3)(µ-RNNNR)2](4). Complexes (3) and (4) undergo reversible one-electron oxidation at a platinum electrode in CH2Cl2. In the presence of PPh3, (4)[or (3)] reacts with [Fe(η-C5H5)2][PF6] to give the isolable paramagnetic salt [{Ir(CO)(PPh3)(µ-RNNNR)}2][PF6] which undergoes reversible one-electron reduction and which has been characterised by e.s.r. spectroscopy.

Tetrachlorobenzene-o-quinone (o-chloranil) and [Ru(CO)2(PPh3)L′](L′=η4-2,3-dimethylbuta-1,3-diene) give the catecholate-bridged dimer [{Ru(CO)2(PPh3)(µ-o-O2C6Cl4)}2](1) which undergoes bridge-cleavage reactions with Group 5 donors, L, and halide ions, X–, yielding the complexes [Ru(CO)2(PPh3)L(o-O2C6Cl4)][2; L = PPh3, AsPh3, P(OPh)3, or pyridine] and [RuX(CO)2(PPh3)-(o-O2C6Cl4)]–(3; X = Cl, Br, or I), respectively. Cyclic voltammetry at a platinum electrode shows that compounds (2) and (3) undergo two one-electron oxidations; chemical oxidation of (2), with AgPF6 or [NO][PF6], and of (3), with [Fe(η-C5H5)2][PF6], yields [Ru(CO)2(PPh3)L(o-O2C6Cl4)]-[PF6][4; L = PPh3, AsPh3, P(OPh)3, or pyridine] and [RuX(CO)2(PPh3)(o-O2C6Cl4)](5; X = Cl, Br, or I), respectively. Complex (5; X = Br or I) may also be prepared directly from (1) and bromine or iodine. E.s.r. spectroscopy suggests that (4) and (5) are semiquinone complexes of ruthenium(II) and, therefore, that the oxidation of (2) and (3) is largely associated with the chelating ligand. However, the hyperfine coupling observed shows the unpaired electron density to be partially delocalised over the metal and axial ligands.

The radical cation [Fe(CO)3(PPh3)2]+(1) undergoes oxidative-substitution reactions with the diamagnetic ligands S2CPPh3 and [S2CNMe2]– to give cis,trans-[Fe(CO)2(PPh3)2(S2CR)]Z(R = PPh3, Z= 2; R = NMe2, Z= 1); the dithiocarbamato-complex is also prepared from Me2NC(S)SSC(S)NMe2. Radical–radical coupling occurs between (1) and NO or NO2 to give [Fe(CO)2(NO)(PPh3)2]+ and mer,trans-[Fe(NO2)(CO)3(PPh3)2]+ respectively. On thermolysis, the nitro-complex yields CO2 and the nitrosyl derivative via cis,trans-[[graphic omitted]O}-(CO)2(PPh3)2]+. The spin trapping of [Fe(CO)3(PPh3)2]+ with ButNO, and the formation of paramagnetic complexes with 1,2-diketones, have been studied by e.s.r. spectroscopy; cis,trans-[Fe(CO)2(PPh3)2(O2C6Cl4)]+, from 3,4,5,6-tetrachloro-p-benzoquinone, has been isolated and fully characterised.

Single-crystal X-ray diffraction studies on the tetraphenylcyclobutadiene iron complexes [Fe(CO){P(OMe)3}2(η-C4Ph4)](1) and [Fe(CO){P(OMe)3}2(η-C4Ph4)]+(2), the latter as its[BF4]– salt, show that significant variations in molecular geometry occur as a consequence of one-electron oxidation. The Fe–P distances increase by 0.115(5)A and the Fe–CO distance by 0.075(8)A, while cyclobutadiene ring carbon–iron lengths show a very small increase of 0.021(8)A. No significant change in the square-planar geometry of the C4 ring occurs on oxidation, but the Fe(CO){P(OMe)3}2 fragment adopts a different conformation relative to the C4Ph4 moiety in (1) as compared with (2). In one of the two distinct cations present in crystals of (2) the Fe(CO){P(OMe)3}2is strongly tilted so that the pseudo-three-fold axis of this conical fragment is not coincident with the four-fold axis of the C4 ring. These structural observations have been analyzed in order to test current theories of the electronic structure of FeL3(η-C4R4) complexes. The principal conclusion is that the highest occupied molecular orbital (1) is iron based and not strongly involved in bonding to cyclobutadiene. This is in full accord with the results of both photoelectron spectroscopic and generalized molecular orbital studies on [Fe(CO)3(η-C4H4)]. The orbital depopulated on oxidation contributes to both Fe–P(OMe)3 and Fe–CO π back bonding. Oxidation diminishes such π bonding and causes changes in P–O bond lengths consistent with P–O σ* orbitals contributing to the π-acceptor function of the P(OMe)3 ligand.

The reaction of [Rh(NO)(NCMe)2(PPh3)2][BF4]2 with Na[RNNNR](R =p-tolyl) in tetrahydrofuran yields [Rh(NO)(PPh3)2(RNNNR)][BF4], X-ray diffraction studies on which have revealed a trigonal-bipyramidal structure with a linear nitrosyl [Rh–N–O 177.5(6)°] and chelating triazenide ligand in the equatorial positions.

The X-ray structure of [Fe(CO) 3 {σ,η 3 -C 8 H 9 (C 3 Ph 3 )}], prepared by hydride ion addition to the product of the reaction between [Fe(CO) 3 (η 4 -COT)] (COT = cyclooctatetraene) and [C 3 Ph 3 ] + , reveals annelation of the ring opened cyclopropenium ion to the coordinated polyolefin.