04136_01a - 1 Ruthenium and osmium 1.1 Introduction...

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Unformatted text preview: 1 Ruthenium and osmium 1.1 Introduction Ruthenium and osmium are the first pair of 'platinum metals' [1—13]. They exhibit oxidation states up to +8, the highest observed for any element, as in MO4 (M = Ru, Os) though this requires the use of the most electronegative elements, fluorine and oxygen, for stability. Generally, the +2 and +3 states are the most important, along with -h4 for osmium; however, there is a considerable chemistry of the MO2+ ('osmyF and 'ruthenyl') and M=N3+groups, as well as the 'classical' hydride complexes OsH6(PR3)2, which also involve osmium(VI). 1.2 The elements and uses Along with iridium, osmium was discovered in 1803 by Smithson Tennant. He took the insoluble residue from the digestion of platinum ores with aqua regia and heated it with sodium carbonate to give soluble yellow OsO4(OH)2". On acidification, distillable OsO4 formed. Noting the smell of the (very toxic) tetroxide, Tennant gave the element its name from the Greek osme (pa ^r] — smell); he also noted that it stained the skin, prefiguring a future use. The last of the metals described in this book to be discovered was ruthenium. As with osmium, it was extracted from the aqua regia-insoluble residue from concentrated platinum ores and was first claimed in 1826 by G.W. Osann but definitely characterized by K.K. Klaus (1844), who oxidized the residue with KOH/KNO3, acidified and distilled off the OsO4 then reacted the residue with NH4Cl. (Aqua regia is a 3 :1 mixture of concentrated HC1/HNO3 (containing some chlorine).) Thermal decomposition of the resulting (NH4)2RuCl6 in an inert atmosphere gave ruthenium, taking its name from ruthenia, the Latin name for Russia. Both of these elements are silver-white lustrous metals with high melting (ruthenium 231O0C, osmium 390O0C) and boiling (3900 and 551O0C, respectively) points. As usual, the 5d metal is much more dense (ruthenium 12.45, osmium 22.59gcm~3); both adopt hep structures; osmium is the densest metal known. The metals are unreactive, insoluble in all acids, even aqua regia. Ruthenium tends to form a protective coating of the dioxide and is not attacked by oxygen below 60O0C nor by chlorine or fluorine below 30O0C. Powdered osmium is slowly attacked by oxygen at room temperature, yielding OsO4 (though not below 40O0C if in bulk). Osmium reacts with fluorine and chlorine at about 10O0C. Both metals are attacked by molten alkalis and oxidizing fluxes. Ruthenium nowadays finds many uses in the electronics industry, particularly for making resistor tracks. It is used as an ingredient in various catalysts and, importantly, in electrode materials, e.g. RuO2-coated titanium elements in the chloralkali industry. Osmium tetroxide is a very useful organic oxidant and, classically, is used as a tissue stain. Both elements are employed in making certain platinum alloys. 1.2.1 Extraction Extraction of ruthenium and osmium is done by solvent extraction [1, 2, 5, 14]. Following the traditional route, however, aqua regia-insoluble residues or anode slimes from nickel refining undergo bisulphate oxidation to remove rhodium, then on alkaline fusion ruthenium and osmium are stabilized as Na2RuO4 and Na2OsO2(OH)4. The ruthenium(VI) can be reduced (alcohol) to RuO2, which is then converted into (NH4)3 RuCl6, giving ruthenium metal in a flow of hydrogen at 10O0C. Osmium can be precipitated and stored as K2OsO2(OH)4 or first converted into OsO4 (by distillation of the osmate with HNO3) which is then reduced with hydrogen or turned into (NH4)2OsCl6, reduced in the same manner as the ruthenium analogue. In the solvent-extraction process, the platinum metal concentrate is solubilized in acid using chlorine oxidant. Ruthenium and osmium are separated by turning them into the volatile tetroxides. 1.3 Halides 1.3.1 Ruthenium halides Ruthenium forms the whole range of trihalides but only fluorides in higher states. RuF3 can be made by iodine reduction of RuF5. It is obtained as a dark brown powder that contains corner-shared RuF6 octahedra [15]. RuCl3 exists in a- and /^-phases: Ru3 (CO) 12 ^^ /3-RuCl3 (brown solid) 0 36O C /3-RuCl3 -^-» Q-RuCl3 V(black crystals) 0 36O C ' The a-form has the Q-TiCl3 structure with 6-coordinate ruthenium and a rather long Ru-Ru distance (3.46A) compared with the /3-form where there are one-dimensional chains, again with octahedrally coordinated ruthenium ( Ru-Ru 2.83A). The /3-form transforms irreversibly to the a-form above 45O0C. Both these forms are insoluble