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A series of crystalline salts based on the complex dianion [Fe(dto)2(NO)]2−(dto = 1,2-dithiooxalate) with hydrogen-bond donor cations have been synthesised. The preparation and structural characterisation of the new crystalline phases [4,4′-H2bipy][Fe(dto)2(NO)]·H2O (1), [3,3′-H2bipy][Fe(dto)2(NO)] (2) and [H2Me4bipyz][Fe(dto)2(NO)]·2H2O (3) (Me4bipyz = 3,3′,5,5'-tetramethyl-4,4′-bipyrazole) are reported, and show that a charge-assisted NHdto synthon is formed in each of compounds 1–3. In compound 1the anion forms a dimer in the solid-state, and variable temperature magnetic susceptibility measurements reveal a weak antiferromagnetic interaction with a best fit superexchange parameter J= –3.18 (2) cm−1, where H= –2JS1S2. [ABSTRACT FROM AUTHOR]

A number of different compounds have been synthesised by reaction of solutions containing Fe(NCE)2(E = S, Se) with solutions containing 3,3′-bipyridine. When the solvent used is methanol, the products are the monomeric compounds trans-{Fe(NCS)2(3,3′-bipy)2(MeOH)2} 1or trans-{Fe(NCSe)2(3,3′-bipy)2(MeOH)2} 2, but when acetonitrile is used the layered coordination network {Fe(NCS)2(3,3′-bipy)2} 3or doubly interpenetrated coordination network {Fe(NCSe)2(3,3′-bipy)2} 4is formed instead. Using methanol to dissolve the Fe(NCE)2and CHX3(X = Cl, Br) to dissolve the 3,3′-bipyridine yields solvated coordination networks of stoichiometry {Fe(NCE)2(3,3′-bipy)2}·2CHX3. The crystal structures of all of these compounds except 4are presented. [ABSTRACT FROM AUTHOR]

para-Phenylenediamine (ppda) may be ground together with metal chloride salts MCl2to produce the coordination network compounds [{(ppda)MCl2}n] {M = Zn (2), Cd (4), Cu (6)}. Its dihydrochloride salt [H2ppda]Cl2can be ground with MCl2to produce the layered salt structures [H2ppda][MCl4] {M = Zn (1), Cd (3), Cu (5)}, which (when M = Zn or Cd) may then be converted into 2or 4respectively by grinding with solid KOH. Alternatively, [H2ppda][MCl4] will react directly with appropriate basic metal compounds (hydroxides and/or carbonates) to give 2or 4, which can then be converted back to the salts by reaction with gaseous HCl. The interconversion of 5and 6is more complicated, with different combinations of starting material giving different products. The crystal structure of 2has been solved ab initiofrom the powder-diffraction data, and is shown to consist of zigzag polymeric chains containing alternating ppda and MCl2units. [ABSTRACT FROM AUTHOR]

[{(4,4′-bipy)ZnCl2}n] (4) is known in three different polymorphs, in the space groups C2/c(4a), Pnma(4b) and Pban(4c). Solution synthesis produces a mixture of 4aand 4b, but solid-state synthesis, either by grinding together 4,4′-bipy and ZnCl2or by heating [4,4′-H2bipy][ZnCl4] 3to thermally eliminate HCl, generates only the 4bform, demonstrating that solid-state synthesis can exert control over polymorph formation; other solid-state preparations of 4such as reaction of basic zinc carbonate with [4,4′-H2bipy]Cl2or reaction of [4,4′-H2bipy][ZnCl4] 3with KOH also increase the amount of 4bgenerated relative to the solution synthesis. It is also possible to form the mixed-metal phases [{(4,4′-bipy)Co1−xZnxCl2}n] 4xas effectively homogeneous solid-solutions by the same techniques, and when x< 0.5 (i.e.there is more cobalt present than zinc) then only the Pbanpolymorph of the solid-solution is observed. When x> 0.5, the amount of this polymorph is still greater than would be expected for a predominantly zinc-containing phase. [ABSTRACT FROM AUTHOR]

