Edoardo Zinato

Chromium(III) cyano-am(m)ine complexes

Abstract The review deals with various aspects of a relatively young family of mixed-ligand complexes comprising mono- and polynuclear species, the latter cyano- or isocyano-bridged. Compared with other chromium(III) acido-am(m)ines, these exhibit atypical ground-state and excited-state properties arising from coordination of the CN− groups. A systematization is presented of the preparative procedures, followed by a collection of the most relevant structural results. The electronic absorption and emission spectra are then discussed, and the solution reactivity (cyanide aquation and isomerization) is reviewed. Particular attention is devoted to the photochemistry and photophysics of these compounds. The uncommon ligand-field state sequences give access to photolabilization patterns and photostereochemistries that are complementary to the usual occurrence, providing significant test cases for the photolysis models. In addition, the long-lived solution phosphorescence makes possible accurate studies of the possible doubletstate decay routes, as well as of energy transfer in supramolecular systems. The available data are summarized in 11 tables, and reference to the literature is made by 117 citations.

Pentacyanochromate(III) complexes: ground- and excited-state chemistry

Abstract An overview is presented of the available information on pentacyanochromate(III) complexes, including some recent work in our laboratory. Several aspects of the chemistry of this little explored class of anionic compounds are considered, such as the preparative methods, the absorption and emission spectra, and some thermal aquation and anation reactions. The photochemistry is given special attention; various photoaquation and photoanation results are discussed in the light of current models.

Synthesis and solution properties of some trans-cyanoacidotetraamminechromium(III) complexes

Abstract The trans-[Cr(NH3)4(CN)X]+ ions, where X = NCS, F and Cl, have been prepared by reaction in methanol suspension of either trans-[Cr(NH3)4(H2O)(CN)]2+ with NCS− and F−, or trans-[Cr(NH3)4(Me2SO)Cl]2+ with CN−. Their perchlorate salts have been characterized by analysis, ion-exchange, conductance, absorption spectra, phosphorescence in room-temperature solution and aqueous chemistry. The solvent dependence of the ligand-field spectrum for X = F is ascribed to hydrogen bonding that affects the CrF interaction. Both π(X) → eg*(Cr) and t2g(Cr) → π*(CN) charge-transfer transitions are observed, consistent with the π-donor and π-acceptor nature of the two axial ligands. All these complexes undergo acid-catalysed CN− aquation. When X = Cl this competes with pH-independent loss of X. The aquation products, fully stereoretentive, confirm the trans isomerism of the new species. The anation and aquation behaviours reveal trans effects of the CN− ligand.

Photoaquation reactions of trans-chloro(dimethylformamide)tetraammine-chromium(III)

Abstract Upon ligand-field irradiation in acidic solution, the trans -Cr(NH 3 ) 4 (DMF)Cl 2+ cation undergoes simultaneous loss of DMF, Cl − and NH 3 . Photosolvation of DMF predominates at all excitation energies. From the wavelength dependence of quantum yields, the lower-lying 4 E state and the upper 4 B 2 state, are inferred to be the main precursors to aquation of the axial ligands and of NH 3 , respectively. The results are examined in the light of the current photolytic models. Differential spectrophotometry indicates that DMF and Cl − replacement takes place with complete trans → cis isomerization. Comparison with the photochemical behavior of trans -Cr(NH 3 ) 4 (H 2 O)Cl 2+ suggests that the photoisomerization of the latter occurs entirely through water exchange. Charge-transfer irradiation destabilizes the coordinated groups less selectively than ligand-field photolysis.

The photochemistry of some binuclear complexes of chromium(III)

Abstract Irradiation of the d-d absorption bands of trans -aquoerythro, [I], and trans -chloroerythro, [II], chromium (III) leads in both cases to the cis -aquoerythro ion with a quantum yield of 0.20, independent of the irradiation wavelength. A second binuclear complex is produced by NH 3 photoaquation of [II], with quantum yields of 0.16 and 0.30 for irradiation of the first (L 1 ), and second (L 2 ), ligand-field bands, respectively. The photoreactions are qualitatively different from the thermal paths which involve cleavage of the OH bridge. The reaction modes are those predicted by the chromium(III) photolysis rules and stereomobility is observed in at least one Cr III center. This behavior conforms to that found in analogous mononuclear species.

MERCURY(II)-PROMOTED FORMATION AND AQUATION OF LINKAGE ISOMERS OF TRANS-DICYANOTETRAAMMINECHROMIUM(III). A POLAROGRAPHIC AND SPECTRAL STUDY

Polarography and spectrophotometry reveal various interactions between trans-Cr(NH3)4(CN)2+ and Hg2+ in acidic aqueous solution. There is evidence for formation of two trinuclear adducts with Cr:Hg proportions of 2:1 and 1:2, accompanied by linkage isomerization (CrNCHg) of the cyanide bridging groups. These processes are followed by Hg2+-induced aquation of CN−. The six Cr(III) species occurring in the reaction sequence are identified by their half-wave reduction potentials. The trans configuration of the final aquation product, Cr(NH3)4(H2O)23+, indicates complete stereoretention during all the reactions, consistent with the observed stoichiometry of CrHg association which differs from that of a cis-dicyano analogue. The linkage isomerization of trans-Cr(NH3)4(CN)2+ is much faster than that of trans-Cr(NH3)4(H2O)(CN)2+, while the rates of the respective Hg2+-promoted aquation reactions are the same, denoting trans effects of CN− in the former process but not in the latter.