in water though /2-RuCl3 dissolves in ethanol [16]. Commercial 'ruthenium trichloride' purporting to be RuCl3^H2O is an ill-defined mixture of oxochloro and hydroxychloro species of more than one oxidation state. Obtained by dissolving RuO4 in hydrochloric acid, it can be purified by repeatedly evaporating to dryness with concentrated HCl. RuBr3 is usually made by brominating the metal while several routes to RuI3 are open Ru RuO4 Ru(NH 3 ) 5 I 3 450°C,20atm HI(aq.) heat — > RuBr3 > RuI3 > RuI3 Black-brown RuBr3 has roughly octahedral coordination of ruthenium (Ru-Br 2.46-2.54 A) with short R u-Ru contacts (2.73 A) [17]. Black RuI3 has a similar structure. Neither is particularly soluble in water. RuF4 can be made as a deep pink solid: K 2 RuF 6 -^^ RuF4 It has a VF4 type puckered sheet structure with 6-coordinated ruthenium; four fluorines bridge, two non-bridging ones are trans with the terminal distances shorter as expected (Table 1.1). It is paramagnetic (//eff = 3.04//B at room temperature). Green RuF5, sublimeable in vacua (650C, l (T 8 torr (1.33 x 10"6Pa)) can be made by fluorination Ru -^ RuF5 Ru -^ BrFjRuF6- ^^ BrF3 + RuF5 It melts at 86.5 C and boils at 2270C. The tetrameric structure (Figure 1.1) is one adopted by a number of pentafluorides with ds-bridges completing the 6-coordination. Table 1.1 Bond lengths (A) in ruthenium fluorides Ru-F (terminal) RuF3 RuF4 RuF5 RuF6 1.82 1.795-1.824 1.824 Ru-F (bridge) 1.982 2.00 1.995-2.007 0 Figure 1.1 The tetrameric structure of RuF5. A second, red form has recently been reported; from mass spectral evidence, it may be a trimer. In the gas phase at 12O0C, it consists mainly of a trimer (with octahedrally coordinated Ru) [18]. RuF6 is made by fluorination of RuF5 under forcing conditions: RuF5 50atm,230°C > RuF6 It is an extremely moisture-sensitive dark brown solid (m.p. 540C); bond lengths have been obtained from an EXAFS study [19]. There is some evidence that RuCl3 reacts with chlorine in the gas phase above 40O0C to form RuCl4 but RuCl4 has not been authenticated as a solid, neither has RuF8, which is claimed to exist at low temperatures. 1.3.2 Osmium halides Unlike ruthenium (and other platinum metals) osmium forms chlorides and bromides in a range of oxidation states [11,12], There are no convincing reports of halides in oxidation states below III: early reports of OsI and OsI2 seem to result from oxide contaminations. Neither is there OsF3, evidence of the greater stability of the +4 state compared with that of ruthenium. Dark grey OsCl3 has the 6-coordinate Ce-RuCl3 structure OsCl4 Cl2,100-500torr — > OsCl3 Black OsBr3 and OsI3 (IJL= 1 .8// B ) are also prepared by thermal methods OsBr4 ^If^ QsBr3 sealed tube 9 Sf)0C (H3O)2OsI6 N2/sealed tube ——> OsI3 There is evidence for OsX3 5 (X = Cl, Br). Figure 1.2 The structure adopted by OsCl4. OsF4, a yellow-brown solid that distills as a viscous liquid, is made by reduction of solutions of OsF5 OsFc 3 H2/Pt gauze UV > OsF4 It is isomorphous with MF4 (M = Pd, Pt, Ir, Rh). Black OsCl4 exists in two forms. A high-temperature form is made by reaction of the elements Os -^-» OsCl4 0 70O C It has 6-coordinate osmium in a structure (Figure 1.2) regarded as being made from a hexagonally packed array of chlorides with osmiums occupying half the holes in alternate layers; Os-Cl bond lengths are 2.261 A (terminal) and 2.378 A (bridge) [2O]. The low-temperature form is made using thionyl chloride as the chlorinating agent. OsO4 ^ OsCl4 reflux Black OsBr4 (PtX4 structure) has 6-coordinate osmium [21] OsCl4 Os 330°C/120atm — > OsBr4 > OsBr4 10atm,470°C — A second form can be made by refluxing OsO4 with ethanolic HBr, then drying the product. The green-blue pentafluoride (m.p. 7O0C, b.p. 2260C) is thermochromic, becoming bright blue at its boiling point (the vapour is colourless). It is synthesized by reducing OsF6: it has the tetrameric structure adopted by RuF5 (Os-F = 1.84 A (terminal) 2.03 A (bridge)) in the solid state [ISc]. s^ T- W^FS OsF6 > OsF5 0 ° 55 C ^ Like RuF5, it is mainly a trimer (OsF5)3 in the gas phase. In contrast to this, very moisture-sensitive black OsCl5, prepared by chlorinating OsF6 using BCl3 as the chlorinating agent, has the dimeric ReCl5 structure (Os-Cl = 2.24 A (terminal) 2.42 A (bridge)). Its magnetic moment is 2.54//B OsF6 -^U OsCl5 Like several other heavy metals, osmium forms a volatile (bright yellow) hexafluoride (m.p. 33.20C, b.p. 470C) ^ Os + 3F2 „ 250-30O0C 1 atm >• OsF6 The solid is polymorphic, with a cubic structure above 1.40C. A bond length of 1.816 A has been obtained from EXAFS measurements at 