Ti-Acceptor ligands

Around 65% of Fe in the human body is a component of hemoglobin. The protein molecule contains four heme sub-units. The resting state has high-spin Fe but coordination of the strong ti acceptor ligand 02 changes it to the low-spin form (see Topic H2Y This is important for the action of... 

Most of the reactions of PH3 with the carbonyls of the transition metals and with a variety of carbonyl derivatives can essentially be classified according to three main types of reactions (i) substitution of CO ligands, of other neutral Ti-acceptor ligands like, e.g. PR3, weak 0-, S-, N-donor ligands, and alkenes, or of anionic ligands, (ii) cleavage of metal-metal bonds, and (iii) oxidative addition. Only in a few cases combinations of two or three of these reaction types or completely different reactions are observed. Most reviews dealing with the chemistry of phosphanes, with complexes of transition metals, or with transition metal carbonyls and derivatives cover only some aspects of the coordination chemistry of PH3 see e.g. [1,4, 14, 16, 22, 24, 27 to 29]. 

A recent review in Advances in Organometallic Chemistry (14) described the synthesis and reactions of a great many stable complexes of transition metals that contain Si—M bonds, in which M is Ti, Zr, Hf, Cr, Mo, W, Mn, Re, Fe, Ru, Co, Ir, Ni, or Pt. Some of these are hydrides, Si—M—H, and nearly all contain -acceptor ligands to stabilize them. 

Various attempts have been made at quantifying the overall a-donorand 7r-acceptor effects of the many different types of phosphine.1,2 Chemists have access to Ti-acceptors such as tris-V-pyrrolyl phosphine, perfluoroalkyl phosphines and phosphites, right through to the potent o-donor, tris(2,4, 6-trimethoxyphenyl)phosphine, 6 (Scheme 2). It is important to note that the donor/acceptor properties of a ligand are dependent on the metal, its oxidation state, and the ancillary ligands on the metal. 

CO acts as both a a-donor (via the lone pair of electrons on carbon) and a 7i-acceptor ligand in transition metal complexes. CO is usually depicted as having a triple bond (one a- and two ti-) between the C and the O as well as lone pairs on both the C and the O. The lone pair on C is used for donation into a suitable metal centred o-orbital. However, the strongest M-CO bonds are formed (in simple terms) when some of the electron density donated by the carbon to the metal is directed back from a filled metal (i-orbital of the correct symmetry into an antibonding ti of the CO. Thus the M-CO bond has two parts, the forward (C M) donation, and the (M C) back donation (Figure 1). 

In the presence of 9,10-phenanthrenequinone (PQ), a chelating ligand with good (T-donor and Ti-acceptor properties, oxidation with ferrocenium at low temperature (-78 °C) leads to the formation of a green solid which decomposes at room temperature to form ethane, metallic gold, and two equivalents of phenanthrenequi-none [64] (Eqs 79, 80). 

The complex shown in Scheme 670 appears to be the first example of an titanium complex. Its reactivity toward cr-donor and 7r-acceptor ligands demostrates that this complex does not behave like a low-valent titanium compound but rather bears a closer resemblance to a Lewis-acidic Ti(iv) complex. Based on theoretical calculations, this behavior can be attributed to a strong and appreciably covalent Ti-cycloheptatrienyl interaction, which leads to highly stabilized frontier orbitals and consequently to a diminishing 7r-electron donor ability.1706... 

Fig. 20.14 Approximate partial MO diagrams for metal-ligand 7r-bonding in an octahedral complex (a) with 7r-donor ligands and (b) with 7r-acceptor ligands. In addition to the MOs shown, cr-bonding in the complex involves the a g and ti MOs (see Figure 20.12). Electrons are omitted from the diagram, because we are dealing with a general ion. Compared with Figure...

Phosphines are typical substituents possessing an amphoteric electronic character as they have both cr-donor and Ti-acceptor properties. The phosphine ligands based on the [l,2,3]triazolo[l,5-fl]pyridine moiety should be interesting due to their unusual electronic and chemical properties. These derivatives have been obtained by trapping the 7-lithio derivative of 3-(2-pyridyl)triazolopyridine 33 with various aliphatic and aromatic chlorophosphines (Scheme 33) (09DT5068). In each case, a different ratio between the two isomers 40a and 41b is obtained. The different isomers cannot be isolated, but can be identified by H-NMR as they show distinct signals. 

The equilibrium between a- and ti-allyl complexes can be influenced by the ligands. Thus strongly basic alkyl phosphine ligands favor the a structure, as has been shown for allyl metal halide complexes of Pt and Ni Soft Jt-acceptor ligands such as CO favor the formation of it-allyl complexes. 

Chiral stationary phases that are currently available can be classified into those containing cavities (cellulose derivatives, cyclodextrins, synthetic polymers, crown ethers, and chiral imprinted gels), affinity phases (bovine serum albumin, human serum albumin, a-glycoprotein, enzymes), multiple hydrogen-bond phases, Ti-donor and Ti-acceptor phases, and chiral ligand exchange phases. This classification scheme was used in a review that gave numerous pharmaceutical examples of separation by... 

The Ti -cyclopentadienyl ligand is an L X-type ligand, and can be considered to occupy three facially oriented coordination sites. The molecular orbitals of the TT-system of a cyclopentadienyl ligand were shown in Chapter 1. These orbitals consist of one filled orbital of cr-symmetry and two filled orbitals of ir-symmetry, along with two imoccupied orbitals of 8-symmetry that can act as electron acceptors. 

Hgands are shown in Figure 4.6. A majority of NO complexes contain a relatively linear M-N-0 linkage. The NO ligand bound in a linear fashion is generally considered to be a poorer a-donor and a better Ti-acceptor than CO. ... 

T-Acceptor ligands G- and Ti-Donor-acceptor ligands (T-Donor ligands ... 

Carbonyl groups, due to their o--donor and Ti-acceptor properties, create a very strong ligand field. Consequently, the values of the splitting parameter for carbonyl complexes are very high, even higher than those of cyanide complexes (Table 2.12). Therefore, high-spin carbonyls are unknown. 

---