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M-M Bonding in Transition M etal Complexes. Prepared by: Dr. Kuldeep Mahiya (Assistant Professor) Department of Chemistry FGM Government College, Adampur. B. Sc -II (3 rd Semester) Paper : Inorganic Chemistry (CH-201) Kurukshetra University, Kurukshetra Session 2018-19. Introduction.
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M-M Bonding in Transition Metal Complexes Prepared by: Dr. Kuldeep Mahiya (Assistant Professor) Department of Chemistry FGM Government College, Adampur B. Sc-II (3rd Semester) Paper : Inorganic Chemistry (CH-201) Kurukshetra University, Kurukshetra Session 2018-19
Introduction • The M-M bonding is quite common in transition metal complexes. In 1844, French chemist M. Eugene Peligot prepared the chromous acetate dehydrate. At the time, it was impossible to know that he had prepared the first compound with a metal−metal multiple bond; it was not until 1970 that the structure of Cr2(OAc)4(OH2)2 was solved crystallographically and a short Cr−Cr distance of 2.36 Å was determined. • F. A. Cotton (who described the first metal-metal multiple bond in 1964), and co-workers have shaped this field from its infancy to its present maturity and have introduced the majority of new compounds and the most significant bonding considerations to the chemical community. • Bonds between metals continue to expand beyond quadruple to bond orders of 5 or 6.
Introduction • The second and third transition series metals have strong tendency to form metal-metal bonds in comparison to their congeners of first transition series. M-M bonding is rare in first transition series elements. • It occurs in few metal carbonyls like Mn2(CO)10, Fe2(CO)9, Co2(CO)8, Fe3(CO)12, Co4(CO)12, etc. • It occurs in some binuclear carboxylate complexes such as Cr2(CH3COO)4(H2O)2.
Introduction • The elements of second and third transition series form carbonyls with M-M bonds such as Ru3(CO)12, Os3(CO)12, Rh4(CO)12, Ir4(CO)12, Rh6(CO)16etc.. • Rhenium forms Re2Cl82-, which contain Re-Re quadruple bond whereas no is Mn2Cl82- is unkown. • The binuclear species such as M2(CH3COO)4(H2O)2 where M = Mo, Ru, Rh, Re are similar to those of Cr2(CH3COO)4(H2O)2. • Niobium and tantalum forms cluster compounds of type [Nb6Cl12]2+ and [Ta6Cl12]2+. These have extensive M-M bonding. However, their analog vanadium does not form such species
M-M Bonding in 3d Metal Carboxylates • Structural features : • Extremely short Cr―Cr distance of 2.36 Å or 236 pm. • Chromium is in +2 oxidation state. So four unpaired electrons per chromium metal but compound is diamagnetic. • This behavior can be explained on the basis of a quadruple bond between the two Cr atoms. • Each Cr uses its six d2sp3 hybrid orbitals for bonding. One is used for formation of Cr-Cr sigma bond and other are used to receive the pairs of electron from five ligand sites (one water and two bi-dentate acetates). • Remaining three unhybridd orbitals are used for formation of two pi and one delta bond as shown in figure. • These types of molecules are known as paddle wheel molecules
Re2Cl82- anion • Structural features : • Extremely short Re―Re distance of 2.24 Å or 224 pm compared with average Re―Re distance of 2.75 Å in rhenium metal and 2.48 Å in Re3Cl9. • Unexpected feature is the eclipsed configuration of chlorine atoms. • Cotton explained both phenomena by invoking a quadruple bond.
Bonding • The electronic configuration of Re is 5d56s2. • Oxidation state is +4 (Re3+, d4 system). • There are 3 × 7 + 12 × 1 + 3 = 36 valence electrons in Re3Cl123−. On each Re–Re linking, each bridging Cl atom forms a 3c–2e Re–Cl bonds with two Re atoms. • The three 3c–2e and nine terminal 2c–2e Re–Cl bonds use up 24 of the valence electrons, leaving 12 electrons for Re–Re bonding. • Thus each Re–Re linkage is a double bond, with bond order equal to 2.
M-M Bonding in Metal Carboxylates M2(RCOO)4(H2O)2 (M = Mo, Ru, Rh, Re)
Nb6Cl122+ • The electronic configuration of Nb is 4d35s2. • There are 6 × 5 + 12 × 1 - 2 = 40 valence electrons in Nb6Cl122+. Each Cl bridging at each edge of octahedron forms twelve 3c–2e Nb–Cl bonds and utilized 24 electrons. • The remaining 16 electrons are used to form 12 Nb-Nb bonds with a bond order of 16/24 = 2/3. • Thus each Nb-Nb bond has a fractional bond order.