1OK, while vapour phase measurements give Os-F of 1.831 A [22]. There is uncertainty about the heptafluoride, claimed to be formed as a yellow solid from fluorination under very high pressure Os 600°C/450atm - > OsF7 Material with the same IR spectrum has been obtained by fluorination of OsO3F2 at 18O0C (50 atm). OsF7 is said to decompose at -10O0C (1 atm fluorine pressure) [23]. As osmium forms a tetroxide, OsF8 might possibly exist, especially in view of the existence of the osmium(VIII) oxyfluorides, but MO calculations indicate the Os-F bond would be weaker in the binary fluoride. It is also likely that non-bonding repulsions between eight fluorines would make an octafluoride unstable [23b]. 1.3.3 Oxyhalides Much less is known about ruthenium oxyhalides than about the osmium compounds. The only compound definitely characterized [24] is RuOF4, synthesized by fluorination of RuO2, condensing the product at -1960C. It loses oxygen slowly at room temperature, rapidly at 7O0C. RuO2 + 2F2 300 400 C " ° ) RuOF4 + ± O 2 > RuOF4 H- SiF4 It has also been made by passing RuF5 vapour down a hot glass tube: RuF5 + SiO2 It gives the parent ion in the mass spectrum and has a simple IR spectrum (z/(Ru=O) 1040Cm"1 and (z/(Ru-F) 720cm"1) similar to that of the vapour (1060, 710, 675cm"1), implying a monomeric structure. Chlorides RuOCl2 and Ru2OClx (x = 5 ,6) have been claimed; various oxo complexes Ru2OX^o are well defined. Although no OsF8 has been described, there are oxofluorides in the +8 state. Table 1.2 Vibrational frequencies* for osmium oxyhalides State* Os=O CW-OsO2F4 Raman IR OsO3F2 Matrix OsOF5 Matrix Vapour OsO2F3 Matrix OsOF4 Matrix OsOCl4 Matrix Gas a Vibrational frequencies (cm ] ) Os-F (term) Os-F (bridge) 942, 932 940, 930 954 (947, 942) 931 960 966.5 964 995, 955 907 1018 1079.5 1028 1032 1028 672, 579, 571 675, 588, 570 646 710, 700, 640 713,638.5 717, 700, 645 720 655 735, 705, 657, 648 685 392 (Os-Cl) 395 397 480-580 (broad) 529, 423 Only IR except for OsO2F4; b solid unless otherwise stated. Deep red OsO2F4 (m.p. 890C) has recently been made [25] OsO4 4 H F/KrF2 -1960C > C^-OsO2F4 L * It is thermally stable but instantly hydrolysed in air (like osmium oxyhalides in general); it has a simple vibrational spectrum (V(Os=O) 940cm"1; z/(Os-F) 680, 590, 570Cm'1) (Table 1.2) and a ds-octahedral structure has been confirmed by an electron diffraction study (Os=O 1.674 A, Os-F 1.843-1.883 A). Several syntheses have been reported for orange-yellow diamagnetic OsO3F2 (m.p. 172-1730C) [26]: OsO4 —^ OsOJ3FL2 4 0 30O C OsO4 -^^ OsO3F2 RT OsO3F2 is a monomer in the gas phase, to which a monomeric D3h structure has been assigned. EXAFS and X-ray diffraction measurements show a 6-coordinate solid-state structure with cw-fluorine bridges (Figure 1.3) (Os=O 1.678-1.727 A, Os-F 1.879 A (terminal), 2.108-2.126 A (bridge)). The other possible osmium(VIII) oxyfluoride OsOF6 has so far eluded synthesis and recent ab initio MO calculations indicate it is unlikely to exist. Emerald green OsOF5 (m.p. 59.50C; b.p. 100.60C) has an octahedral structure like OsF6 but is rather less volatile (Os=O 1.74 A, Os-F 1.72 A (trans) 1.76-1.80 A (cis)) [27]. It is paramagnetic (/xeff = 1.47/xB at 298K) and ESR (a) (b) Figure 1.3 The structure O fOsO 3 F 2 in (a) the gas phase and (b) the solid state. studies in low-temperature matrices indicate delocalization of the unpaired electron 11.5% from the osmium 5dxy orbital to each equational fluorine. Syntheses include OsO3F2 ^ ^ OsOF5 18O0C On heating a 3 :1 OsFJOsO4 mixture at 150-20O0C, a mixture OfOsOF5 and OsO4 is obtained that can be separated by using the greater volatility of OsOF5. OsO2F3 is a yellow-green solid, disproportionating at 6O0C to OsO3F2 and OsOF4, from which it may be made: OsOF4 + OsO3F2 -^-^ 2OsO2F3 12h 10O0C OsO4 + OsF6 -^> 2OsO2F3 17h 1 SO0C Matrix isolation studies suggest isolated D3h molecules, but the pure solid has a more complicated IR spectrum indicating both bridging and terminal fluorines [28]. Blue-green OsOF4 (m.p. 8O0C) is a byproduct in the synthesis of OsOF5 and can also be made in small quantities by reduction of OsOF5 on a hot tungsten wire. In the gas phase it has a C4v pyramidal structure (Os=O 1.624 A, Os-F 1.835 A); crystallography suggests a solid-state structure similar to tetrameric OsF5; the more complex IR spectrum of the solid is in keeping with this [29]. Oxychlorides are less prolific, apart from the red-brown OsOCl4 (m.p. 320C). This probably has a molecular structure in the solid state as the IR spectra of the solid, matrix-isolated and gas-phase molecules are very