Pyridine (C 5 H 5 N) is being the simplest six-membered heterocycles, closely resembles its structure to benzene. The “N” in benzene ring has its high electronegativity influence on resonance environment and produces markedly different chemistry from its carbon analog. The presence of nitrogen and its lone pair in an aromatic environment makes pyridine a unique substance in chemistry. The sp lone pair orbital of “N,” directed outward the ring skeleton, is well directed to have overlap with vacant metal orbital in producing an σ bonding interaction. This causes pyridine to be a ligand and has been utilized with all transition metals in producing the array of metal complexes. A rich literature of metal complexes is now available with pyridine and its derivatives. Innumerable complexes have been synthesized with academic as well as industrial importance. To shed a light on ligating capability of pyridine, transition metal complexes with pyridine, and its derivative is presented in this chapter.

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Pyridine: A Useful Ligand in Transition Metal Complexes

Publisher Summary The cyanate, thiocyanate, and selenocyanate ions are pseudohalides (106) and have also been called chalcogenocyanates. They have the general formula NCX - (X = 0, S, Se, or Te) and they are all potentially ambidentate— that is, they can form a coordinate bond to a Lewis acid through either N or X. Thus, the thiocyanate ion, for example, will form either N- or S-bonded complexes depending on the nature of the metal, and this preference may be modified by the presence of other ligands or by whether the complex is in the solid state or in solution. The chalcogenocyanates can also be present in a variety of bridging modes. There are many examples of such varied coordination behavior of the thiocyanate ion, and several different, sometimes conflicting, explanations have been put forward. The remaining chalcogenocyanates have not been studied to the same extent, but they also show similar characteristics and have attracted a largely similar set of explanations.

Coordination Chemistry of the Cyanate, Thiocyanate, and Selenocyanate Ions

Several nickel(II) complexes containing pyridine (py), pyrazine (pyz) or methylpyrazine (mepyz) have been synthesized and characterized by vibrational and electronic spectroscopy, differential scanning calorimetry, and magnetic susceptibility studies to cryogenic temperatures. A comparison of the magnetic properties of the polymeric diazine-bridged complexes, Ni(pyz)2X2 (X = Cl or NO3), Ni(pyz)(p-CH3C6H4SO3)2, Ni(mepyz)(NO3)2, and Ni(pyz)3(CH3SO3)2•CH3OH with those of the related monometallic systems, Ni(py)4X2 (X = Cl, p-CH3C6H4SO3 or CH3SO3) and Ni(mepyz)4(NO3)2•H2O provides evidence for weak antiferromagnetic coupling between metal centers mediated by bridging diazine ligands in the former group of compounds. The magnetic properties were analyzed employing a model for S = 1 which takes into account zero-field splitting and employs a molecular field term to account for weak magnetic exchange. The compounds Ni(pyz)Cl2 and Ni(py)Cl2 show metamagnetic behaviour with critical fields of 13 and 2 kOe, respect...

Antiferromagnetism and metamagnetism in 1,4-diazine and pyridine complexes of nickel(II)

The stepwise thermal decomposition of [Ni(py)4]Cl2 was studied by means of a derivatograph. The stepwise mode of thermal decomposition allowed the preparation of new complexes. X-ray powder diffraction and IR methods were used to study the structures of the decomposition intermediates. [Ni(py)4]Cl2 was applied as a model substance. The decompositions of a great number of complex compounds of similar type have been reported, but only a few of them have been investigated structurally by single-crystal X-ray diffraction techniques due to the difficulty of obtaining large single-crystals.

Characterization of [Ni(Py)4]Br2 and its thermal decomposition

Inclusion compounds are very interesting materials for thermoanalytical investigations. Stable guest molecules are packed in the cages, channels or layers in the host matrix, thermal dissociation reactions are reversible and have the clear-cut stoichiometry. The clathrates with a matrix of coordination compounds (of the type [MA 4 X 2 ].nA) were synthesized (M=transition metals; A= Py, 4-MePy; X=NO 3 , NCS, NCO; n=0.67 or I or 2). Four Py or MePy molecules are tightly bound in the coordination sphere by M-N bonds and others are confined in the matrix cavities by van der Waals forces. Processes of thermal dissociation of the clathrates were studied. Q-thermogravimetry (for checking the stable intermediate phases) and the home-made gas-flow reactor (as EGA-device for kinetic studies) were used. The compensation dependence (IgA i =aE i +b) was observed for the most of clathrates series, it was explained by the identity of the reaction mechanism.

Processes of thermal dissociation of clathrates on the base of coordination compounds

On the basis of the thermal investigation of MpynX
 z
 mixed complexes (where M=Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+; py=pyridine; orα-, Β- andγ-picoline; X=Cl−, Br−, I−, OCN−, SCN−, NO
 3
 −
 , SO
 4
 2−
 ; andn=2,3,4,6;z=1,2), factors influencing the stability of the metal-pyridine-N bond have been determined. On the basis of the measured magnetic susceptibilities the symmetry of the coordina tion sphere of the central atom has been established in the intermediate products formed by the thermal decomposition of cobalt(II) complexes.

é. Mocsári-Fülöp

Papers

Thermal analysis of metal complexes, III