Ligands cr-donor

Novel ruthenium-amidinate complexes of the type (j -CgHsRlRufamidina-te)X (R = Me, OMe, F X = Cl, Br, OTf) and [Ru(amidinate)(MeCN)4][PF6] have been synthesized by photochemical displacement of the benzene ligand in (j -CgHglRufamidinatelX by substituted arenes or MeCN. The acetonitrile ligands of [Ru(amidinate)(MeCN)4][PF6] are easily replaceable by other cr-donor ligands (L) such as pyridines, phosphines, and isocyanides to afford the corresponding derivatives [Ru(amidinate)(MeCN) (L)4 ][PF6] n — 1, 2). These reactions are summarized in Scheme 142. ... 

A T structure with the strongest ct-donor D trans to the empty site (I in Scheme 1) is preferred in the case of three pure cr-donor ligands. The presence of a ir-acceptor ligand also makes the T structure more stable. When one of the ligands is a tt-donor, X, a Y structure of type II (Scheme 1) is observed. This structure permits the formation of a w bond between the empty metal d orbital and the lone pair of X. No such tt bond is present in the T structure since all symmetry adapted d orbitals are filled. This partial M—X multiple bond stabilizes Y over T. 

Fischer carbenes characteristically contain a number of electron-withdrawing carbonyl ligands while the typical Ru, Os, or Ir carbene complexes described above frequently contain several cr-donor ligands. The metal centers in these former compounds, then, are rather electron-deficient, with nucleophilic attack at Ca being a favorable reaction. 

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. 

A. Hejl, T. M. Trnka, M. W. Day, and R. H. Grubbs, Terminal Ruthenium Carbido Complexes as cr-Donor Ligands, Chem. Commun. 2002, 2524-2525. 

Answer 6.15 Transition metals need both vacant orbitals to accept electron pairs from the cr-donor (ligand) and also filled d orbitals for jt back-donation (back-bonding) to the ligand (note that neither the very early nor the very late transition metals form stable binary carbonyls). CO is a rather weak a-donor, and lanthanides tend to complex only with good a-donors. For tt-bonding to occur, the metal must possess suitable orbitals to 7r-bond to the ligands, and the inner 4f orbitals are unsuited to this, unlike the dxy, dxz, and dyz orbitals of transition metals. 

The presence of sufficiently powerful cr-donor ligands can make a metal a proton acceptor. The trialkylphosphine ligands in Pt(PEt3)3, and h -C5HjCo(PMe3)2 make the metals sufficiently basic to deprotonate ElOH and H O ... 

As can be seen by comparing HCo(CO) with HCo(CO)jL and HV(CO) with HV(CO)jL, cr-donor ligands decrease the acidity of hydrides the data on HCo(CO)jL and HCoL show that phosphines decrease the acidity more than do phosphites. Comparison of H20s(C0) and H(CH3)Os(CO) suggests that replacement of a hydride by an alkyl also decreases acidity. 

It is extremely common for coordination compounds also to exhibit strong charge-transfer absorptions, typically in the ultraviolet and/or visible portions of the spectrum. These absorptions may be much more intense than d-d transitions (which for octahedral complexes usually have e values of 20 L moF cm or less) molar absorp-tivities of 50,000 L mole cm or greater are not uncommon for these bands. Such absorption bands involve the transfer of electrons from molecular orbitals that are primarily ligand in character to orbitals that are primarily metal in character (or vice versa). For example, consider an octahedral d complex with cr-donor ligands. The ligand electron pairs are stabilized, as shown in Figure 11-15. 

Good cr-donor ligands at the same equatorial sites would improve back-donation from the filled metal orbital to a of the entering molecule, assisting its cleavage,... 

The epoxidation of alkenes is strongly retarded by cr-donor ligands such as DMF or HMPT which prevent complexation of the alkene on the metal." ... 

The phosphaalkyne P=CBu bonded in the r] mode, has a Pl value (—0.04 V) [18[ that is very close to that of CO (Pl = 0 V). Since the former is not expected to be a good cr-donor, this similarity is accounted for by considering that it is also not an efficient rr-acceptor [18[. A similar case is that of the N2 ligand (Pl = —0.07 V [10]). Since Pl reflects the net jt-electron acceptor minus cr-donor character, similar values can be found for both a strong it-acceptor/strong cr -donor ligand and a weak JT-acceptor/weak cr-donor ligand. 

The water molecule is a good cr-donor ligand, while jr-backbonding is negligible. For this reason, higher-valent transition metals form the more stable metal complexes, but the nature of the metal by itself is important, too (Table 1). 

With harder cr-donor ligand systems, such as those containing nitrogen and oxygen donors, and the metal center in a normal oxidation state, a formal one-electron reduction to an end-on coordinated superoxo species occurs with a bent... 

Carbon monoxide binds weakly as a cr-donor ligand to four-coordinate co-balt(II) systems. Despite a bout of artifactual excitement, CO has never been observed to bind significantly to five-coordinate Co systems with a nitrogenous axial base to yield octahedral six-coordinate species. - The sulfur analogue thiocarbonyl (CS), although not stable as a free entity, binds very strongly to iron-porphyrin species in a linear manner. ... 

It is important to distinguish between outer-sphere and inner-sphere complexes. In inner-sphere complexes the surface hydroxyl groups act as cr-donor ligands, which increase the electron density of the coordinated metal ion. Cu(II) bound in an inner-sphere complex is a different chemical entity from Cu(II) bound in an outer-sphere complex or present in the diffuse part of the double layer. The inner-spheric Cu(II) has different chemical properties for example, it has a different redox potential with respect to Cu(I), and its equatorial water is expected to exchange faster than that in Cu(II) bound in an outer-sphere complex. As we shall see, the reactivity of a surface is affected, above all, by inner-sphere complexes. 

A series of technetium(I) /Tdiketonates have been prepared by refluxing 32 in the corresponding neat /9-diketone. Due to the instability of the CO ligands trans to a cr-donor ligand, one of the COs can be substituted by an additional amine (L), forming compounds of the composition [Tc(OaO)(CO)3L] (61) [74,75],... 

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