similar, and the volatility is consonant with this [3O]. Syntheses include heating osmium in a stream of oxygen/chlorine ('oxychlorination') and by: OsO4 -^ OsOCl4 Table 1.3 Bond lengths in MX67" (A) n 0 1 2 3 RuF6 1.824(EX) 1.845(EX) 1.85(X) 1.916(EX) RuCl6 RuBr6 OsF6 1.816(EX) 1.831 (ED) 1.882(EX) 2 .29(X) 2.318(X) 2.375(X) 1.927(EX) 2.514(X) OsCl6 OsBr6 2.284 (X) 2.303(X) 2.332 (X) 2.336(X) ~2.5 (X) ED, electron diffraction; X, X-ray; EX = EXAFS. Electron diffraction measurements on the vapour indicate a C4v square pyramidal structure (Os=O 1.663A,o Os-Cl 2.258 A; O-Os-Cl 108.3° Cl-Os-Cl 84.4°) with osmium 0.709 A above the basal plane. OsOCl2 can be made as dark olive green needles from heating OsCl4 in oxygen [31]. There are also reports of OsO0JCI3 (probably Os2OCl6) and a corresponding bromide [32]. 1.3.4 Halide complexes The complexes of ruthenium and osmium in the same oxidation state are generally similar and are, therefore, treated together; the structural (Table 1.3) and vibrational data (Table 1.4) have been set out in some detail to demonstrate halogen-dependent trends. No complexes have at present been authenticated in oxidation states greater than +6, whereas oxyhalide complexes exist where the +8 state is known; this parallels trends in the halides and oxyhalides. Oxidation state +6 Reaction of NOF with OsF6 produces NO+OsFf, along with some NO + OsF 6 . Table 1.4 Vibrational frequencies in MX6 species (cm ] ) (M = Ru, Os; X = halogen) n O 1 2 3 RuF6 RuCl6 RuBr6 RuI6 OsF6 OsCl6 OsBr6 OsI6 675, 735 660, 630 609, 581 328, 327 209, 248 (Cs) (K) -, 310 (K) 184, 236 (PhNHf) 731, 720 688, 616 (XeF+) 608, 547 (Cs) 375, 325 (Et4N) 344, 313 211,227 152, 170 (Cs) (Bu4N) (K) 313, 294 201, 200 144, 140 (Co(en)3) (Co(en)3) (Co(en)3) The first figure given for each species is ^1 ( A lg ), the second is z/3 (T1 ^ ). Data are for ions in solution except where a counter-ion is indicated. Oxidation state H-5 Fluorination of a mixture of alkali metal halide and an appropriate ruthenium or osmium halide affords cream MRuF6 (M = alkali metal, Ag; /xeff = 3.5-3.8//B) or white MOsF6: RuCl 3 OrOsF 4 —^-> M RuF 6 OrMOsF 6 BrF 3 OrF 2 ° + ° They contain octahedral MF6 (Table 1.3) [33]; in XeF RuF6 the attraction of XeF+ distorts the octahedron by pulling one fluorine towards it, so that there is one long Ru-F distance of 1.919 A compared with the others of 1.778-1.835 A (EXAFS measurements indicate KRuF6 has regular octahedral coordination (Ru-F 1.845 A)) [19]. Magnetic moments are as expected for d3 ions. Disproportionation occurs on hydrolysis: MF6 -^ MO4 + MF^" Octahedral OsCl^ has been isolated as Ph4As, Ph4P and Ph4N salts (/^eff values of 3.21 and 3.03//B have been reported) [34]: OsCl5 H- Ph4AsCl ffl f F —^ Ph4AsOsCl6 (X = Br, I) Os(CO)2X4 —^-> Et4NOsCl6 0 12O C OsCl^ is reduced to OsCl6" in contact with most solvents (e.g. CH2Cl2); the redox potential for OsCl6/OsCl6" is 0.8 V and for OsBr6/OsBr6" it is 1.20V. PbO2 can be used to form a transient OsBr^ ion by oxidizing OsBr6"; it will also oxidize OsCl6" to OsCl6 . Cation size can affect bond lengths in OsCl6 ; Os-Cl is 2.284 A and 2.303 A in the Ph4P and Bu4N salts, respectively. Oxidation, however, has a more significant effect, so that Os-Cl in (Ph4P)2OsCl6 is 2.332 A. Oxidation state +4 All MX6" have been isolated except RuI6". MF6" can be made by hydrolysis of MF^", as already mentioned, but other methods are available: RuCl3 H- BaCl2 —^ BaRuF6 Yellow Na2RuF6 has the Na2SiF6 structure while M2RuF6 adopts the K2GeF6 structure (M = K to Cs). EXAFS indicates Ru-F is 1.934 A in K2RuF6 while in K2OsF6 Os-F is 1.927 A [35]. Magnetic moments are as expected for a low spin d4 ion (K2RuF6 2.86/^B, Cs2RuF6 2.98//B, K2OsF6 1.30//B, Cs2OsF6 1.50/^B); the lower values for the osmium compounds are a consequence of the stronger spin-orbit coupling for the 5d metal. Various routes are available for the chlorides [36]: Ru or Os -^-> M 2 RuCl 6 OrM 2 OsCl 6 MCl M 2 RuCl 5 (OH 2 ) OsO4 ^^ HCl (aq.) ——> M2RuCl6 > M2OsCl6 ^ ^ ^1 conc.HCl/MCl EtOH The last synthesis uses ethanol as the reducing agent. Soluble Na2OsCl6 has been used to make the less soluble salts of other alkali metals by metathesis. Typical colours are red-brown to black (Ru) and orange to dark red (Os). K2RuCl6 has the K2PtCl6 structure. Magnetic moments for the ruthenium compounds are 2.7-3.0/xB; the osmium compounds have substantially lower moments (1.51/^B for K2OsCl6) but on doping into K2PtCl6 the moment of OsCl6" rises to 2.1 //B, 'superexchange' causing a lowered value in the undiluted salts. Bromides and iodides can be made (except X = I for Ru). RuBr5(H2O)2- ""^. RuBiT Br2 K2RuCl6 —^-+ K2RuBr6 OsO4 H Br( q } ^ ' ) M2OsBr6 (M - alkali metal) OsO4 J^ M2OsI6 + M These compounds tend to be black in colour. Magnetic moments of 2.84 and 1.65/iB have been reported for K2RuBr6 and K2OsI6, respectively. OsCl6" is a useful starting material for the synthesis of a range of osmium complexes (Figure 1.4). The mixed halide species O sX 6 _ w Y^~ or OsX0Y^Z2" (a + b + c = 6) have been studied in considerable detail [37]. Reaction of OsCl6" with BrF3 affords stepwise substitution OsCl6- -> OsCl5F2' -> CW-OsCl4F^- -»/0C-OsCl3Fi- -> Cw-OsCl2F^- -» OsClFi;- -> OsF^ with the stronger trans-effect of chloride directing the position of substitution. This can likewise be utilized to synthesize the trans- and raer-isomers, for example CW-OsCl2F4" -^->racr-OsCl3F^ The isomer(s) obtained depend on the reaction time; thus reaction of K2OsCl6 with BrF3 at 2O0C affords 90% CW-OsF4Cl2" after 20min whereas Os(phen)2Cl2+ Os(terpy)Cl3 terpy Os2(OAc)4Cl2 OsCl62' phen Os(py)2CU + fac-Os(Py)3Cl3 PY Os(NH3)5N22+ 1. Zn dust 2. NH3(aq)/O2 Os(NH3)63+ Os(PR3)3Cl3 Os(NO)Cl52" OsH4(PR3)S Figure 1.4 Reactions of OsCIg". after 1Oh the mixture contains 30% ds-OsF4Cl2~, 40% OsF5Cl2- and 30% OsF^". Mixtures can be separated by chromatography or ionophoresis; within this series, the ds-isomers are eluted before the trans (on diethylaminoethyl cellulose) whereas in ionophoresis, the trans-isomers move 3-5% faster. Such octahedral anions are, of course, amenable to study by vibrational spectroscopy; as the anion symmetry descends from O/j(MX6~), the number of bands increases as the degeneracy of vibrations is removed. Pairs of isomers can be distinguished; thus for OsF2Cl4", the more symmetric trans-isomer (D4J1) gives rise to fewer stretching vibrations (5) than the ds-isomer (C2v), which has 6. Moreover the centre of symmetry in the trans-isomer means there are no IR/Raman coincidences. The Os-F vibrations can be associated with bands in the 490-560Cm"1 region and Os-Cl stretching vibrations in the 300-360Cm"1 region (Figure 1.5). Other series of mixed hexahalide complexes have been made. Thus from K2OsI6 and concentrated HBr: OsI6" -» OsBrI§~ -> C^-OsBr2I4" -> /^c-OsBr3!3" -> C^-OsBr4I2" -> OsBr5I2" -> OsBr6" As before the rrarcs-isomers can be obtained using OsBr6" and concentrated HI; similarly, starting from OsCl6" and concentrated HI, the sequence OsCl5I2", /JWw-OsCl4Ii", WeT-OsCl3I2", /ra^-OsC!2I4", OsClli" and OsI6" is obtained. A more drastic synthesis of this type has been achieved by taking mixed crystals K2OsBrJK2SnCl6 and using the nuclear process 190 Os(n,7)191Os, when all the mixed species 191OsCl^Br6I71 were obtained. Mixed species with three different halogens have been made OsF5Cl2" c ncHBr 50°C,30min ° ) ^-OsF4ClBr2" Figure 1.5 The vibrational spectra of the cis (a) and trans (b) isomers of [OsCl2F4]2 in their caesium salts. (Reproduced with permission from Z. Naturforsch., Teil B, 1984, 39, 1100.) Table 1.5 Bond lengths (A) in dipyridinio methane salts OsF5Cl2/^-OsF3Cl2T W^r-OsF3Cl2- Cw-OsF2Cl2T 1.944 1.976 2.278 2.307 /TYWs-OsF2Cl2T 1.926 1.948 2.316 2.338 2.337 Os-F trans to F trans to Cl Os-Cl trans to F trans to Cl 1.918 1.959 2.329 1.948 2.320 The crystal structure of the caesium salt shows Os-F, Os-Cl and Os-Br bonds of 1.94, 2.43 and 2.49 A, respectively. The complex exhibits strong IR bands at 552, 320 and 222cm"1, assigned to Os-F, Os-Cl and Os-Br stretching, respectively (compare z/3 of OsX^" at 547cm"1 (F), 313cm"1 (Cl) and 227 cm"1 (Br)) [38]. Bond lengths in the dipyridinio methane salts [(C5H5N)2CH2][OsF5Cl],/^and ^-[(C5H5N)2CH2][OsF3Cl3] and cis- and fra^-[(C5H5)2CH2][OsF2Cl4] show the mutual ^raws-influence of chlorine and fluorine; thus Os-Cl bonds trans to fluorine are shorter than those trans to chlorine, while Os-F bonds trans to chlorine are longer than those trans to fluorine (Table 1.5) [38c]. Oxidation state +3 Halide complex ions of ruthenium and osmium in the +3 state are known for all except OsFg" [39]. Syntheses include: RuCl3 -^X K3RuF6 fuse (p, = 1 .25/XB) K2RuCl5(H2O) ^ K3RuCl6 l RuX3 ff H NT-I !"'"'V" [ 5 3l * cone. HX/N2 > (C6H5NH3)3RuX6 (M = 2.09 /*„, X = Br) A general synthesis for the osmium compounds is OsXiC nC HX/N2 °' . (Coen3)3+OsX6 (X = C l,Br,I) Magnetic moments reported for the OsX^" salts are 1.70, 1.67 and 1.61//B for X = Cl, Br and I, respectively, consonant with the low-spin d5 configuration. A number of dinuclear complexes have been synthesized [40] Os2(OCOMe)4Cl2 -^^ Os2CIi" ^ ' EtOH 0s cl > " iSrOs^ Os2X82" Os2X102" (a) (b) Figure 