What makes a good ligand

Thus the energy difference between the t 2g and e g orbitals can range between the energy of a rather weak to a rather strong covalent bond.

For a given metal in one oxidation state e. The 4d and 5d elements are similar in their size and their chemistry. This trend reflects the spatial extent of the d-orbitals and thus their overlap with ligand orbitals. The 3d orbitals are smaller, and they are less effective in bonding than the 4d or 5d. The 4d and 5d orbitals are similar to each other because of the lanthanide contraction.

At the beginning of the 5d series between 56 Ba and 72 Hf are the fourteen lanthanide elements 57 La - 71 Lu. Although the valence orbitals of the 5d elements are in a higher principal quantum shell than those of the 4d elements, the addition of 14 protons to the nucleus in crossing the lanthanide series contracts the sizes of the atomic orbitals.

The important result is that the valence orbitals of the 4d and 5d elements have similar sizes and thus the elements resemble each other in their chemistry much more than they resemble their cousins in the 3d series. For example, the chemistry of Ru is very similar to that of Os, as illustrated below, but quite different from that of Fe.

Both Os and Ru form volatile, molecular tetroxides MO 4. OsO 4 is used in epoxidation reactions and as a stain in electron microscopy. A handful of complexes with other oxidation states are reported. V II pyridine complexes were prepared with a range of monodentate and bidentate anionic ligands. Along with basic pyridine moiety, various substituted derivatives were found in the coordination sphere of V II centers [ 14 , 15 , 16 ].

The inclusion of oxo group helped in stabilizing V IV and V V states and a range of pyridine complexes were synthesized. The complexes could be synthesized from halides salts of respective metals and pyridine in a neutral environment. The tetravalent and pentavalent complexes are harvested in a low-temperature environment to prevent decomposition through disproportionation. The higher valent pyridine complexes possess superior stability than their lower valent counterpart. Chromium, molybdenum, and tungsten pyridine complexes could be obtained from their inorganic salts as well as carbonyl and nitrosyl complexes.

Neutral environment remain a preferred choice to ensure the stability of synthesized complexes. The large range of oxidation states 0-VI of these metals has produced innumerable pyridine complexes. The common coordination number remains six in Cr and Mo, but it could be in higher numbers in tungsten pyridine complexes. The Cr IV and upper oxidation state mostly found with an oxo group. The representative chromium-pyridine complexes are [Cr py acac 3 ] [ 25 ], [Cr O py Br 3 ] [ 26 ], [Cr O 3 py ] [ 27 ] and so on, reflects the above facts.

The oxo group continues to stabilize higher valent molybdenum and tungsten pyridine complexes too. Representative pyridine complexes of Cr, W and Mn. Manganese and rhenium forms complexes with pyridine in different oxidation states spreading over 0 to VII.

The coordination number commonly varied from four to eight. However, the manganese forms pyridine complexes only in zero to quadrivalent oxidation states, whereas rhenium pyridine complexes exist in seven oxidation states. The lower valent pyridine complexes of these metals are composed of carbonyl and nitrosyl counterpart.

The higher valent rhenium accommodates oxo ligands along with anionic monodentate and chelating ligands. The Mn I complexes quickly react with air and oxygen. Thus their preparation is carried out in a neutral atmosphere. The higher valent pyridine complexes are stable in normal condition and could be prepared in alcoholic or aqueous media.

Manganese halides and manganese oxide are remaining preferred starting materials for synthesizing pyridine complexes. The rhenium-pyridine complex preparation has also origin at rhenium halides, such as ReI 4 , K 2 [ReCl 6 ] are few to mention. The higher valent rhenium pyridine compound also derived from K[ReO 4 ].

Technetium-pyridine complexes are rare [ 39 , 40 ]. Representative pyridine complexes of Re, Fe and Ru. Fe II has produced quite a significant number of pyridine complexes with the comparison to other first transition metals.

A range of pyridine derivatives were included in these Fe-pyridine complexe. Though four and five coordinated complexes are also seldom found, the pyridine complexes were prepared by interaction of pyridine and an inorganic salt of iron. In this group, the versatility of complex formation continues with ruthenium and osmium too. This is evident from numerous pyridine complexes reported with these metals.

Ruthenium displays nine oxidation states 0-VIII. Few complexes even reported with fractional oxidation number. The common coordination number in such complexes ranges from 4 to 6, though higher coordination numbers are claimed in higher oxidation states. Ruthenium pyridine complex preparation involves high-temperature reflux of ruthenium salt with pyridine in an organic solvent preferably in the oxygen-free environment.

These resultant pyridine complexes are often labile and subject to the decomposition. Ru III pyridine complexes often show up disproportion to Ru II and Ru IV states and this decomposition route appears to be a synthetic procedure for new complex preparation. The osmium pyridine complex preparations could be achieved by reaction of K 2 [OsCl 6 ] and pyridine. OsO 4 also proved to react with pyridine and produce higher valent pyridine complexes.

In a reaction of [Os 3 CO 12 ] with pyridine in neat or with a pyridine saturated hydrocarbon solvent resulted series of complexes [ 48 ], such as [HOs 3 py CO 10 ], [HOs 3 py CO 9 ], [H 2 Os 3 py 2 CO 8 ], [Os 2 py 2 CO 6 ], and so on, K[OsCl 4 bpy ] gave rise to pyridine complex [Os py Cl 3 bpy ] by treatment with aqueous pyridine in boiling condition or treatment in pyridine—glycerol mixture [ 49 ].

These complexes are fairly stable except the labile Os II complexes. Representative pyridine complexes of Os, Co and Rh. The Co II could accept one to six pyridine ligands in its coordination sphere and the resultant complexes are mostly six coordinated.

One such complex contains six pyridine ligands Figure 6 and formulated as [Co py 6 ]I 2 [ 50 ]. Most of the cobalt pyridine complexes are mixed-ligand molecule. They could either prepared from alcoholic or aqueous solution with reaction of cobalt salt and appropriate quantity of pyridine.

When cobalt iodide treated with excess of 3-ethyl pyridine a brown complex [Co 3-Et-py 4 Cl 2 ] resulted [ 51 ]. Several other substituted pyridine employed in a similar fashion resulting in numerous complexes.

Cobalt I complexes usually resulted from reduction of Co II -pyridine complexes. Rhodium also used its various oxidation states to have pyridine coordination.

Rh III dominates the spectra of pyridine complexes with octahedral geometrical preference, whereas square planar geometry is common finding with Rh I complexes. RhCl 3 is the most common starting material for preparation of pyridine complexes. The preparation of iridium pyridine complexes can be achieved from the array of starting materials.

The number of pyridine moiety varies around iridium center and it could reach maximum six. Representative pyridine complexes of Ir and Ni. Ni II complexes could be tetra, penta, and hexacoordinated. Ni II pyridine complexes offer easy preparation in the organic solvent by the combination of nickel salt and pyridine and are stable against aerial oxidation, but Ni I pyridine complexes are sensitive to air and moisture.

Though nickel can be coordinated up to six pyridines the stability of these complexes are very low [ 56 ]. The palladium and platinum have many similarities in complex formation.

Majority of the complex contain one or two pyridine ligand in metal coordination sphere. Tetra pyridine complexes such as [Pd py 4 ] PF 6 2 , [ 57 ], [Pt py 4 Br 2 ] [ 58 ] were also synthesized. Representative pyridine complexes of Pd, Pt and Cu. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. What makes a ligand stronger than another?

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