Monometallic complexes of the tetrakis(pyrazolyl)borate ligand [ML2{B(pz)4}] {M = Rh, Ir; L2= η-cod, η-nbd, (CO)2, (CO)(PPh3)} have two free pyrazolyl rings which can be coordinated to a second ML2unit to give the dimeric compounds [L2M{μ-B(pz)4}ML2]+, and to a metal halide to give heterobimetallic species [L2M{μ-B(pz)4}M′Cl2]. 1H NMR spectroscopy shows that [(η-cod)Rh{μ-B(pz)4}Rh(η-cod)]+1+, [(η-nbd)Rh{μ-B(pz)4}Rh(η-nbd)]+2+, [(η-cod)Ir{μ-B(pz)4}Ir(η-cod)]+3+and [(CO)2Rh{μ-B(pz)4}Rh(CO)2]+4+are fluxional at room temperature. Cooling a solution of [(η-cod)Rh{μ-B(pz)4}Rh(η-cod)]+1+to −90 °C slows the fluxional process, which involves inversion of the two B-(N–N)2-M six-membered rings. Attempts to synthesise the asymmetric complexes [(η-cod)Rh{μ-B(pz)4}Rh(η-nbd)]+7+, [(η-cod)Rh{μ-B(pz)4}Ir(η-cod)]+8+and [(η-cod)Rh{μ-B(pz)4}Rh(CO)2]+9+produced a mixture of [L2M{μ-B(pz)4}ML2]+, [L′2M′{μ-B(pz)4}M′L′2]+and the desired species. The heterobimetallic complexes [L2Rh{μ-B(pz)4}M′Cl2] (M′ = Co, L2= η-cod 10; M′ = Co, L2= η-nbd 11; M′ = Co, L = CO 12; M′ = Co, L2= (CO)(PPh3) 13; M′ = Zn, L2= η-cod 14; M′ = Zn, L2= η-nbd 15; M′ = Pd, L2= η-cod 16) possess square planar Rh(i) linked to square planar Pd(ii) or tetrahedral Zn(ii) and Co(ii) centres. The paramagnetic Co(ii) complexes give 1H NMR spectra with signals shifted over a range of 75 ppm. The UV-Vis spectra of 10–13show four bands, one MLCT at Rh and three d-d transitions arising from the spltting of the 4T1(P) excited state due to approximate C2vsymmetry at Co. [ABSTRACT FROM AUTHOR]

The complexes [(η-C5Me5)(ON)LMn(μ-CN)M(CO)(RCCR)Tp′]+(L = CNXyl, M = Mo; L = CNBut, M = Mo or W, R = Ph or Me) and trans- or cis-[(dppm){(EtO)3P}(OC)2Mn(μ-CN)M(CO)(PhCCPh)Tp′]+, and their linkage isomers [(η-C5Me5)(ON)LMn(μ-NC)M(CO)(PhCCPh)Tp′]+and trans- or cis-[(dppm){(EtO)3P}(OC)2Mn(μ-NC)M(CO)(PhCCPh)Tp′]+, undergo two one-electron oxidations. The complexes [(η-C5Me5)(ON)LMn(μ-XY)M(CO)(RCCR)Tp′]+(XY = CN or NC) are oxidised first at the N-bound metal centre and then at the C-bound centre. For [(dppm){(EtO)3P}(OC)2Mn(μ-XY)M(CO)(PhCCPh)Tp′]+, the transisomers are first oxidised at manganese whereas the cisisomers are first oxidised at M. Thus, the order of one-electron oxidation of the two series of binuclear monocations is influenced by linkage isomerisation of the cyanide bridge and cis-transisomerisation of the Mn(CO)2group. IR spectroscopic changes on reaction of Ag+with [(η-C5Me5)(ON)(ButNC)Mn(μ-CN)W(CO)(MeCCMe)Tp′]+are consistent with one-electron at the N-bound tungsten centre. Likewise, trans-[(dppm){(EtO)3P}(OC)2Mn(μ-NC)M(CO)(PhCCPh)Tp′]+(M = Mo or W) give the stable dications [(dppm){(EtO)3P}(OC)2Mn(μ-NC)M(CO)(PhCCPh)Tp′]2+. Significantly longer Mn-P bond distances in trans-[(dppm){(EtO)3P}(OC)2Mn(μ-NC)Mo(CO)(PhCCPh)Tp′]2+than in trans-[(dppm){(EtO)3P}(OC)2Mn(μ-NC)Mo(CO)(PhCCPh)Tp′]+are consistent with one-electron oxidation first at Mn(I); the alignment of the (CN)Mn(CO)2{P(OEt)3}(dppm) fragment relative to the alkyne in trans-[(dppm){(EtO)3P}(OC)2Mn(μ-NC)Mo(CO)(PhCCPh)Tp′]+suggests it acts as a π-acceptor, in contrast to related species such as trans-(NC)Mn(CO)2{P(OEt)3}(dppm) and (NC)Mn(NO){P(OPh)3}(π-C5H4Me) which behave as simple N-donors. [ABSTRACT FROM AUTHOR]