1.6 The structures of the diosmate ions Os2Xg" (a) and Os2XiQ (b). Oxidation with halogens gives the decahalogenodiosmate (IV) ( 2—) ions (Figure 1.6): Os2Xi- —£-> Os2XJo CH2Cl2 (X - Cl, Br) The short Os-Os bonds in Os2X2." correspond to triple bonds and give rise to stretching vibrations associated with bands around 28OCm-1 in the Raman spectrum (Table 1.6). The Os2Xg" ions participate in various redox processes: at 235 K Os2Cl2T undergoes reversible oxidation to Os2CIg" (n = 1,0), the bromide behaves similarly. At high temperatures, the Os-Os bond is broken and OsCl^ is formed. Os2CIg" can also be cleaved with Bu1NC to form transOsCl4(CNBul)2 [41]. In addition to the doubly bridged Os2X2^, triply bridged Os2Br^ can be made (Figure 1.7): OsBrr ~^? CF3CO2H ^0J . OsjBrfo ^ ^j CF3CO2H ^ ' Os2Br9- ^ ^ It can be reduced electrochemically to Os2Br9" (n = 2,3), with Os2Br"o (n = 3,4) similarly accessible. Rb3Os2Br9 has Os-Os 2.799 A [42]. Table 1.6 Characteristics of Os2Xg' Counter-ion Os2Cl81Os2Br81Os2I8" Bu4N Bu4N (Ph3P)2N Os-Os(A) 2.182 2.196 2.212 ^y1n(Os-Os)(Cm"1) 285 287 270 Figure 1.7 The structures of the diosmate ions Os2Xg . In the case of ruthenium, the Ru2X9 system with confacial octahedra is important M 2 RuCl 5 (H 2 O) -^-+ M3Ru2Cl9 in vacua 9 SO0P (M = alkali metal) RuCl;!- -J^L-, Ru2Br^These evidently have some Ru-Ru bonding with Ru-Ru distances of 2.73 and 2.87 A in Cs3Ru2Cl9 and (l-methyl-3-ethylimidazolinium)3Ru2Br9, respectively; the magnetic moments of (Bu4N)3Ru2X9 of 0.86//B (Cl) and 1.18//B (Br) are lower than expected for low spin d5 and indicate some metal-metal interaction. Ru2X9" again forms part of a redox-related series Ru2X9" (n — 1-4) obtainable in solution by low-temperature electrochemistry [42]. 1.3.5 'Ruthenium blues' [43] It has long been known (Claus, 1846) that reduction (e.g. Zn, H2 with Pt catalyst) of some ruthenium salts gives a blue solution, which on treatment with HCl or oxidation turns green. Various claims have been made for the species present: RuCl4", Ru2Cl3+, Ru2Cl+ and R u 5 CIf 2 . A cluster (Cl3Ru(/x-Cl)3Ru(/Lt-Cl)3RuCl3)4~ has been isolated and characterized from such a solution [44]. At present it seems likely that the compound in solution is a cluster, that the ruthenium valency is between 2 and 2.5 and that more than one species is present. The blue solutions have been found to catalyse alkene isomerization and hydrogenation and have very considerable synthetic utility (Figure 1.8). [RuCl2(PhCN)3I2 CiS-RuL2Cl2+ (L=phen, bipy) PhCN Ru2Cl3(PMe2Ph)6 PMe2Ph Ru(PPh3)3Cl2 Ruthenium b lue RuL32+ Ph2PC2H4PPh2 trans-Rudppe2Cl2 Ru(NH 3 J 5 N 2 Ru(Tl-C5H5), Ru Py4Cl2 Figure 1.8 Syntheses using 'Ruthenium blue'. 1.3.6 Oxyhalide complexes Various anionic complexes have been made [26a]: OsO3F2 + KF -55L» K[OsO3F3] OsO4 C sF(aq °) Cs2[OsO4F2] EXAFS measurements on KOsO3F3 indicate the presence of/^c-OsO3 F^" with Os-O 1.7OA, Os-F 1.92 A; in Cs2OsO4F2, m-OsO4F^~ has Os=O 1.70 A and Os-F 2.05 A. Reaction O fPh 4 PCl with OsO4 gives Ph4P+OsO4CP, the anion having a tbp structure with a very long equatorial Os-Cl bond (2.76 A) [45]. Both ruthenium and osmium form /r<ms-MO2X4~ species (X = Cl, Br), for example RuO4 -^U M2RuO2Cl4 K2[OsO2(OH)4] -^U K2OsO2Cl4 Typical bond lengths are M-O 1.709 A (Ru) 1.750 A (Os) and M-Cl 2.3882.394 A (Ru) 2.379 A (Os) in [(Ph3P)2N]RuO2Cl4 and K2OsO2Cl4, respectively. Characteristic z/(M=O) bands can be seen in the vibrational spectra owing to both the symmetric and asymmetric stretches: for OsO2X4- the symmetric stretch is at 904 (X = Cl) and 900 (X = Br) cm"1, with corresponding values for the asymmetric stretch of 837 and 842cm"1 (in the potassium salts). In solution [(Ph3P)2N]2 RuO2Cl4 loses chloride to form [(Ph3P)2N] RuO^Cl3, which has a tbp structure with two axial chlorines (Ru-Cl 2.372.39A); the equatorial bond lengths are 1.66-1.69 A (Ru-O) and 2.13A (Ru-Cl) [46]. The dimeric M2OCl^o ions contain linear M —O—M units (Figure 1.9); in Cs4Os2OCl10 the Os-O-Os stretching vibration is at 852cm"1 in the IR spectrum [47J while its crystal structure reveals Os=O 1.778 A, Os-Cl 2.367-2.377 A (cis to O) and 2.433 A (trans to O). In K4Ru2OCl10, Ru-O is 1.801 A, Ru-Cl is 2.363 (cis) and 2.317 A (trans). The shortness of the M-O bridge bonds is explained by the formation of two M —O—M threecentre MOs. Figure 1.10 shows the formation of one of these by overlap of T-TC"1! (M - Rb, Cs) Figure 1.9 The dimeric [M2OCl10]4" ions (M = Ru, Os). Figure 1.10 The three-centre molecular orbitals in [Os2OCl10]4 . osmium 5d and oxygen 2p orbitals; each MO contains two osmium electrons and two from the oxygen occupying the bonding and non-bonding MOs. These two MOs account for two of the four electrons belonging to each Os4+ ion (d4); the remaining two occupy the dxy orbital (unused in the MO scheme) explaining the diamagnetism of these MIV compounds. 