Three-electron donors

The nitrogen atom in the (almost) linear metal-thionitrosyl complexes is 5p-hybridized and the NS ligand behaves as a three-electron donor. The N-S bond distances in metal complexes vary between 1.45... 

Counting NO as a three-electron donor, [IrCl(NO)(PPh3)2]+ is, therefore, a 16-electron species isoelectronic with Vaska s compound, isolable as a red crystalline hexafluorophosphate (m.p. 211°C, i/(N-0) 1870 cm-1) or similar perchlorate and tetrafluoroborate a trans-structure is indicated by spectroscopic data, and it is presumed to have a linear Ir-N-0 grouping. 

Dihydro-l,3-diborolenes are accessible as pentaorgano derivatives " and serve as ideal precursors of the diborolyl radieal. The formation of the radical and its interaction with a metal center is formally described as a metal oxidation. The resulting 2,3-dihydro-1,3-diborolyI ligand is either a three-electron donor or an anionic four-electron donor. l,3,4,5-Tetraethyl-2-methyl-2,3-dihydro-l,3-diborole reacts with [T -CpNi(CO)2] or (T7 -Cp>2Ni in hot mesitylene to yield a sandwich complex and a triple-decked complex ... 

Such a mechanism would have to involve the nitrosyl ligand acting in a non-innocent manner, changing from a three-electron donor to a one-electron donor in the intermediate complex. Such participation of the nitrosyl ligand has precedent in related systems (108). 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

In method (ii), the group X (e.g., a halogen) may function as a three-electron donor when present as a bridging group, but as a one-electron donor when combined to one metal center. The simple act of bridgeopening creates a vacant coordination site on one metal atom. Behavior of this sort is invoked to explain the stereospecific incorporation of I3CO in the complex Os3(CO)10Cl2. 

For the dibridged species, such as the halogen derivatives HOs2(CO)i0X (X = Cl, Br), we have used this type of behavior to explain the stereospecific incorporation of, 3CO into the molecules (160). The halogen group varies from a three-electron donor in bridged molecules to a one-electron donor in the terminally bonded species. 

Figure 16.2 Proposed P-gp recognition elements type I contains two electron donor groups separated by 2.5 0.3 A and type II consists of two or three electron donor groups with a spatial separation of the outer two groups of 4.6 0.6 A. A donates a hydrogen bonding acceptor group (electron donor group). Reprinted with permission from ref. [40], Copyright 1998 Blackwell Publishing Ltd.

Detailed structure-activity relationships of P-glycoprotein are not yet available. Some understanding is provided by Seelig [12] who has compared structural features in P-glycoprotein substrates. This analysis has indicated that recognition elements are present in structures and are formed by two (type I) or three electron donor groups (type II) with a fixed spatial separation. The type I element consists of two electron donor groups separated by 2.5 A, whilst the type II elements has a spatial... 

Ru(03)(N0)(NCS)(PPh3)3 As Ru(O3)(NO)(NCS)(PPh3)3/O3/xylene/80°C this oxidised PPh3 to PPh30. The mechanism may involve the coordinated NO ligand, which can function in effect as a one-electron donor (NO") when the Rn-N-O unit is bent and, as here, a three-electron donor when linear [911]. 

Electrochemical techniques can also give interesting information in the case of catenanes of higher complexity, as shown by the results obtained by investigating a series of catenanes made of up to seven interlocked rings.20 The three basic components of these catenanes are the tetracationic cyclophanes 124+ and 154+, and macrocycle 16 containing three electron donor DMN units (Fig. 13.16). For space reasons, only the electrochemical behavior of catenanes 174 1, 188+, 194+, and 204+ (Fig. 13.17), compared to those of their molecular components, is reported. 

The carbyne ligand may be viewed as a three-electron donor, similar to the nitrosyl ligand, with a pair of electrons in an sp orbital and a single electron in a p orbital. Donation of the sp electrons and pairing the p electron with one from the metal atom gives a a bond and a 7r bond, respectively. The second rbond results from donation of an electron pair from the metal atom to the empty p orbital of the ligand. 

Few ligands are as common and important to organometallic chemistry as the allyl group. A recent crganometallics text devotes an entire chapter lo the use of its complexes in organic synthesis.8 It can function as a one-electron donor (monuhaplo) or as a three-electron donor (tnhapto). 

When this occurs, the electron contribution of the ligand is increased by two. In die neutral atom scheme, it is now a three-electron donor and by the oxidation state method, a four-electron donor. The complex Rh CO C has two bridging chloro ligands ... 

Liao, Martire, and Sheridan (10) used the methods of Cadogan-Purnell and Martire-Riedl with three electron donor systems. The association constants obtained were in excellent agreement. 

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. 

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