Scandium coordination number

Scandium coordinates fairly readily with several other types of oxygen donors, especially those with a little negative charge on the oxygen atom, as in Ph3P— 0. It usually takes a coordination number of six, judging from the stoichiometry of the compounds formed and from structures in the limited number of cases where there are single crystal X-ray data available. 

The coordination number of six which is common for d-transition metals and scandium, is rather rare in the case of lanthanides. Authentic six-coordinate rare earth complexes... 

Scandium is more like an element of the first transition series (or like aluminum) than like the rare earths. For example, it characteristically has a coordination number of 6 (6-coordinate radius, 0.89 A compared to 1.00-1.17 A for the lanthanides), although coordination numbers 8 and 9 are known. Examples of CN 8 are Na5[Sc(C03)4] -6H20 and the tropolonate, [SeT4] . There is but one example of CN 9, namely the tricapped trigonal prismatic [Sc(H20)9]3+ ion found in Sc(CF3S03)3-9H20.38 The aqua ion in solution, [Sc(H20)6]3+, is appreciably hydrolyzed and OH-bridged di- and trinuclear species are formed. 

Relatively few complexes of N-donor ligands are known. One of the best characterized is with the tridentate ligand terpyridyl, which reacts with scandium nitrate to form Sc(terpy )(N03 )s here scandium is connected to 9 atoms, but one Sc-O bond is considerably longer than the others, so a coordination number of 8.5 has been assigned. A range of porphyrin and phthalocyanine complexes exist syntheses often involve the high-temperature routes typical of the transition metals but recently a high-yield low-temperature route has been utilized to make octaethylporphyrin (OEP) complexes ... 

Higher coordination numbers of 8 -F 1 are adopted in the LT-YF3 type by the trifluorides of the larger ions TP+, bP+ and the smaller rare-earth ions Sm to Ln. The tysonite or LaF3 type with CN 9 + 3 is found for the trifluorides of the larger 4f and the 5f elements (see Scandium, Yttrium the Lanthanides Inorganic Coordination Chemistry). 

Overall, then, the existence of the three scandium-containing ions [Sc(H20)x] + (x = 6,7,8) in five different solid salts indicates that in itself X-ray diffraction data on solids cannot be relied upon to indicate the coordination number in aqueous solution. Ultimately, it is the solubihty of a particular salt that determines which compound crystalhses out from aqueous solution. 

The REPO4 structure for the heavier rare earth elements (terbium to lutetium, yttrium, and scandium) belong to the tetragonal system. The coordination number of the central ions is... 

The fact that this structure is observed for all M(03SCF3)3 9H20 (M = Sc, Y, La-Lu), irrespective of ionic radius, reflects the role of hydrogen bonding between the coordinated water molecules and the triflate groups in stabilizing the structure and has no implications for the coordination number of scandium in aqueous solution. The high coordination number of nine is the maximum yet observed for scandium. 

The structural chemistry of scandium(III) has been reviewed37 with emphasis on the coordination number and geometry of coordination complexes. This review covers the period 1926-1998. 

Coordination numbers from 3 to 12 have been observed in solid compounds of scandium and yttrium and some specific examples are given in Tables 1 and 2. Some discussion of these properties will be introduced in individual sections. The structural chemistry of scandium(III) has been reviewed with coordination numbers from 3 to 9 being highlighted.37... 

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. 

Many of the early reports of complexes with bidentate oxygen ligands involved acac complexes and the /3-diketonate and related ligands were discussed in CCC (1987).210 The /3-diketonate complexes of scandium and yttrium and the lanthanides were also discussed in CCC (1987).1 Many reported systems contained adducts formed by the metal /3-diketonate complexes. In solution some of the complexes with bidentate ligands can expand their coordination number to seven by coordination to a suitable donor ligand. A solvent extraction method has been used to show that Sc(acac)3 does not form an adduct with trioctylphosphine oxide but tris(tropolonato)scandium(III) and tris-(ethylmatolato)scandium(III) do coordinate with trioctylphosphine oxide.211... 

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