1.4 Oxides and related anions The oxides are dominated by the very volatile and toxic tetroxides. Yellow RuO4 (m.p. 25.40C, b.p. 4O0C) is isomorphous with OsO4; electron diffraction measurements indicate that it is tetrahedral in the gas phase (Ru-O 1.706 A) [48a]. It is light sensitive and thermodynamically unstable with respect to RuO2 (from which, however, it can be made) and can be explosive. Because of the lesser stability of ruthenium(VIII) compared with osmium (VIII), RuO4 is a stronger oxidizing agent than OsO4 (and therefore less selective); solutions in CCl4 are stable [48b]. A convenient synthesis involves periodate oxidation of RuCl3 or RuO2: RuCl3 or RuO2 Nal 4 ° > RuO4 RuO4 reacts with pyridine to form RuO3(py), probably a dimer Py2(O)2Ru(//-O)2Ru(O)2Py2, an aerobically assisted oxidant [48c]. RuO2 can be made by high-temperature oxidation of ruthenium. It has the rutile structure (Ru-O 1.942 A and 1.984 A) and forms blue-black crystals [49b]. Recently RuO3 has been made as a brown solid by photolysis: RuO4 370-440 nm > RuO3 In matrices, RuO2 is bent (149°) while RuO3 is trigonal planar. Copper-coloured OsO2 also has the rutile structure: it can be made from the metal and NO at 65O0C. OsO4 is obtained on oxidation of any osmium compound or by direct synthesis at 300-80O0C from the elements [5O]. Its solubility in CCl4 and volatility make it easy to purify; it forms pale yellow crystals (m.p. 40.460C, b.p. 1310C). Like RuO4 it forms tetrahedral molecules with Os-O0 1.684-1.710A, O-Os-O 106.7-110.7° in the solid state; Os-O 1.711 A in the gas phase [51]. It is soluble in water as well as in CCl4 and is very toxic (TLV 2.5ppm), affecting the eyes. (Its use as a biological stain involves its reaction with tissue.) Gas-phase vibrational data for OsO4 are ^1 — 965.2, V2- 333.1, 1/3 = 960.1 and i/4 = 322.7cm"1. Photoelectron spectra have been interpreted with a MO scheme, shown in Figure 1.11 [52]. OsO4 will add to C=C bonds but will only attack the most reactive aromatic bonds; thus benzene is inert, but it will attack the 9,10 bond in phenanthrene and will convert anthracene to 1,2,3,4-tetrahydroxytetrahydroanthracene. It can be used catalytically in the presence of oxidizing agents such as NaClO3 or H2O2 [53]. Figure 1.11 A molecular orbital diagram for OsC^. (Reprinted with permission from Inorg. Chem., 1992, 31, 1588. Copyright American Chemical Society.) L4.1 Anions Alkalis reduce RuO4 to RuO4 ; various salts have been prepared RuCl3.xH20 -^U RuO4 -^^ Pr4NRuO4 K2CO3 On heating it decomposes in a similar manner to KMnO4: 2KRuO4 -» K2RuO4 + RuO2 H- O2 The anion in KRuO4 has a slightly flattened tetrahedral structure (Ru-O 1.73A). Organic-soluble salts like Pr4NRuO4 are selective mild oxidants that will oxidize alcohols to carbonyl compounds but will not affect double bonds [54a]. ESR indicates that RuO4 (gx = 1.93; gy = 1.98; gz = 2.06) has a compressed tetrahedral geometry with the electron in dz2 [54b]. RuO4", which is believed to be tetrahedral in solution, is formed from RuO4 and excess concentrated aqueous KOH, isolable as black crystals of K2RuO4.H2O, which is actually K2[RuO3(OH)2]. The anion has a tbp structure with axial OH groups (Ru=O 1.741-1.763 A, Ru-OH 2.0282.040 A) [55]. In contrast to ruthenium, osmium exists in alkaline solution as OsO4(OH)2", believed to be cis and isolable as crystalline salts: OsO4 H- 2KOH -> K2OsO4(OH)2 Similarly, instead of forming OsO4", reduction of OsO4 with ethanolic KOH yields K2[OsO2(OH)4]. The osmiamate ion, OsO3N", is isoelectronic with OsO4. The yellow potassium salt is the most convenient one to prepare; other, less soluble, salts, can be made by metathesis: OsO4 + NH3 -^* KOsO3N ICOl-T The crystallographic study of the potassium salt is complicated by disorder but in CsOsO3N Os=N is 1.676 A and Os=O 1.739-1.741 A. Assignments of the vibrational spectrum OfOsO3N" is assisted by isotopic substitution: the higher frequency absorption is shifted significantly on 15N substitution whereas the band just below 900cm"1 is scarcely affected (Table 1.7); conversely the latter band is shifted by some 50cm"1 on replacing 16O by 18O [56]. Nitrido salts are discussed later (section 1.12.2). Table 1.7 Vibrational data for osmiamate ions (in cm"1) V1 (Os=N) OsO3NOsO315N" K [Os18O3N]1029 998 1024 U2 (Os=O) 898 896 844 1.5 Other binary