The d4halide complexes [MX(CO)(η-RCCR)Tp′] {R = Me, M = W, X = F; R = Ph, M = Mo or W, X = F or Cl; Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} undergo two-electron oxidation in the presence of a halide source to give the d2monocations [MX1X2(η-PhCCPh)Tp′](R = Me, M = W, X1= X2= F; R = Ph, M = Mo, X1= X2= F or Cl; M = W, X1= X2= F or Cl; X1= F, X2= Cl). Each monocation (R = Ph) shows two reversible one-electron reductions (the second process was not detected for R = Me) corresponding to the stepwise formation of the neutral d3and monoanionic d4analogues, [MX1X2(η-PhCCPh)Tp′] and [MX1X2(η-PhCCPh)Tp′]−respectively; the potentials for the two processes can be ‘tuned’ over a range of ca.1.0 V by varying M and X. Chemical one-electron reduction of [MX2(η-PhCCPh)Tp′]膫 [MX2(η-PhCCPh)Tp′] (M = Mo or W, X = F or Cl). X-Ray structural studies on the redox pairs [WX2(η-PhCCPh)Tp′]z(X = F and Cl, z= 0 and 1) show the alkyne to bisect the X–W–X angle in the d2cations but align more closely with one M–X bond in the neutral d3molecules, consistent with the anisotropic ESR spectra of the latter; the solution ESR spectrum of [MoF2(η-PhCCPh)Tp′] showed equivalent fluorine atoms, i.ethe alkyne oscillates at room temperature. The successful isolation of [MX2(η-PhCCPh)Tp′]橷 [MX2(η-PhCCPh)Tp′] completes a series in which d6to d2alkyne complexes are linked in a redox family tree by sequential one-electron transfer and substitution reactions. The implications for such trees in the production of new species and the selective synthesis of paramagnetic complexes acting as synthetically useful ‘alkyne radicals’ are discussed. [ABSTRACT FROM AUTHOR]

The preparation of the complex salts [H2im]2[MCl4] (H2im = imidazolium, M = Co, 1; Zn, 2; Cu, 3) and coordination compounds [MCl2(Him)2] (M = Co, 4; Zn, 5; Cu, 6) by a range of solid-state and solid-gas reactions is reported. Compounds 4–6and the related [{MCl2(4,4′-bipy)}n] (M = Co, 7; Zn, 8) were prepared by the solid state reactions of metal hydroxide or carbonate salts (or their equivalent) with the hydrochloride salt of the appropriate ligand (imidazole or 4,4′-bipy). [ABSTRACT FROM AUTHOR]

