Trihapto ligands

CbCin perpeiidi .uLriiip1jrc 165 Ru atoms, and reduced tr-bonding is indicated by a much shorter (127.5 pm) C-C distance. 

The classic example is (Co2(CO)6(C2Ph2)] which is Formed by direcl displacement of the 2 bridging carbonyls in (Co2(CO)ii] to give the structure sketched below  

The possibility that the allyl group CH2=CH-CHi- can act as an ligand was recogni2ed independently by several groups in I960 and since then the held bas flourished, partly because of its importance in homogeneous catalysts and partly because of the novel sleric possibilities and inferconversions that can be studied by proton nmr spectroscopy. Many synthetic routes are available of which the following are representative. 

Similarly many other ij -allyl carbonyl complexes convert to ij -allyl complexes with loss of I CO. 

Many -allyl complexes are fiux ional (p. 914) at room temperature or slightly above. 

---

Reactions with Stannylenes SnX2 Displaying Trihapto-Ligand Properties... 

Experimental evidence pointing to intermolecular methanolysis, or at least, not excluding this termination path has been reported as well. An in situ NMR investigation showed in fact that the Pd" acyl complexes [Pd(C(0)Me)(NCMe)(dppf)]OTs and [Pd(C(0)Me)(dppomf)]OTs, containing dihapto and trihapto ligands, respectively, undergo fast methanolysis at room temperature (dppomf = l,T-bis(diphe-... 

Complex (69) is diamagnetic. Nickel is coordinated to two phosphorus atoms of the phosphines and to three carbon atoms of the C3PI13 fragment which acts as a trihapto ligand. 

Hydrated nickel(II) salts in the presence of the ligand p3 react with both white phosphorus and yellow arsenic, breaking up the structure of the tetraatomic P4 and As4 molecules to form triatomic cyclo-triphosphorus and cyc/o-triarsenic species. These fragments act as trihapto ligands yielding double sandwich complexes (equation 99).292 The compounds are air-stable both in the solid state and in solution. Upon reduction with NaBH the complexes of equation... 

The allyl group most commonly functions as a trihapto ligand, using delocalized tt orbitals as described previously, or as a monohapto ligand, primarily ct bonded to a metal. Examples of these types of coordination are shown in Figure 13-24. 

The allyl ligand can bind to a metal in a monohapto fashion like an alkyl, in which case it is a 1-electron X ligand. Most of the time, however, it is bonded to the metal as a Jt 3-electron LX trihapto ligand. 

A Pt(II) complex containing an acac(2-) ion as a trihapto ligand, Pt( / -acac(2-)) P(p-ClC6H4)3 2, was prepared in a 49% yield by the reaction of Pt(acac)2 with tris(p-chlorophenyl)phosphine in chloroform under nitrogen at room temperature, and was characterized by H, and P NMR spectroscopy. It is surprising that this reaction proceeds under such very mild conditions. As is discussed shortly, a type-33 complex (Scheme 5) [Pt(PR3)2(acac)](acac) seems to be involved as... 

Homoleptic complexes have been obtained also with tetrakis(l-pyrazolyl)borates, e.g., [ B(pz)4 2Cd] and [ B(3-Mepz)4 2Cd] (both P2 jc, Z = 2) in both compounds structure analyses the ligands have been shown to coordinate trihapto, i.e., with one pz ring free. In both cases Cd has a distorted octahedral environment, with averaged structural data very similar to those for the tris(l-pyrazolylhydridoborate complexes.201 Variable-temperature 3H NMR studies of these and of mixed complexes with tris- and bis(l-pyrazolylhydridoborates indicate fluxional behavior (coalescence temperatures and barriers for the dynamic processes are given). 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

The first organometallic compound of the transition metals to be characterized (1827) was Zeise s salt, K[(C2H4)PtCl3]-H20 (Fig. 18.1). It forms when K2[PtCl4] in aqueous ethanol is exposed to ethylene (ethene) a dimeric Pt—C2H4 complex with Cl bridges is also formed. In both species, the ethylene is bonded sideways to the platinum(II) center so that the two carbon atoms are equidistant from the metal. This is called the dihapto-or T]2 mode. A ligand such as an allyl radical with three adjacent carbons directly bonded to a metal atom would be trihapto- or t 3, and so on. 

The power of the 18-electron rule for predicting structures of complexes involving unsaturaiec ligands can be illustrated with W(CO)2(C Ho2- If both CSH, ligands were pentahapto. the compound would have 20 electrons two more than the optimum for stability. However, if one of the ligands is presumed to be pentahapto and the other trihapto, we have an 18-electron complex ... 

A typical structure of these so called triple-decker sandwich complexes is that of the complex [(p3)Ni -(j)3-P3) Ni(p3)](BPh4)2-2.5Me2CO (73). In each complex the trihapto P3 (or trihapto As3) groups form a bridge between the two Ni(p3) residues.292 Each nickel atom is thus six-coordinated by three phosphorus atoms from the ligand p3 and by three phosphorus atoms from the cyclo-P3 (or arsenic atoms from the cyclo-As ), which acts as a three-electron-donor ligand. 

The trihapto arrangement, which is by r the mosi common, usually ha.s equal C—C bond lengths in accord with ddoadization over the three TrorbilaLs. The three MOs of (he allyl radical (Fig. 15.27) can overlap with crand ttorbitals of ihe mel.il. but Ihe v interaction of the ligand with the metal orbital is not very significant. 

The [Mn(CO)5] ion displaces CF from allyl chloride to give an 18-electron product containing ir) -C3H5. The allyl ligand switches to trihapto when a CO is lost, preserving the 18-electron count. 

---