The structure and selected reactions of the title compounds are reviewed. Some highlights are as follows (L = conjugate base of phenylazoacetaldoxime). While PtL₂ exist in cis and trans forms, PdL₂ occurs only in thetrans form. In the crystalline state cis-PtL₂ consists of weakly held dimers (Pt. Pt 3.151 A) buttrans-PtL₂ consists of stacks of discrete molecules in the solid state.Trans- PdL₂ displays a surprisingly large tetrahedral distorsion. The different metal-ligand bonds undergo selective cleavage by different reagents. Thus HCl and tertiary phosphines respectively cleave the M-N (oxime) and M-N (azo) bonds. Phosphine cleavage leads to interesting situations such as cone angle-dependent equilibria and fluxional behaviour. A rich redox chemistry consisting of successive reductions of the azo groups and of the metal are revealed electrochemically.Cis-PtL₂ unliketrans- PtL₂ undergoes a facile one-electron oxidation at low potentials. Another very curious reaction of cis-PtL₂ is the process in which an aromatic ring is thermally hydroxylated at the expense of an oximato oxygen atom. The meaning and significance of the observations are discussed.

Theg-tensor theory of low-spind⁵ configuration in octahedral fields having axial (tetragonal or trigonal) and rhombic distorsions is summarised. Illustrations taken from literature concern the following ruthenium(III) species: Ru(H₂O)₆³⁺ (trigonal), Ru(bpy)₃³⁺ (trigonal), the Creutz-Taube cation (axial + rhombic) and RuCl₂(HL) (L) (axial + rhombic: HL = isonitrosoketone). The rationalisation of distorsion parameters in terms of bonding is considered.

Synthetic ferroverdins, NaFe¹¹(RQ)₃ afford trinuclear M[Fe(RQ)₃]₂ upon reaction with bivalent ions (M²⁺). A probable bonding mode is considered. Ferric ions oxidise ferroverdin to ferriverdin, Fe^III(RQ)₃. Bothfac andmer isomers can be observed for ferro- and ferriverdins but at equilibrium the preferred isomers arefac for ferro- andmer for ferriverdin. Mismatch in geometry with oxidation state gets spontaneously corrected by isomerisation. Equilibrium constants, isomerisation rates, redox potentials and EPR spectra are examined. A study on the pattern of isomer population as a function of oxidation for other 3d analogues of ferro- and ferriverdin is initiated.

The reaction of Mn(CH₃COO)₃ 2H₂O with the carboxyl-rich ligand pyridine-2,6-dicarboxylic acid (H₂L) in methanol affords a high-spin (S = 2) hydratedbis-complex. Structure determination has revealed the solid to be [Mn^III(H₂ L)(L)] [Mn^IIIL₂] 5H₂ O: space group P−1;Z = 2;a = 7.527(3)ų,b= 14.260(4)Å,c = 16.080(6)Å,α = 91.08(3)°,β = 103.58(3)°,γ= 105.41(3)° andV= 1611.2(10)ų. Each ligand is planar and is bonded in the tridentate O₂N fashion. The MnO₄N₂ coordination spheres show large distortions from octahedral symmetry. The lattice is stabilised by an extensive network of O…O hydrogen-bonding involving water molecules and carboxyl functions. Upon dissolution in water, protic redistribution occurs and the complex acts as the mono-basic acid Mn(HL)(L) (pK, 4.3 ±0.05). The deprotonated complex displays high metal reduction potentials: Mn^IVL₂-Mn^IIIL₂⁻, 1.05V; Mn^IIIL₂⁻Mn^IIL₂²⁻-, 0.28V vs. SCE

Reactions involving the coupled transfer of electrons and protons are called electroprotic reactions. In this article we briefly describe some of our experiences with electroprotic reaction as a tool for executing interesting chemical transformations.

The reaction of Schiff monobases of 2,6-diformyl-4-methylphenol with ruthenium(II) and osmium(II) has afforded a new family of orthometalated organometallics. Synthesis, characterization and reactivity are briefly discussed in this article.

The title transformations are oxygen atom transfer, twin isomerization and regiospecific imine oxidation.Bispyridyldiazole ligands have furnished new oxygen atom transfer reagents of coordination type Re^VOCl₃(NN) which undergo a slower transfer to PPh₃ than the corresponding azole reagents. The rate of twin isomerization (linkage and geometrical) of meridional azole complexes of coordination type Re^III(OPnP)Cl₃(NN) to facial Re^III(PnPO)Cl₃(NN) decreases rapidly asn increases in the interval 1–4 (PnP is Ph₂P(CH₂)_nPPh₂). An α-diimine chelate of type Re^V(NPh)Cl₃(NN) is shown to undergo facile oxidation to the corresponding iminoamide complex Re_VI(NPh)Cl₃(NN) upon treating with dilute nitric acid. The reaction proceeds via regiospecific nucleophilic addition of waterto the more polarized imine function.

The reaction of [RhCl₂(HPhL)(PhL)] with M^II(ClO₄)_2.6H₂O in presence of alkali has furnished trinuclear [RhCl₂(PhL)₂]₂M(H₂O)$_2.$H₂O (HPhL is phenylazobenzaldoxime; M = Mn, Co, Ni). A similar reaction with M^I(PPh₃)₂NO₃ yielded binuclear [RhCl₂(PhL)₂]M(PPh₃)₂ (M = Cu, Ag). In these molecules the oximato group acts as a bridge between Rh^III (bonded at N) and M^II or M^I (bonded at O). In structurally characterized [Rh^IIICl₂(PhL)₂]₂Mn(H₂O)₂.H₂O the centrosymmetric distorted octahedral MnO6 coordination sphere is spanned by four oximato oxygen atoms and two water molecules lying in trans position. In the lattice the neighbouring molecules are held together by H₂O$\cdots$H₂O$\cdots$H₂O hydrogen bonds generating infinite zigzag chains. The manganese atoms lie parallel to the C-axis, the shortest Mn…Mn distance being 7.992 Å. Magnetic exchange interactions if any are small as seen in room temperature magnetic moments. The manganese system displays a strong EPR signal near 𝑔 = 2.00. In the complex [RhCl₂(PhL)₂]Cu(PPh₃)₂ the copper atom is coordinated to two oximato oxygen atoms and the two phosphorus atoms in a distorted tetrahedral geometry. The softness of the phosphine ligand is believed to sustain the stable coordination of hard oximato oxygen to soft Cu^I. The coordination sphere of the Rh^III atom in both the complexes is uniformly trans-RhN₄Cl₂.