The structures adopted by a range of poly(pyrazolyl)borate complexes [ML2Tpx] [M = Rh, Ir; L2 = diene; Tpx = Bp′ {dihydrobis(3,5-dimethylpyrazolyl)borate}, Tp′ {hydrotris(3,5-dimethylpyrazolyl)borate}, Tp {hydrotris(pyrazolyl)borate}, B(pz)4 {tetrakis(pyrazolyl)borate}] have been investigated. Low steric hindrance between ligands in [Rh(η-nbd)Tp] (nbd = norbornadiene), [Rh(η-cod)Tp] (cod = cycloocta-1,5-diene) and [Rh(η-nbd)Tp′] results in κ3 coordination of the pyrazolylborate but [M(η-cod)Tp′] (M = Rh, Ir) are κ2 coordinated with the free pyrazolyl ring positioned above and approximately parallel to the square plane about the metal. All but the most sterically hindered Tpx complexes undergo fast exchange of the coordinated and uncoordinated pyrazolyl rings on the NMR spectroscopic timescale. For [Rh(η-cod){B(pz)4}], [Rh(η-dmbd)Tp′] (dmbd = 2,3-dimethylbuta-1,3-diene) and [Rh(η-cod)TpPh] {TpPh = hydrotris(3-phenylpyrazolyl)borate} the fluxional process is slowed at low temperatures so that inequivalent pyrazolyl rings are observed. The bonding modes of the Tp′ ligand (but not of other pyrazolylborate ligands) can be determined by 11B NMR and IR spectroscopy. The 11B chemical shifts (for a series of Tp′ complexes) show the general pattern, κ3 < −7.5 ppm < κ2 and the ν(BH) stretch κ3 > 2500 cm−1 > κ2. The electrochemical behaviour of the pyrazolylborate complexes is related to the degree of structural change which occurs on electron transfer. One-electron oxidation of complexes with Tp′, Tp and B(pz)4 ligands is generally reversible although that of [Ir(η-cod)Tp] is only reversible at higher scan rates and that of [Ir(η-cod){B(pz)4}] is irreversible. Of the complexes with the more sterically hindered TpPh ligand, only [Rh(η-nbd)TpPh] shows any degree of reversible oxidation. The ESR spectra of a range of Rh(ii) complexes show coupling to both 14N and 103Rh nuclei in most cases but what appears to be coupling to rhodium and one hydrogen atom, possibly a hydride ligand, for the oxidation product of [Rh(η-nbd)TpPh]. [ABSTRACT FROM AUTHOR]

The new square-planar platinum–diimine–dithiolate compounds [Pt(mesBIAN)SS] have been synthesised {mesBIAN = bis(mesityl)biazanaphthenequinone; SS = 1,2-dithiooxalate (dto) 1, maleonitriledithiolate (mnt) 2, 1,2-benzenedithiolate (bdt) 3, 3,4-toluenedithiolate (tdt) 4 and 1,3-dithia-2-thione-4,5-dithiolate (dmit) 5}, and the X-ray crystal structures of 3 and 5 determined. Cyclic voltammetry reveals that all the compounds form stable anions, and ESR spectroscopy of these anions shows that the SOMO is based upon the mesBIAN ligand; compounds 2–5 also show a reversible oxidation wave in their CV. Computational studies reveal that charge-transfer processes from orbitals that are combinations of metal and dithiolate ligand to a mesBIAN π*-based LUMO are responsible for the low energy absorptions seen in the UV/visible spectra of these compounds, and that the reverse process is responsible for the observed room-temperature solution luminescence of [Pt(mesBIAN)Cl2] and 1, 2 and 5. Compounds 3 and 4, containing aromatic thiolates, were not found to luminesce under the same conditions. Resonance Raman experiments have shown the origin of band-broadening of the lowest-energy absorption band in the absorption spectra of 2–5 to be due to vibronic structure within one electronic transition. [ABSTRACT FROM AUTHOR]

The reaction of [Mn(CN)L′(NO)(η5-C5R4Me)] with cis- or trans-[MnBrL(CO)2(dppm)], in the presence of Tl[PF6], gives homobinuclear cyanomanganese(i) complexes cis- or trans-[(dppm)(CO)2LMn(µ-NC)MnL′(NO)(η5-C5R4Me)]+, linkage isomers of which, cis- or trans-[(dppm)(CO)2LMn(µ-CN)MnL′(NO)(η5-C5R4Me)]+, are synthesised by reacting cis- or trans-[Mn(CN)L(CO)2(dppm)] with [MnIL′(NO)(η5-C5R4Me)] in the presence of Tl[PF6]. X-Ray structural studies on the isomers trans-[(dppm)(CO)2{(EtO)3P}Mn(µ-NC)Mn(CNBut)(NO)(η5-C5H4Me)]+ and trans-[(dppm)(CO)2{(EtO)3P}Mn(µ-CN)Mn(CNBut)(NO)(η5-C5H4Me)]+ show nearly identical molecular structures whereas cis-[(dppm)(CO)2{(PhO)3P}Mn(µ-NC)Mn{P(OPh)3}(NO)(η5-C5H4Me)]+ and cis-[(dppm)(CO)2{(PhO)3P}Mn(µ-CN)Mn{P(OPh)3}(NO)(η5-C5H4Me)]+ differ, effectively in the N- and C-coordination respectively of two different optical isomers of the pseudo-tetrahedral units (NC)Mn{P(OPh)3}(NO)(η5-C5H4Me) and (CN)Mn{P(OPh)3}(NO)(η5-C5H4Me) to the octahedral manganese centre. Electrochemical and spectroscopic studies on [(dppm)(CO)2LMn(µ-XY)MnL′(NO)(η5-C5R4Me)]+ show that systematic variation of the ligands L and L′, of the cyclopentadienyl ring substituents R, and of the µ-CN orientation (XY = CN or NC) allows control of the order of oxidation of the two metal centres and hence the direction and energy of metal–metal charge-transfer (MMCT) through the cyanide bridge in the mixed-valence dications. Chemical one-electron oxidation of cis- or trans-[(dppm)(CO)2LMn(µ-NC)MnL′(NO)(η5-C5R4Me)]+ with [NO][PF6] gives the mixed-valence dications trans-[(dppm)(CO)2LMnII(µ-NC)MnIL′(NO)(η5-C5R4Me)]2+ which show solvatochromic absorptions in the electronic spectrum, assigned to optically induced Mn(i)-to-Mn(ii) electron transfer via the cyanide bridge. [ABSTRACT FROM AUTHOR]

