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The synthesis of the highly encapsulating pyrazolylborate ligand hydrotris(3-p-cumenyl-5-methylpyrazolyl)borate (L = Tp(Cum,Me)) and of its zinc hydroxide complex L.Zn-OH (1) are described. 1 is converted by H(2)S into the hydrosulfide complex L.Zn-SH (2). Both 1 and 2 seem to be contaminated with traces of the isomeric species 1' and 2' containing L' with one 3-methyl-5-p-cumenyl substituent. Thermal condensations of 1' and 2 yield the molecular zinc oxide and sulfide complexes L'.Zn-O-Zn.L' (3') and L.Zn-S-Zn.L (4). The hydroxide complex 1 has been found to react readily with cumulated double-bonded species: CO(2) is incorporated in alcoholic solutions to form the alkylcarbonate complexes L.Zn-OCOOR (5). Similarly, CS(2) in ethanol forms the O-ethyl dithiocarbonate complex L.Zn-SC(S)OEt (6). SO(2) is converted to a bridging sulfito ligand in L.Zn-O-SO-O-Zn.L (7), and phenyl isothiocyanate is bound as a thiocarbamidato ligand in L.Zn-SC(O)NHPh (8). Complexes 1, 2, 2', 3', 4, 5, and 6 have been confirmed by structure determinations and complexes 7 and 8 by spectral data.

The synthetic and structural chemistry as well as some physical and chemical properties of cyanide-bridged complexes containing two or more metal building blocks are discussed. Emphasis is laid on molecular shape and design and their consequences, including the effects of cyanide-isocyanide isomerism. Redox phenomena and molecular magnetism have been dealt with in relation to electronic communication across the cyanide bridges and metal-metal charge transfer. Long-range interactions between remote metal centers are discussed, and the few cases where they lead to remote chemical reactions are listed.Prussian Blue, [Fe 4 {Fe(CN) 6 } 3 ]ؒxH 2 O, which actually is the oldest known co-ordination compound, 1 has attracted the skills of many scientists seeking an understanding of its formation and composition, 2 structure, 3 colour 4 and physical properties like magnetism 5 and electrical conductivity. 6 Although it was soon obvious that the linking of two different metal ions by the cyanide ligand is the basis of all these phenomena, this did not initiate a variation of the theme. The systematic synthetic and physical exploration of Prussian Blue-like compounds had to await the popularity of the chemistry of materials. Only in recent years has a number of attractive two-and three-dimensional co-ordination polymers comprised of M᎐CN᎐MЈ units been prepared and subjected to the appropriate measurements. 7 While bulk magnetism and conductivity require extended interactions their basic fundamental property, the electronic communication between two metals across the cyanide bridge, is local and can be studied in simple dinuclear complexes. But classical co-ordination chemistry was not interested in this phenomenon, and hence the literature of more than 25 years ago contains only rare examples of cyanide-bridged systems 8 amongst the multitude of ligand-bridged dinuclear complexes. 9 It took the emerging discussion of mixed valence 10-12 and innersphere electron transfer 13,14 to move the M᎐CN᎐MЈ systems into the limelight. They were among the first to be studied in this respect, 15,16 and since then the electronic interactions between two metals across a cyanide bridge have proven to be a fertile area of research.In the last 15 years many di-and tri-nuclear complexes with bridging cyanide ligands have been investigated for the electronic communication and the electron transfer between their metal centers by means of electronic spectroscopy, electrochemistry, IR spectroscopy, fast kinetic methods and molecular orbital (MO) theory. Among others, the research groups of

A series of new pyrazolylborate-zinc-thiolate complexes Tp(Ph,Me)Zn-SR and Tp(Me,Me)Zn-SR, including two homocysteine derivatives, were prepared and structurally characterized. Their reactions with methyl iodide in nonpolar media resulted in the formation of the thioethers MeSR, including two methionine derivatives, and Tp(R',Me)Zn-I in all cases. Methylation of the thiolates could also be achieved with dimethyl sulfate and trimethylsulfonium iodide but not with trimethyl phosphate or N-methylpyridinium salts. The accumulated evidence indicates that the methylation occurs intramolecularly, i.e., at the zinc-bound thiolates: (i) The reactions occur readily in nonpolar media. (ii) Thiolate exchange at Tp(Ph,Me)Zn-SR with [PPN]SR' is slower than thiolate alkylation. (iii) The methylation of Tp(Ph,Me)Zn-SBn with MeI is a clean second-order reaction with k'' = 1.75 x 10(-2) M(-1) s(-1) at 300 K.

The 1-substituted tris(2-thioimidazolyl)hydroborate ligands Tt(R) were prepared as the potassium salts from KBH(4) and the corresponding 1-R-2-thioimidazole for R = t-Bu and C(6)H(4)-p-CH(CH(3))(2) (Cum). Their reactions with zinc salts yielded the tetrahedral complexes Tt(R)Zn-X with X = F, Cl, ONO(2) and (Tt(t)()(-)(Bu))(2)Zn. With zinc perchlorate the labile perchlorate complexes Tt(R)Zn-OClO(3) were obtained. They served as starting materials for the incorporation of substrates which are relevant for the chemistry of horse liver alcohol dehydrogenase: Ethanol led to [Tt(t)()(-Bu)Zn.EtOH] ClO(4).EtOH, p-nitrophenol (NitOH) yielded Tt(Cum)Zn-ONit. Pyridine-2-carbaldehyde and salicylic aldehyde were incorporated as N(pyridine) and O(phenolate) coligands with possible additional O(aldehyde) coordination. Substituted pyridyl methanols (R-PyCH(2)OH) yielded the trinuclear complexes [(Tt(t)()(-Bu))(2)Zn(3)(R-PyCH(2)O)(2)] (ClO(4))(2) with bridging Tt and pyridylmethoxide ligands. Preliminary experiments on the functional modeling of alcohol dehydrogenase have shown that TtZn complexes promote both the dehydrogenation of 2-propanol and the hydrogenation of pentafluorobenzaldehyde.

Cyanide-bridged trinuclear complexes containing square-planar platinum at the center were synthesized and identified by structure determinations. Their chemical building blocks were cis- and trans-PtL2X2, Cp(dppe)FeX, Cp(PPh3)2RuX (with X = Hal or CN), Pt(CN)4 2-, and Fe(CN)6 3-. Several of the intermediate dinuclear complexes were isolated and used for reference purposes. The molecular structures, the ν(CN) IR data, the cyclic voltammograms, and the UV−vis−near-IR spectra were used to probe the effects of the geometry at platinum (cis vs trans), of cyanide−isocyanide isomerism (Pt(CN)2 vs Pt(NC)2), of the nature and ligation of the terminal metals (Fe vs Ru), and of one- or two-electron oxidation. The redox properties and the observed intervalence transfers indicate that there is electronic communication between the outer metal atoms along the linear M−CN chains containing trans-configured platinum, but not along the bent chains containing cis-configured platinum.

Dedicated to Professor Helmut Behrens on the occasion of his 60th birthdayThe types of sulfur bonding-as sulfane or sulfide-encountered in the molecules of maingroup elements are almost unknown in the chemistry of metal complexes, where the sulfur atoms function instead as two-electron donors by bridging two metal atoms, as four-electron donors by bridging three or four metal atoms, or as six-electron donors by incorporation between four metal atoms. In such complexes, the metal-metal bond can be modified over a wide range by chemical or electrochemical variation of the number of electrons present. The readiness with which polynuclear complexes containing metals and sulfur undergo redox reactions is also utilized by Nature in the active sites of some redox proteins.

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