Interaction of cyanopentaamminechromium(III) with mercuric ion. Electrochemical, spectral and preparative study of adduct formation and linkage isomerism

Abstract The reaction of Cr(NH 3 ) 5 (CN) 2+ with Hg 2+ was investigated in aqueous medium by various electrochemical techniques, as well as by absorption and emission spectrophotometry. Evidence was found for formation in solution of a stable 2:1 adduct, the polarographic and spectral properties of which indicate linkage isomerization (CrNCHg) of the cyano bridging group. In addition, two solids containing 4:1 and 2:1 Cr:Hg adduct species were isolated and characterized. The kinetics of adduct formation and subsequent Hg 2+ promoted CN − aquation were studied by monitoring the phosphorescence of cyanopentaamminechromium(III) end by polarography, respectively. The former reaction obeys the rate law: −d[Cr]/d t = k 1 [Cr]+ k 2 [Cr] 2 , with k 1 independent of the [Hg 2+ ]:[Cr] ratio in the 5–700 range, and k 2 decreasing as this ratio increases. The aquation rate is independent of both [Hg 2+ ] and [H 3 O + ]. The factors affecting the two reaction stages are briefly discussed.

Preparation of some dimethylformamide- and dimethylsulfoxide-Cr(III)-ammine complexes and aquation kinetics of Cr(NH3)5(DMF)3+ and trans-Cr(NH3)4(DMF)Cl2+

Abstract The Cr(NH 3 ) 4 L 3+ , trans -Cr(NH 3 ) 4 LCl 2+ , and trans -Cr(NH 3 ) 4 staggered3+ (L = DMF, DMSO) complexes have been synthesized and characterized by IR and electronic spectroscopy. The solvent influence on the ligand-field absorption spectra is described. The aquation kinetics of Cr(NH 3 ) 5 (DMF) 3+ and trans -Cr(NH 3 ) 4 (DMF)Cl 2+ have been investigated for various acidities and ionic strengths. For dimethylformamidepentaamminechromium(III) the only reaction is replacement of DMF by water. The trans -chloro(dimethylformamide) species undergoes simultaneous release of DMF and Cl − , with comparable rates and with retention of configuration. Successive chloride loss from trans -Cr(NH 3 ) 4 (H 2 O)Cl 2+ complicates the kinetic study. Despite the different charge and ligand environment, the DMF aquation parameters are virtually identical for the two complexes.

A SPECTROPHOTOMETRIC AND ELECTROCHEMICAL STUDY OF THE COMPETITIVE INTERACTIONS OF MERCURY(II) WITH TRANS-CYANOISOTHIOCYANATOTETRAAMMINECHROMIUM(III)

Abstract The reaction of trans -Cr(NH 3 ) 4 (NCS)(CN) + with Hg 2+ develops in three stages: (1) immediate CrNCSHg bonding; (2) formation of CrNCHg bridges at a measurable rate, occurring with cyano-isocyano rearrangement about Cr(III); (3) slower Hg(II)-promoted aquation of cyanide. The processes were studied by absorption and emission spectrophotometry, as well as by polarography and potentiometry. The identified species participating in this sequence are: (1) [ trans -(NC)Cr(NH 3 ) 4 (NCS)] 2 Hg 4+ and trans -Cr(NH 3 ) 4 (CN)(NCSHg) 3+ ; (2) [ trans -(SCN)Cr(NH 3 ) 4 (NC)] 2 Hg 4+ ; (3) trans -Cr(NH 3 ) 4 (H 2 O)(NCS) 2+ . The quotients for the consecutive equilibria of step (1), involving soft-acid/soft-base HgS interactions, are estimated as K 1 = (4±1) × 10 6 M −2 and K 2 = 50±40 at 25°C and μ = 0.10 M (NaClO 4 ). Under the same conditions, the pseudo-first-order rate constant for NC→NC linkage isomerization, measured by the disappearance of the phosphorescence of Cr(III) in process (2), is k 1 = (7.5±0.1) × 10 −3 s −1 . Aquation (3) is conducted to proceed in two steps, whereby Hg(II) causes successive detachment of cyanide from the two CrNCHg bridges, with rate constants k 1 = (5.9±0.7) × 10 −4 s −1 and k 2 = (1.8±0.2) × 10 −4 s −1 at 25°C and μ = 0.10 M. The trans configuration of the Cr(III) units is completely retained throughout all reactions. The absorption data place the Hg 2+ -bound ligands in the spectrochemical series in the order: NH 3 ∼NCHg + >NCSHg + >NCS − .

Solution phosphorescence of trans-cyanoacidotetraammine-chromium(III) complexes. Comparison with models for doublet-state deactivation

Abstract The trans -Cr(NH 3 ) 4 X(CN) 2+ ions, where X denotes CN − , NH 3 , NCS − , H 2 O, F − and Cl − , exhibit fairly intense and long-lived emissions compared with those of analogous species not containing cyanide ligands. The phosphorescence decay rates in acidic aqueous solution around room temperature, and their activation parameters, are examined for the whole series. The lifetime values at 20 °C vary between 40 and 0.26 μs, and obey Adamsons empirical emission rules. The activation energies are in the 25–55 kJ mol −1 range. The data are discussed in terms of the possible processes controlling the doublet-state lifetime under ambient conditions.