The salts [NEt4][Ru(CN)(CO)2L(o-O2C6Cl4)] {L = PPh3 or P(OPh)3}, which undergo one-electron oxidation at the catecholate ligand to give neutral semiquinone complexes [Ru(CN)(CO)2L(o-O2C6Cl4)], react with the dimers [{Ru(CO)2L(µ-o-O2C6Cl4)}2] {L = PPh3 or P(OPh)3} to give [NEt4][(o-O2C6Cl4)L(OC)2Ru(µ-CN)Ru(CO)2L′(o-O2C6Cl4)] {L or L′ = PPh3 or P(OPh)3}. The cyanide-bridged binuclear anions are, in turn, reversibly oxidised to isolable neutral and cationic complexes [(o-O2C6Cl4)L(OC)2Ru(µ-CN)Ru(CO)2L′(o-O2C6Cl4)] and [(o-O2C6Cl4)L(OC)2Ru(µ-CN)Ru(CO)2L′(o-O2C6Cl4)]+ which contain one and two semiquinone ligands respectively. Structural studies on the redox pair [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)]− and [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)] confirm that the C-bound Ru(CO)2(o-O2C6Cl4) fragment is oxidised first. Uniquely, [(o-O2C6Cl4){(PhO)3P}(OC)2Ru(µ-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)]− is oxidised first at the N-bound fragment, indicating that it is possible to control the site of electron transfer by tuning the co-ligands. Crystallisation of [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-CN)Ru(CO)2{P(OPh)3}(o-O2C6Cl4)] resulted in the formation of an isomer in which the P(OPh)3 ligand is cis to the cyanide bridge, contrasting with the trans arrangement of the X–Ru–L fragment in all other complexes of the type RuX(CO)2L(o-O2C6Cl4). [ABSTRACT FROM AUTHOR]

