4- Coordinated monomeric complexes

These complexes can be prepared from phosphorus ligands that are themselves substituted by electron-withdrawing groups. It is often beneficial, however, to employ a low ratio of phosphorus ligand to rhodium since in several instances excess ligand cleaves the halo bridges and forms four-coordinate monomeric complexes. Table 2 lists those complexes of this structure that have been isolated. 

Bis(indenyl)lanthanide complexes are also described. According to X-ray crystallographic investigations (C4H7-OCH2C9H6)2LnCl (Ln = Nd, Sm, Dy, Ho, Er, Yb) (Figure 39) are unsolvated nine-coordinate monomeric complexes with a... 

Four-coordinate monomeric complexes are formed with either tetrahedral or planar stereochemistries. With acetylacetone, the beryllium compound (2) has been studied crystallographically and has the expected tetrahedral configuration. Crystalline Cu(AA)2 shows a nearly planar (13) arrangement however, some intermolecular perturbation does occur. 

There is much discussion on the nature of the aluminum species present in slightly acidic and basic solutions. There is general agreement that in solutions below pH 4, the mononuclear Al " exists coordinated by six water molecules, ie, [ ( 20) ". The strong positive charge of the Al " ion polarizes each water molecule and as the pH is increased, a proton is eventually released, forming the monomeric complex ion [A1(0H)(H20) ]. At about pH 5, this complex ion and the hexahydrated Al " are in equal abundance. The pentahydrate complex ion may dimerize by losing two water molecules... 

Treatment of GaH3(quin) (quin = quinuclidine) with 1 equivalent of the sterically bulky formamidine as shown in Scheme 40 resulted in formation of a monomeric amido-gallane complex containing a monodentate amidinate ligand. Reaction of this species with a second equivalent of the amidine led to displacement of the quinuclidine ligand and formation of a five-coordinate monohydride complex, which could also be prepared directly by a one-pot reaction of GaHsfquin) with the free amidine in a molar ratio of 1 2 ... 

The coordination of [Me2Sn(IV)f to captopril (cap) [(2S)-l-[(2S)-2-methyl-3-sulfanyl propanoyl]pyrrolidine-2-carboxylic acid] in aqueous solution was studied by means of pH-metric titration, electrospray mass spectrometry, H NMR, and Mossbauer spectroscopies in the 2-11 pH range. The results obtained proved that only monomeric complexes are formed in solution. In the acidic pH... 

The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. 

The value of the tris(pyrazolyl)hydroborato complexes [TpRR ]ZnOH is that they are rare examples of monomeric four-coordinate zinc complexes with a terminal hydroxide funtionality. Indeed, [TpBut]ZnOH is the first structurally characterized monomeric terminal hydroxide complex of zinc (149). As such, the monomeric zinc hydroxide complexes [TpRR ]ZnOH may be expected to play valuable roles as both structural and functional models for the active site of carbonic anhydrase. Although a limitation of the [TpRR ]ZnOH system resides with their poor solubility in water, studies on these complexes in organic solvents... 

IR spectroscopy is often used for distinguishing between unidentate and bidentate coordination of carboxylate (02CR) ligands. For monomeric carboxylate derivatives the separation between the symmetric and asymmetric C02 stretching bands, At = [t asym(C02) - t syJCOa)], provides a useful indication of the coordination mode complexes which exhibit values of greater than 200 cm-1 invariably possess unidentate coordination. Deacon, G. B. Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. 

Nin-amido complexes such as (117) react with small electrophiles by insertion either in the Ni—N bond (e.g., with C02 to form (118)) or in the N—11 bond. With unsubstituted aryl groups (Ar = Ar = Ph), both a monomeric complex (117) or a dimeric species (119) is formed, depending on the amount of PMe3 added. Using bulky borylamide ligands, an almost linear, two-coordinate Nin complex could be obtained and structurally characterized.467 The N—Ni—N angle in (120) is 167.9°. 

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