Unidentate ligand

Table 2. Coordination Compounds Possessing Unidentate Ligands...

Four-coordinate complexes provide good examples of the early use of preparative methods for establishing stereochemistry. For complexes of the type [Ma2b2], where a and b are unidentate ligands, a tetrahedral structure cannot produce isomerism whereas a planar structure leads to cis and trans isomers (see below). The preparation of 2 isomers of [PtCl2(NH3)2], for instance, was taken as good evidence for their planarity. ... 

Where unidentate ligands are present, the ability to effect the resolution of an octahedral complex (i.e. to separate 2 optical isomers) is proof that the 2 ligands are cis to each other. Resolution of [PtCl2(en)2] therefore shows it to be cis while of the 2 known geometrical isomers of [CrCl2en(NH3)2] the one which can be resolved must have the cis-cis structure since the trans form would give a superimposable, and therefore identical, mirror image ... 

Reactions of group C, which involve interaction between two unidentate ligands in adjacent coordination positions, have so far only been reported for the pentacyanides, most of which undergo reactions to the type ... 

As already pointed out, the presence of a macrocyclic ligand, which occupies the four equatorial coordination sites of an octahedral complex, will tend to limit insertion and elimination reactions of the Co—C bond to those which require only that single coordination site. The presence of unidentate ligands in the pentacyanides, on the other hand, will offer greater opportunities for reactions which require a second, adjacent site. 

Fe(ptz)6] (BF4)2 and [Zni Fej.(ptz)6] (BF4,)2. The solid iron(II) complex of the unidentate ligand ptz = 1-propyltetrazole shows a reasonably sharp spin-state transition at about 130 K [112]. Optical excitation into the spin-allowed -> absorption band produces via LIESST the HS T2 state which remains trapped at temperatures below 50 K. Relaxation of the metastable J2 state to the LS state has been studied by following the changes of... 

The major difference in the dithiocarhamato chemistry of tantalum and niobium is the existence of the Ta(V) species Ta(R2C tc)5, whereas the comparable niobium compound is unknown. The preparative methods for Ta(R2C tc)5 involve either a CS2 insertion in Ta(NR2)j (R = Me )) or a reaction between TaClj and sodium or ammonium dithiocarbamate (R = Et (7), R2 = ( 112)4 ( ))- The structure of this interesting diamagnetic compound is not elucidated, but from infrared data the presence of one or more unidentate ligands is suggested 11,15). 

FIG. 6. Plot of log Kx values for Ag+ complexes with unidentate ligands vs log Kx values for corresponding complexes with CH3Hg+. Data at 25°C and ionic strength zero in aqueous solution, from Ref. (11). Ligands are classified as soft ( ) intermediate (O) hard, N-donors (O) hard, O-donors ( ), or F ( ). Formation constant data from Ref. (11). 

Fig. 7. Plot of log K values for Bi3+ complexes with unidentate ligands vs log Kt values for corresponding complexes with CH3Hg+. Data at 25°C and corrected to ionic strength zero, from Ref. (22).

Equation (3) has good quantitative predictive power and is a successful extra-thermodynamic relationship like the Hammett sigma function. No other approach to modeling complex formation equilibria, including HSAB itself, can predict log values for unidentate ligands so accurately. 

Equation (3) usually models formation constants for Lewis acids with unidentate ligands well, as shown for log K1 values for [Cu(tetb)]2+ with unidentate ligands ... 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

The exchange proreeds in three steps. The first step is substitution of the free ligand at one end of the chelate to give the intermediate I(fca). The second is intramolecular proton transfer between the unidentate ligand in l(kb). The third is the reverse of the first (k a). Consequently, application of the steady-state approximation to the intermediate 1, whose concentration is reasonably assumed to be very low, provides... 

Figure 12 [115] shows a series of complex formation titration curves, each of which represents a metal ion-ligand reaction that has an overall equilibrium constant of 1020. Curve A is associated with a reaction in which Mz+ with a coordination number of 4 reacts with a tetradentate ligand to form an ML type complex. Curve B relates to a reaction in which Mz+ reacts with bidentate ligands in two steps, first to give ML complexes, and finally close to 100% ML2 complexes in the final stages of the titration. The formation constant for the first step is 1012, and for the second 108. Curve C refers to a unidentate ligand that forms a series of complexes, ML, ML2. .. as the titration proceeds, until ultimately virtually 100% of Mz+ is in the ML4 complex form. The successive formation constants are 108 for ML, 106 for ML2, 104 for ML3, and 102 for ML4 complexes. 

Organochromium compounds with various coordination modes of the ligand, namely CpCr(CO)2S2P(OPr )2 (bidentate), CpCr[S2P(OPr )2]2 (one bident-ate and one unidentate) and Cr[S2P(OlV)2]3 (bidentate) are formed in reactions of [CpCr(CO)3]2 with (Pr 0)2(S)PSSP(S)(0Pr )2. Variable temperature NMR shows rapid interchange between bi- and unidentate ligands in solutions of CpCr[S2P(OPr )2]2.61... 

Some pyridine-containing ligands of this type have been used to mimic the protein environment in non-heme iron metal proteins. The ligands L (10 and 11) tend to bind strongly to five positions of the coordination sphere leaving the sixth position available to bind unidentate ligands X [FeLX]w+. 

The first experimental information on the kinetic parameters for water exchange on a tetravalent metal ion was published in 2000 for U4+ and Th4+ (265,268,271). The coordination numbers for these two complexes were determined by EXAFS to be 10 1. Based on the high coordination number (there are no complexes known with unidentate ligands and coordination numbers larger than 10) a limiting associative mechanism (A) is unlikely and a d-activated mechanism is probable. Surprisingly,... 

Now we examine the replacement of a coordinated unidentate ligand L by a different unidentate ligand L,. Either L or Lj will be H2O. The reaction can be examined in either direction. Figure 4.2... 

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