Two series of stable cyanide-bridged linkage isomers, namely [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-XY)MnL(NO)(η-C5Me5)] (XY = CN or NC, L = CNBut or CNXyl) and [(o-O2C6Cl4)L(OC)2Ru(µ-XY)M(CO)(PhC&z.tbd;CPh)Tp′] {M = Mo or W, L = PPh3 or P(OPh)3, Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} have been synthesised; pairs of isomers are distinguishable by IR spectroscopy and cyclic voltammetry. The molecular structure of [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-NC)Mo(CO)(PhC&z.tbd;CPh)Tp′] has the catecholate-bound ruthenium atom cyanide-bridged to a Mo(CO)(PhC&z.tbd;CPh)Tp′ unit in which the alkyne acts as a four-electron donor; the alignment of the alkyne relative to the Mo–CO vector suggests the fragment (CN)Ru(CO)2(PPh3)(o-O2C6Cl4) acts as a π-acceptor ligand. The complexes [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-XY)Mn(NO)L(η-C5Me5)] undergo three sequential one-electron oxidation processes with the first and third assigned to oxidation of the ruthenium-bound o-O2C6Cl4 ligand; the second corresponds to oxidation of Mn(i) to Mn(ii). The complexes [(o-O2C6Cl4)L(OC)2Ru(µ-XY)M(CO)(PhC&z.tbd;CPh)Tp′] are also first oxidised at the catecholate ligand; the second oxidation, and one-electron reduction, are based on the M(CO)(PhC&z.tbd;CPh)Tp′ fragment. Chemical oxidation of [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-XY)MnL(NO)(η-C5Me5)] with [Fe(η-C5H4COMe)(η-C5H5)][BF4], or of [(o-O2C6Cl4)L(OC)2Ru(µ-XY)M(CO)(PhC&z.tbd;CPh)Tp′] with AgBF4, gave the paramagnetic monocations [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-XY)MnL(NO)(η-C5Me5)]+ and [(o-O2C6Cl4)L(OC)2Ru(µ-XY)M(CO)(PhC&z.tbd;CPh)Tp′]+, the ESR spectra of which are consistent with ruthenium-bound semiquinone ligands. Linkage isomers are distinguishable by the magnitude of the 31P hyperfine coupling constant; complexes with N-bound Ru(o-O2C6Cl4) units also show small hyperfine coupling to the nitrogen atom of the cyanide bridge. [ABSTRACT FROM AUTHOR]

The triazenide-bridged tetracarbonyls [(OC)2Rh(µ-p-MeC6H4NNNC6H4Me-p)2M(CO)2] (M = Rh or Ir) undergo oxidative addition of iodine across the dimetal centre, giving the [RhM]4+ complexes [I(OC)2Rh(µ-p-MeC6H4NNNC6H4Me-p)2M(CO)2I], structurally characterised for M = Ir. The anionic tricarbonyl iodide [I(OC)Rh(µ-p-MeC6H4NNNC6H4Me-p)2Rh(CO)2]− forms [I2(OC)Rh(µ-p-MeC6H4NNNC6H4Me-p)2Rh(CO)I]− by initial one-electron transfer whereas the analogous tricarbonyl phosphine complexes [(OC)(Ph3P)Rh(µ-p-MeC6H4NNNC6H4Me-p)2M(CO)2] (M = Rh or Ir) undergo bridge cleavage, giving mononuclear [Rh(p-MeC6H4NNNC6H4Me-p)I2(CO)(PPh3)] and dimeric [I(OC){RNNN(R)C(O)}M(µ-I)2M{C(O)N(R)NNR}(CO)I] (M = Rh or Ir, R = C6H4Me-p) in which CO has been inserted into a metal–nitrogen bond. [ABSTRACT FROM AUTHOR]

The d4 halide complexes [MX(CO)(η-RC&z.tbd;CR)Tp′] {X = F, Cl, Br or I; R = Me or Ph; M = Mo or W; Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} undergo one-electron oxidation to the d3 monocations [MX(CO)(η-RC&z.tbd;CR)Tp′]+, isolable for M = W, R = Me. X-Ray structural studies on the redox pairs [WX(CO)(η-MeC&z.tbd;CMe)Tp′]z (X = Cl and Br, z = 0 and 1), the ESR spectra of the cations [WX(CO)(η-RC&z.tbd;CR)Tp′]+ (X = F, Cl, Br or I; R = Me or Ph), and DFT calculations on [WX(CO)(η-MeC&z.tbd;CMe)Tp′]z (X = F, Cl, Br and I; z = 0 and 1) are consistent with electron removal from a HOMO (of the d4 complexes) which is π-antibonding with respect to the W–X bond, π-bonding with respect to the W–C(O) bond, and δ-bonding with respect to the W–Calkyne bonds. The dependence of both oxidation potential and ν(CO) for [MX(CO)(η-RC&z.tbd;CR)Tp′] shows an inverse halide order which is consistent with an ionic component to the M–X bond; the small size of fluorine and its closeness to the metal centre leads to the highest energy HOMO and the lowest oxidation potential. In the cations [MX(CO)(η-RC&z.tbd;CR)Tp′]+ electronegativity effects become more important, leading to a conventional order for Cl, Br and I. However, high M–F π-donation is still facilitated by the short M–F distance. [ABSTRACT FROM AUTHOR]