Heteroleptic ligands

The amido and imido derivates of phosphorus-chalcogen ligands have been of much interest to coordination chemists due to the fact that they possess both hard (nitrogen) as well as soft (chalcogen) donor sites and are therefore able to act as heteroleptic ligand systems.56... 

Heteroleptic complexes of uranium can be stabilized by the presence of the ancillary ligands however, the chemistry is dominated by methyl and benzyl ligands. Examples of these materials include UR4(dmpe) (R = alkyl, benzyl) and U(benzyl)4MgCl2. The former compounds coordinate "soft" chelating phosphine ligands, a rarity for the hard U(IV) atom. 

Another class of heteroleptic alkyl complexes contains TT-donating ancillary ligands such as RU[N(Si(CH2)3)2]3 (R = 4)- The hydride... 

A number of general features in Table 1-3 is apparent. Complexes may be cationic, neutral or anionic. Ligands may be simple monatomic ions, or larger molecules or ions. Many ligands are found as related neutral and anionic species (for example, water, hydroxide and oxide). Complexes may contain all of the same type of ligand, in which case they are termed homoleptic, or they may contain a variety of ligand types, whereby they are described as heteroleptic. Some ligands such as nitrite or thiocyanate can coordinate to a metal ion in more than one way. This is described as ambidentate behaviour. In such cases, we commonly indicate... 

This review deals with the chemistry and coordination complexes of isoelectronic analogues of common oxo-anions of phosphorus such as PO3, POl", RPOl" and R2POy. The article begins with a discussion of homoleptic systems in which all of the 0x0 ligands are replaced by imido (NR) groups. This is followed by an account of heteroleptic phosphorus-centered anions, including [RN(E)P(/<-NR )2P(E)NR]2-, [EP(NR)3]3-, [RP(E)(NR)2] and [R2P(E)(NR )] (E=0,S, Se, Te). The emphasis is on the wide variety of coordination modes exhibited by these poly-dentate ligands, which have both hard (NR) and soft (S, Se or Te) centers. Possible applications of their metal complexes include new catalytic systems, coordination polymers with unique properties, and novel porous materials. 

Tris(imido)phosphonates [R P(NR)3] and bis(imido)phosphinates [R 2P(NR)2] are not, strictly speaking, homoleptic anions. In this article, however, homoleptic refers to anions that contain only imido (NR) ligands, in addition to the H or R substituent directly attached to phosphorus (if any), whereas the term heteroleptic is used for anions that involve both imido and 0x0 (or chalcogenido) ligands. 

Properties of nickel poly(pyrazol-l-yl)borate complexes such as solubility, coordination geometry, etc., can be controlled by appropriate substituent groups on the pyrazol rings, in particular in the 3- and 5-positions. Typical complexes are those of octahedral C symmetry (192)°02-604 and tetrahedral species (193). In the former case, two different tris(pyrazolyl)borate ligands may be involved to give heteroleptic compounds.602,603 Substituents in the 5-position mainly provide protection of the BH group. Only few representative examples are discussed here. 

Homo- and heteroleptic complexes of Cd alone and of Cd and Hg with the ligand dicyanamide (dca) N(CN)2-, homologous to cyanamide NCN2-, have been studied in various solvents (formation constants of the complexes [M(dca) ](" 2> (M = Cd, Hg l < n < 4)), with the result that the complexes of Hg are more stable than those of Cd. Otherwise, obviously no studies on the isolated compounds M(dca)2 or on homoleptic complexes derived therefrom have been published. 

Scheme 3 shows the details of the synthetic strategy adopted for the preparation of heteroleptic cis- and trans-complexes. Reaction of dichloro(p-cymene)ruthenium(II) dimer in ethanol solution at reflux temperature with 4,4,-dicarboxy-2.2 -bipyridine (L) resulted the pure mononuclear complex [Ru(cymene)ClL]Cl. In this step, the coordination of substituted bipyridine ligand to the ruthenium center takes place with cleavage of the doubly chloride-bridged structure of the dimeric starting material. The presence of three pyridine proton environments in the NMR spectrum is consistent with the symmetry seen in the solid-state crystal structure (Figure 24). 

Reaction of the bis-chelate complex 149 and various bis(arylalkyl)barium complexes generates heteroleptic barium complexes with one chelate and one reactive arylalkyl ligand 164. The homoleptic and heteroleptic barium complexes both induce living polymerization of styrene to atactic polystyrene in cyclohexane solution. The fact that no stereocontrol is observed during polymerization despite the presence of the chiral carbanionic ligands is... 

Two equiv. of 6,6-di(cyclopropyl)fulvene react at 60 °C over a period of a week with Ca[N(SiMe3)2]2-(THF)2 bis in THF to yield the metallocene 170. The heteroleptic amido complex 171 is detected as an intermediate with 111 and 13C 1H NMR spectroscopy. A 1 1 reaction of the calcium amide and 170 also produces 171 in solution, an equilibrium involving these three derivatives exists (Equation (30)). The calcocene 170 crystallizes at — 20 °C from THF as colorless cuboids. The metal center is surrounded by the four ligands in a distorted tetrahedral manner, and the cyclopentadienyl group and the propylidene fragment are coplanar with each other.393... 

Although reports on silver(i) cr-alkynyl complexes have appeared for more than a century, the number of examples was still very limited prior to the past decade, and many of them were referred to as insoluble homoleptic polymeric [Ag(C=CR)]oo. Molecular alkynylsilver(i) complexes were often heteroleptic in nature and were achieved commonly through the stabilization by an extra coordination with strong cr-donor ligands such as amines, phosphines, and arsines. 

The combination of equimolar amounts of tris(trimethylsilyl)methyllithium and zinc bromide in a THF/diethyl ether mixture, Scheme 27, furnished tris(trimethylsilyl)methylzinc bromide, as a lithium bromide/ether adduct.43 The compound, which may also be formulated as a lithium alkyldibromozincate, showed no ligand redistribution reactions. It is monomeric in solution and can be treated with 1 equiv. of an organolithium reagent to afford heteroleptic diorganozinc compounds. 

The reactions of a benzylzinc chloride TMEDA adduct with either benzyllithium or benzyl(trimethylsilyl)lithium TMEDA adduct yielded both homoleptic dibenzylzinc (37, Figure 16) and heteroleptic monobenzylzinc compounds as TMEDA adducts. The heteroleptic diorganozinc compounds do not disproportionate as long as TMEDA is present, but removal of the chelating nitrogen ligand in the gas phase does cause disproportionation. 

Many 1,4-diazabutadiene adducts of dialkylzinc compounds show unusual reactivities (see also Section 2.06.10.7), such as the intramolecular electron transfer from zinc to the chelating ligand and the subsequent dimerization of these radicals.127 In solution, the carbon-carbon coupled dimer [MeZn(ButN=CHCH=NBut)2ZnMe] 71 is in equilibrium with its radical monomers (Scheme 56). Addition of potassium to a THF solution of the dimer produced cleanly 72, the first heteroleptic alkyldiamidozincate. 

The use of homoleptic compounds, where all coordinating atoms are as chemically indistinguishable as possible. Indeed, with heteroleptic complexes it can easily be the case that the error in the parameterization of the effect of a type of ligand can be compensated and thus shadowed by another error in the parameterization of a different ligand, resulting in flawed parameters for both of them. 

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