Coordination number nine-coordinated complex

The lanthanum complex (Scheme 9) has the coordination number nine with two NO3 ions bidentately bound in the solid state, while in solution only one nitrate group is coordinated. H2dapsox is symmetrically pentadentately coordinated in the a-oxyazine form (Scheme 9). 

A study of hydration of the complex [EuL2(H20) ](BF4)3 where L= (C N-CO-CHr OCH(Me)- 2 shows by lifetime studies of a similar sort that in aqueous acetone, x = 0.9 0.4 which gives a coordination number of nine. This complex was prepared from Eu(MeCN) (BF3)3, itself prepared from the action of NOBF4 on europium metal in acetonitrile. 

Note that the cis isomer lacks an improper axis of rotation and is therefore chiral, but that the trans isomer has a plane of symmetry and will be achiral in the absence of an asymmetric carbon in the phosphine ligand-28 As in the case of the previously encountered cyclopentadienyl complex (page 476), it can be argued whether the coordination number is 5 or 9. In either semantic interpretation these compounds are of considerable interest since isomerism in nine-coordinate complexes Is even less well documented than in those with coordination number 5. 

The formation of luminescent lanthanide complexes relies on a number of factors. The choice of coordinating ligand and the method by which the antenna chromophore is attached to it, as well as the physical properties of the antenna, are important. In order to fully coordinate a lanthanide ion, either a high-level polydentate ligand such as a cryptate 1 or a number of smaller ligands (such as 1,3-diketones, 2) working in cooperation are required. Both 1 and 2 are two of the simplest coordination complexes possible for lanthanide ions. In both cases there are no antennae present. However, the number of bound solvent molecules is decreased considerably from nine (for lanthanide ions in solution) to one to two for the cryptate and three for the 1,3-diketone complexes. 

By the lUPAC (International Union of Pure and Applied Chemistry) definition, a transition metal is an element, an atom of which contains an incomplete d shell, or that gives rise to a cation with an incomplete d shell. Transition metals have a total of nine atomic orhitals, but only some of these are used for bonding to ligands. Coordination number denotes the number of donor atoms associated with the central atom and dictates the shape, or stereochemistry, of the coordination complex. The most common coordination numbers for the first transition metal series are four (tetrahedral and square-planar) and six (octahedral). Metals in the early second and third series are larger and thus can have higher coordination numbers, forming more stmcturally complex molecules. 

A number of chloride complexes include Pr(terpy)Cl3.8H2O X-ray diffraction established the presence of a [Pr(terpy)C1(H20)5] + ion. Lanthanide nitrates react with 1 mole of terpy in MeCN to give [La(terpy)(N03)3(H20) ] (Ln = La, = 2 Ln = Ce-Ho, n = 1 Ln = Er-Lu, n = 0) with coordination numbers decreasing from 11 (La) to 9 (Er-Lu) if these are crystallized from water [Ln(terpy)(N03)2(H20)4] NO3 (Ln = La-Gd) and [Ln(terpy)(N03)2(H20)3]N03.2H20 (Ln = Tb-Lu) are formed these have both biden-tate and ionic nitrate and nine- and eight-coordinate lanthanides, respectively. Thiocyanate complexes Ln(terpy)2(NCS)3(Ln = Pr, Nd) are also nine coordinate. 

Large coordination numbers. For example, the coordination number of 3d transition metals is generally four or six. However, the most common coordination number of lanthanide complexes is eight or nine. This number is close to the sum of the 6s, 6p, and 5d orbitals. Another fact responsible for the large coordination number of lanthanide complexes... 

The classical tridentate ligand terpyridyl (terpy) forms a nine-coordinate complex with scandium nitrate [Sc(terpy)(N03)3].52 This compound with chelating nitrates exhibits a relatively rare high coordination number for scandium. 

The higher stability of inner shell complexes that employ just the nine 3d—4s—4p orbital set suggests that there may be some special stability associated with the systems that employ nine orbitals that can accommodate no more than 18 electrons. There arose the 18-electron rule, which suggests that coordination complexes whose total number of valence... 

High coordination numbers are common in the f block. The didentate chelate ligand 2,4-dioxopentan-3-ido (acac ) forms six-coordinate octahedral complexes with first-row d-block metal ions (like Cr(III), shown at left), but eight or nine coordinate complexes with f-block lanthanide ions (like Eu(III), shown at right). 

---