Soft donors

Adducts are formed with hard and soft donors, including 7r-acids such as CO, PF3 and PPh3. DMSO bonds through S for R = Me and Et, but through O when R = CF3. 

The thiocyanate K2[Pt(SCN)6].2H20 contains octahedrally coordinated platinum, confirming the readiness of platinum(IV) to bind to a soft donor atom like sulphur [182]. 

Condensation of dicesium 2-thioxo-l,3-dithiole-4,5-diselenolate with fo/s-alkylating polythioethers under high dilution conditions afforded the TTF-containing macrocycles possessing soft donor sites and 12-, 15-, and 18-membered rings <%JCS(P1)1995>. 

It is interesting to notice that the complex 58 is also able to react with soft donor hgands such as triphenylphosphine resulting in the formation in very mild conditions of the unexpected orthometallated complex 59 (Scheme 23) [89,91, 92]. The ligand here is linked to the metal through both an aromatic and an ylidic carbon. Other transformations are realized from 58 leading, including the compounds of the previous scheme, to four different structures for the bis-ylide (i) C,C-chelate (ii) C,C-orthometallated (iii) C,C-orthometallated and free ylide (iv) C,C,C-terdentate (Scheme 23). 

A combination of P- and N- donors is another useful approach to potentially reactive (and cataly-tically active) Ni species. Similar to O- donors, N is a hard donor capable of stabilizing metal ions in higher oxidation states, whereas the soft donor P is best suited to stabilize medium or low oxidation states. A neutral bidentate P,N ligand combining a hard dimethylamino and a soft phosphine donor in /V,/V -dimethyl-2-(diphenylphosphino)aniline (241) affords the neutral trigonal bipyramidal and the... 

Phosphine selenides are stronger soft donors than the sulfides.55 The solid 1 1 adduct of /Pr3P Sc with I2 is ionic (iPr3PSe)2I+ 13 reaction of... 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

The many laboratories involved in this work allowed a large experimental program to be set up to test the behavior of these soft donor extractants in countercurrent separations. The trivalent actinide-lanthanide separation process was named SANEX and four chemical systems have been explored (Fig. 12.20) ... 

The phosphine-functionalized tpy ligand 4 -Ph2Ptpy has both hard and soft donor sites the ligand is prone to oxidation to 4 -Ph2P(0)tpy and this affects the method of preparation of [Ru(4 -Ph2Ptpy)2]. " The complex [RuL(4,4 -Me2bpy)(NCS)] where L = 4-phosphonato-2,2 6, 2"-terpyridine, shows two ground-state pKi values at 6.0 and <4.0. The emission spectrum of the complex and the excited-state lifetimes are pFI-dependent. ... 

The metal ions of Tables 1 and 2 have been chosen to represent as wide a variation of bonding character and charge as the available experimental material allows. A natural limitation is that complexes of any measurable strength are not formed in aqueous solution between typically hard acceptors and soft donors, or vice versa. 

On the whole, the thermod3mamic functions found for the stepwise formation of complexes in aqueous solution agree very well with the models proposed for complexes of different character. Thus for interactions between hard acceptors and hard donors, postulated to be mainly electrovalent, the expected stepwise decrease of ASn generally occnrs, often accompanied by a similarly expected decrease of AHn- With interactions between soft acceptors and soft donors, postulated to be mainly covalent, virtually constant values of AHn from step to step are often found, while in other cases values of AHn becomes step by step less exothermic. Both modes of behaviour are compatible with the current model. 

A more complete and much more rigorous description of bonding in complexes would be provided by a quantum mechanical treatment. Such a treatment is especially needed in the case of departures from the ionic model and increasing contribution of covalent bonding (ion pairs, soft donors and acceptors). However only a few studies have been reported. They are mainly concerned with cation hydration and use either semi-empirical 19—21) or non-empirical methods 22—24). A non-empirical treatment of cation NH3 systems has also been performed recently (25). However the present state of the computations is still far from providing a complete description of the system including the medium. The latter may be taken into account by a Bom-type "solvaton (27,26). Heats of hydration may then be calculated (27). A discussion of this aspect of the problem is deferred to a later date, awaiting especially a more complete analysis of non-empirical calculations. In the course of the discussion of... 

In general the former stereochemistry is adopted by the complexes of the type [CoXLJY studied by Venanzi and coworkers [69), where L is a tripod hgand with soft donor atoms, such as QP and QAs. The magnetic moments of these complexes usually fall in the interval 1.9 to 2.1 BM showing that there is one unpeured electron (69). 

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