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.

Reaction of 2-(phenylazo)pyridine (pap) with [Ru(PPh₃)₃X₂] (X = Cl, Br) in dichloromethane solution affords [Ru(PPh₃)₂(pap)X₂]. These diamagnetic complexes exhibit a weakdd transition and two intense MLCT transitions in the visible region. In dichloromethane solution they display a one-electron reduction of pap near − 0.90 V vs SCE and a reversible ruthenium(II)-ruthenium(III) oxidation near 0.70 V vs SCE. The [Ru^III(PPh₃)₂(pap)Cl₂]⁺ complex cation, generated by coulometric oxidation of [Ru(PPh₃)₂(pap)Cl₂], shows two intense LMCT transitions in the visible region. It oxidizes N,N-dimethylaniline and [Ru^II(bpy)₂Cl₂] (bpy = 2,2′-bipyridine) to produce N,N,N′,N′-tetramethylbenzidine and [Ru^III(bpy)₂Cl₂]⁺ respectively. Reaction of [Ru(PPh₃)₂(pap)X₂] with Ag⁺ in ethanol produces [Ru(PPh₃)₂(pap)(EtOH)₂]²⁺ which upon further reaction with L (L = pap, bpy, acetylacetonate ion(acac⁻) and oxalate ion (ox²⁻)) gives complexes of type [Ru(PPh₃)₂(pap)(L)]ⁿ⁺ (n = 0, 1, 2). All these diamagnetic complexes show a weakdd transition and several intense MLCT transitions in the visible region. The ruthenium(II)-ruthenium(III) oxidation potential decreases in the order (of L): pap > bpy > acac⁻ > ox²⁻. Reductions of the coordinated pap and bpy are also observed.

Salicylaldehyde thiosemicarbazone (H₂saltsc) reacts with [M(PPh₃)₃X₂] (M = Ru, Os; X = Cl, Br) to afford complexes of type [M(PPh₃)₂(Hsaltsc)₂], in which the salicylaldehyde thiosemicarbazone ligand is coordinated to the metal as a bidentate N,S-donor forming a four-membered chelate ring. Reaction of benzaldehyde thiosemicarbazones (Hbztsc-R) with [M(PPh₃)₃X₂] also affords complexes of similar type, viz. [M(PPh₃)₂(bztsc-R)₂], in which the benzaldehyde thiosemicarbazones have also been found to coordinate the metal as a bidentate N,S-donor forming a four-membered chelate ring as before. Reaction of the Hbztsc-R ligands has also been carried out with [M(bpy)₂X₂] (M = Ru, Os; X = Cl, Br), which has afforded complexes of type [M(bpy)₂(bztsc-R)]⁺, which have been isolated as perchlorate salts. Coordination mode of bztsc-R has been found to be the same as before. Structure of the Hbztsc-OMe ligand has been determined and some molecular modelling studies have been carried out determine the reason for the observed mode of coordination. Reaction of acetone thiosemicarbazone (Hactsc) has then been carried out with [M(bpy)₂X₂] to afford the [M(bpy)₂(actsc)]ClO₄ complexes, in which the actsc ligand coordinates the metal as a bidentate N,S-donorformingafive-membered chelate ring. Reaction of H₂saltsc has been carried out with [Ru(bpy)₂Cl₂] to prepare the [Ru(bpy)₂(Hsaltsc)]ClO₄ complex, which has then been reacted with one equivalent of nickel perchlorate to afford an octanuclear complex of type [Ru(bpy)₂(saltsc-H)₄Ni₄](ClO₄)₄.

Reaction of N-(2′-hydroxyphenyl)pyrrole-2-aldimine (H₂L) with [M(PPh₃)₃Cl] (M = Rh, Ir) affords complexes of type [M(PPh₃)₂(L)Cl]. Structures of both complexes have been determined by X-ray crystallography. In both complexes, the N-(2′-hydroxyphenyl)pyrrole-2-aldimine ligand is coordinated to the metal centre, via dissociation of the acidic protons, as a dianionic tridentate N,N,O-donor. The M(L)Cl fragment constitutes an equatorial plane with the metal at the centre and the two PPh₃ ligands occupying axial positions. The complexes are diamagnetic (low-spin d⁶, S = 0) and show intense MLCT transitions in the visible region. Cyclic voltammetry of the [M(PPh₃)₂(L)Cl] complexes shows reversible M(III)-M(IV) oxidation near 0·5 V vs SCE and quasi-reversible ligand-centred oxidative response near 1·0 V vs SCE.

Reaction of 2-(2'-carboxyphenylazo)-4-methylphenol (H₂L) with [M(PPh₃)₂Cl₂] (M = Pd, Pt) affords mixed-ligand complexes of type [M(PPh3)(L)]. Structures of both the complexes have been determined by X-ray crystallography. Both the complexes are square planar, where the 2-(2'-carboxyphenylazo)-4-methylphenol is coordinated to the metal center, via dissociation of the two acidic protons, as a dianionic tridentate O,N,O-donor, and the fourth position is occupied by the triphenylphosphine. These complexes show intense MLCT transitions in the visible region.

