Block Complexes

The /block consists of the 4/metals, La-Lu, and the 5/metals, Ac-Lr. The common terms lanthanide and actinide derive from the names of the first elements of each series, and the symbol Ln, not assigned to any particular element, designates the lanthanides as a class the older term, rare earths, is sometimes encountered. The actinides are radioactive, and only Th and U are sufficiently stable to be readily handled outside 

The absence of ligand field effects makes the magnetism of an/-block complex identical to that of the parent ion. In the d block, a complex such as CP2WCI2 is typically diamagnetic as a result of d-orbital splitting—in contrast, 5f CP2UCI2 has two unpaired electrons. 

The trend in radius, shown for the M(III) ion in Table 15.2, is the result of the increasing number of protons in the nucleus causing the electron shells to contract in the lanthanide contraction-, the/electrons added are deep-lying and inefficient at screening the nuclear charge. In most of chemistry, when we move from one element to the next, the changes in atomic size and preferred valency are abrupt. Here, in contrast, the radius varies smoothly and the M(III) valence state remains preferred, so we have nice control over the M-L bond length. As this 

As the ionic radius changes, the preferred coordination number can change. For the aqua ions [Ln(H20) ] +, n is 9 for the larger ions, L-Eu, and 8 for the smaller ions,Tb-Lu. For Gd +, n = 8 and n = 9 ions have about the same energies. The later lanthanide ions, being smaller, have a slightly greater Lewis acidity. 

Among the 5/elements, we look at Th, with its strongly preferred 5/° Th(IV) state, and U with 5f U(III), 5f (IV), 5 f (V), and 5f (VI) states all accessible. In the actinides, the complexes have somewhat more covalency in their bonding than do the 4/ elements, in line with their higher electronegativities (U, 1.38), and in the case of reduced states of U, a significant tendency to back bond. Tlie 5/ level is somewhat more available for bonding than is 4/in the lanthanides. 

TABLE ISJZ Lanthanide Electronic Conjurations and Ion Radii  

Element Atom Config. M(II) Config. M(U1) Config. M 1V) Config. Radius M(in) (A) 

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Thenoyltrifluoroacetone and carboxin and its derivatives specifically block Complex II, the succinate-UQ reductase. Antimycin, an antibiotic produced by Streptomyees griseus inhibits the UQ-cytochrome c reductase by blocking electron transfer between bn and coenzyme Q in the Q site. Myxothiazol inhibits the same complex by acting at the site. 

Again, however, this is strictly applicable only for free ions. Even though spin-orbit coupling is much less important for the first row of the d block, this formula provides a far less good approximation for d -block complexes than Eq. (5.6) does for lanthanide complexes. The reason is that the ground, and other, terms in these d complexes differ grossly from those of the corresponding free ion. These differences are one result of the crystal field. 

Overall, then, the magnetic moments of all lanthanoid complexes are well reproduced without reference to the ligand field inter alia, we can infer that the ligand-field splittings in /-block complexes are no greater than about kT at room temperature. 

The Magnetochemistry of Homo- and Hetero-Tetranuclear First-Row d-Block Complexes Keith S. Murray... 

Based on Mononuclear f-Block Complexes. The Polyoxometalate Case. Inorg. Chem., 53, 9976-9980. 

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