compounds Ruthenium and osmium form no stable binary hydrides, but very recently heating mixtures of the metals with alkaline earth metal hydrides under pressure in a hydrogen atmosphere have been shown to give oxygen- and moisture-sensitive hydrides M 2 RuH 6 (M = Mg, Ba), M2OsH6 (M = Mg to Ba) and Li4OsH6. Thesoe contain MH6" (K2PtCl6 structure) with Ru-D 1.673 and Os-D 1.682 A in the corresponding deuterides [57]. LiMg2RuH7 has RuH6" with Ru-D 1.704 A in the deuteride. The mineral laurite is the mixed sulphide (Ru5Os)S2; this and RuS2 and OsS2 have the pyrite structure as does RuQ2 (Q = Se, Te). These can be made from the reaction of the chalcogen with the metals, while RuCl3 will also react with Se and Te. MP2, MAs2 and MSb2 all have a compressed form of the marcasite structure, while the carbides MC have trigonal prismatic coordination in the WC structure. Several borides are known: MB2 has nets of boron atoms. Ru11B8 has branched chains while Ru7B3 has isolated borons. 1.6 Aqua ions [58] Diamagnetic, low-spin d6 Ru(H2O)6+ has been made by reduction of RuO4 with activated Pb (or Sn) followed by ion-exchange purification. The pink tosylate salt contains octahedral Ru(H2O)6+ (Ru-O 2.122 A); though the solid is air stable, it is readily oxidized in solution by oxygen and ClO^. The hexaqua ions also occur in the red diamagnetic Tutton salts M2Ru(H2O)6(SO4):, (M = NH4, Rb) RuO4 -^ R u(H 2 O) 2+ ^lM^(NH4)2Ru(H20)6(S04)2 Ru (H2O)6(BF4)2 has been isolated but decomposes on standing. Aerial oxidation of Ru(H2O)6+ produces lemon-yellow Ru(H2O)6+ (Ru-O 2.029 A in the tosylate salt) Ru(H 2 O)^ + -» R u(H 2 O)^ + + e~ E^ = -0.205 V The yellow alum CsRu(H2O)6(SO4)2.6H2O has also been synthesized with /xeff = 2.20//B at 30OK; the Ru-O distance is 2.010 A. Vibrational spectra of octahedral Ru(H2O)6+ (« = 2, vv = 424 cm"1, 1/3 = 426cm"1; n = 3, V1 — 532cm"1, i/3 = 529cm"1) have been interpreted in terms of the force constants l ^lmdynA" 1 (n — 2) and 2 .98mdynA" 1 (n — 3), showing a stronger bond for the ruthenium(III) species. The ruthenium(II) aqua ion reacts with nitrogen at room temperature under high pressure (200 bar) forming yellow-brown [Ru(H2O)5N2J2+, isolated as a tosylate salt, showing z /(N=N) at 2141 cm"1 in its IR spectrum [59]. The ruthenium(IV) aqua ion, best made by electrochemical oxidation of Ru(H2O)^+, but also made by the reaction of RuO4 with H2(VHClO4, is tetranuclear, formulated as [Ru4O6(H2O)12J4+, though this may be protonated [6O]. FAB mass spectra of a pyrazolylborate complex show Ru4O6-containing fragments. No simple osmium aqua ion has been definitely isolated and characterized, though in alkaline solution (and the solid state) the osmium(VIII) species OsO4(OH)2" is well characterized (sections 1.4.1 and 1.12.1). Osmium(II) is probably too reducing to exist as Os(H2O)6+, but Os(H2O)6+ and a polynuclear Os+(aq.) species are likely. 1.7 Compounds of ruthenium(O) Apart from Ru(CO)5 and other carbonyls, there are mixed carbonylphosphine species and a few simple phosphine complexes like Ru(PF3)5 and Ru[P(OMe)3J5 [6Ia]. Photochemistry of Ru(CO)3 (PMe3)2 and the ruthenium(II) compound Ru(CO)2(PMe3)2H2 in low-temperature matrices affords [Ru(CO)2 (PMe3 ) 2 ---S] (S = Ar, Xe, CH4) [6Ib]. These monomers all have 18-electron tbp structures. The phosphine complex Ru(dmpe)2 has been studied in matrices [62]. Ru(diphos)2 (diphos = depe, dppe, (C2F5)2P(CH2)2P(C2F5)2) has similarly been formed by photolysis of Ru(diphos)2H2 in low-temperature matrices. They probably have square planar structures and undergo oxidative addition with cobalt, C2H4 and hydrogen [63]. Additionally a number of nitrosyls such as Ru(NO)2(PPh3)2 (section 1.8.5) exist. 1.8 Complexes of ruthenium(II and III) Because of the relationship between compounds in the adjacent oxidation states 4-2 and -f-3, they are grouped together here; the section is subdivided by ligand, concentrating on some classes of complex important in their diversity and in current research interest. 1.8.1 Ammine complexes Orange Ru(NH3)Jj+ can be obtained by various routes (see Figure 1.12). As expected for the +2 state of a heavy metal, it is reducing: Ru(NH 3 )^ + -> R u(NH 3 )^ + + e~ E^ = +0.214 V Historically, the most important ruthenium(II) ammine species is [Ru(NH3)5N2]2+, the first stable dinitrogen complex to be isolated (1965). It was initially obtained by refluxing RuCl3 in hydrazine solution (but many Next Page ...
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04136_01a - 1 Ruthenium and osmium 1.1 Introduction...

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