Reaction of 1,3-diaryltriazenes (abbreviated in general as HL-R, where R stands for the para-substituent in the aryl fragment and H stands for the dissociable hydrogen atom, R = OCH₃, CH₃, H, Cl, NO₂) with [Rh(PPh₃)₂(CO)Cl] in ethanol in the presence of NEt₃ produces a series of trisdiaryltriazenide complexes of rhodium of type [Rh(L-R)₃], where the triazenes are coordinated to rhodium as monoanionic, bidentate N,N-donors. Structure of the [Rh(L-OCH₃)₃] complex has been determined by X-ray crystallography. The complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. They also fluoresce in the visible region under ambient condition while excited at around 400 nm. Cyclic voltammetry on these complexes shows a Rh(III)-Rh(IV) oxidation (within $0.84-1.67$ V vs SCE), followed by an oxidation of the coordinated triazene ligand (except the R = NO₂ complex). An irreversible reduction of the coordinated triazene is also observed for all the complexes below $-1.03$ V vs SCE.

Reaction of 2-(naphthyl-1'-azo)-4-methylphenol with [Ir(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt3) affords an organoiridium complex of type [Ir(PPh₃)₂(L)(H)], where L represents the coordinated 2-(naphthyl-1'-azo)-4-methylphenolate ligand. A similar reaction carried out in toluene affords the [Ir(PPh₃)₂(L)(H)] complex along with a similar complex of type [Ir(PPh₃)₂(L)Cl]. Structures of both the [Ir(PPh₃)₂(L)(H)] and [Ir(PPh₃)₂(L)Cl] complexes have been determined by X-ray crystallography. In both the complexes, 2-(naphthyl-1'-azo)-4-methylphenol is coordinated to iridium, via C-H activation at the 2' position of the naphthyl ring, as a dianionic tridentate C, N, O-donor and the two triphenylphosphines are trans. The organoiridium complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ir(PPh3)2(L)(H)] and [Ir(PPh3)2(L)Cl] complexes shows a reversible Ir(III)-Ir(IV) oxidation respectively at 0.55 and 0.73 V vs SCE. An irreversible oxidation of the coordinated 2-(naphthyl-1'-azo)-4-methylphenolate ligand is observed above 1.0 V vs SCE and an irreversible reduction of the same is observed near $-1.0$ V vs SCE.

Reaction of a group of N-(aryl)picolinamides (pic-R) with [Ru(PPh₃)₂(CO)₂Cl₂] in refluxing 2-methoxyethanol in the presence of a base affords hydrido complexes of two types (1-R and 2-R), which are geometric isomers. Similar reaction with N-(naphthyl)picolinamide (pic-nap) yields an organoruthenium complex (3) via formation of a hydrido intermediate. Reaction of 2-(arylazo)phenols (ap-R) with [Ir(PPh₃)₃Cl] in refluxing ethanol affords a mono-hydrido intermediate (4-R), a di-hydrido intermediate (5-R) and an organoiridium complex (6-R) as the final product, where the azo-ligand is coordinated as CNO-donor. Reaction of ap-R ligands with [Rh(PPh₃)₃Cl] yields organorhodium complexes (7-R) analogous to 6-R, but without any hydrido intermediate. N-(2'-hydroxyphenyl)benzaldimines (hpbz-R) react with [Rh(PPh₃)₃Cl] to yield a group of organorhodium complexes (8-R), where the hpbz-R ligands are coordinated in CNO-fashion. Upon interaction with [Ir(PPh₃)₃Cl] 2-(2',6'-dimethylphenylazo)-4-methylphenol (dmap) undergoes a methyl C-H activation and affords organoiridium complex 9, while 2-(2'-methylphenylazo)-4-methylphenol (mmap) undergoes a phenyl C-H activation and gives organoiridium complex 10. Reaction of benzaldehyde thiosemicarbazones (bztsc-R) with [Pd(PPh₃)₂Cl₂], carried out with the expectation of inducing CNS-mode of coordination, actually has yielded complexes (11-R) where the bztsc-R is coordinated in an uncommon NS-mode forming a fivemembered chelate ring associated with a restricted rotation around the imine (C=N) bond. These palladium complexes are found to catalyse C-C cross coupling reactions very efficiently. Crystal structures of selected complexes of each type have been determined by X-ray crystallography.

Reaction of pyrrole-2-aldehyde thiosemicarbazone (abbreviated as H₂L, where H₂ stands for the two potentially dissociable protons) with [Pd(PPh3)2Cl2] in ethanol in the presence of NEt3 afforded two complexes, [Pd(PPh₃)(HL_NS)Cl] and [Pd(PPh3)(LNNS)], where the thiosemicarbazone ligand is coordinated to the metal centre respectively as monoanionic N,S-donor (depicted by HL_NS) and dianionic N,N,S-donor (depicted by L_NNS). Similar reaction with Na2[PdCl4] afforded a bis-complex, [Pd(HL_NS)₂]. Crystal structures of all the three complexes have been determined.With reference to the structure of the uncoordinated thiosemicarbazone (H₂L), the N,S-coordinationmode observed in [Pd(PPh₃)(HL_NS)Cl] and [Pd(HL_NS)₂] is associated with a geometrical change around the imine bond.While the N,N,S-mode of binding observed in [Pd(PPh₃)(L_NNS)] takes place without any such geometrical change. All three complexes display intense absorptions in the visible and ultraviolet regions, which have been analyzed by TDDFT method.