ML2 complexes

There are many ways in which a linear ML2 complex may be obtained from the complexes already studied. For example, four coplanar ligands can be removed in an octahedral complex (2-70). 

Notice that the w-type orbitals have just a single nodal plane that contains 

We have not yet considered the second orbital in the e block of the octahedron, z. It is stabilized by the elimination of the four weak antibonding interactions in the xy plane, but this stabilization should be rather small, as the two principal antibonding interactions, along the z-axis, are still present. 

We should mention a few complexes in which the d block is not completely filled. For example, complexes with two borylamide ligands [N(Mes)(BR2)J are known for M=Mn (d ), Feld ), Co (cP), andNi (d ), and they adopt an almost linear geometry. These are all high-spin complexes, which is understandable given the small separation of the energy levels in the d block. 

Other structures exist for ML complexes besides those considered so far, though they are less common for stable species. We shall study five 

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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. 

In the case of a solution such as electroless Ni-P, Ni2+ is usually complexed by citrate, and the stability constants are ca. 104 and 2 x 108 (overall value) for the ML and ML2 complexes [67], Thus, pM will change relatively slowly with pH. On the other hand, the stability constant for the Pd-EDTA complex system (ML type only) is reported to be 1024 [67], i.e. Pd2+ is strongly complexed by EDTA. The Pd2+ pM value changes drastically, in a practical electroless deposition sense, over a rather narrow pH range. Consequently, in the case of an electroless Pd solution with EDTA as complexant, the solution may go from a condition of near spontaneous plating out to one where deposition virtually ceases. 

Intramolecularity in lariat ether complexation was demonstrated in three ways. First, when solution cation binding constants were determined. Kg was shown to be independent of cation and macrocycle concentrations (within certain limits). The efficacy of complexation of a single cation by two macrorings [(ML2) complex formation] should be concentration dependent. Second, ammonium cation binding constants were determined for the series (see above) of monoaza-15-crown-5 and -18-crown-6 compounds having (CH2CH20)jiClH3 (n 0 to 8) sidearms (31). 

Metal complexes with fluoro-/3-diketones have been comprehensively reviewed.1585 The introduction of electron-withdrawing groups in the chelate ring increases the Lewis acidity strength of the ML2 complexes, and consequently the bis adducts of the fluoro-/3-diketonato complexes are more stable than the corresponding complexes with /3-diketones. As an example of a nickel complex with 1,1,1-trifluoroacetylacetone which does not have a counterpart in the nickel acetylacetonate complexes we can mention the hexanuclear complex Ni6Lio(OH)2(H20)2.1586... 

The structural and bonding features of this complex comply with the trigonal in-plane conformational preference observed in d10 (olefin)ML2 complexes. In molecular orbital terms, the dominant bonding interaction is between the b2 HOMO of the ML2 fragment and the ethylene n LUMO283. 

Predict the crystal field energy-level diagram for a linear ML2 complex that has two ligands along the z axis ... 

ML2 complexes with and these complexes show both aromatic stacking interactions and helical distortions. An nnusual S,N-chelate, with no carbon atoms in the four-membered chelate ring set, is (21) it forms a spirocyclic tetrahedral bis Co complex. 

Mixed donor didentate ligands containing selenium are less well known than their sulfur analogues. A typical example of an Se,N donor is (41), which coordinates as a didentate chelate another is (42), a selective analytical reagent for platinum group metals. The monoanionic (43) forms neutral ML2 complexes (M = Zn, Cd, Hg) with an overall N2Sc2 donor set, with the imine N selected as a much better donor than the alternate ether However, Se,0 donor chelation is displayed by the butane-2,4-dionate analogue... 

A series of closely related Schiff-base ligand complexes based on cyclohexyl- or alkyl diamines have been prepared and characterized.786-789 Crystallographic studies have confirmed that the ML2 complexes were eight coordinate,790 while the related complexes MC12L and MCl2L(donor) adopted cis- or trans-octahedral or seven-coordinate geometry.791... 

Steps 1, 2, and 3 are counted from the top of the figure clockwise, e.g. step one corresponds to addition of hydrogen to ML2 complex, where L is a ligand. For an irreversible reaction eq. (5.72) is essentially simplified to... 

A theoretical study of the reactions between methane and ML2 complexes (where M = Pd, Pt L = PH3, CO, = PH2CH2CH2PH2), led to the conclusion that for 14-electron ML2 complexes, a smaller L-M-L angle, a better electron-donating ligand, and a heavier transition metal center should be a potential model for the oxidative addition of C-H bonds [63a]. 

The pseudotetrahedral complexes ZnL2X2 (X = Cl, Br or I L = 2,6-dimethylthiopyrone) have been prepared the ligand is coordinated via sulfur. Thiazolidine-2-thione (HL) reacts with MCI2 (M = Zn, Cd or Hg) as its deprotonated thiol to yield polymeric ML2 complexes in which the ligand is N,S-bound. ... 

Figure 2.12. Derivation of the d-block orbitals for a linear ML2 complex from the d orbitals of an octahedral ML6 complex.

ML2 complexes usually adopt a linear (or essentially linear) geometry (see 2.7). Knowledge of the orbital structure for strongly bent ML2 species is therefore useful mainly when one wishes to consider them as fragments in larger complexes. For example, the ethylene)Ni(PR3)2] complex (2-94) can be described in terms of a bent ML2 entity interacting with a molecule of ethylene. 

Figure 2.16. Derivation of the d-block orbitals for a bent ML2 complex from the d orbitals of a butterfly ML4 complex. The hybrid s-p orbital (2ai) and the metalp orbital (lying above the d block) are also indicated.

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