3-VBT For Coordination Chemistry [PDF]

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VALENCE BOND THEORY FOR COORDINATION CHEMISTRY



Setia Budi Jurusan Kimia, Universitas Negeri Jakarta



Valence bond theory • The formation of coordination compounds involves reaction between Lewis bases (ligands) and Lewis acid (Central atom) with the formation of coordinate covalent bond • A set of hybrid orbitals is produced to explain the bonding • [Ag(CN)2]- , based on experimental result, has bonding angle of 180 oC



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s, p and d orbitals



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Orbitals hybridization • Hybridization: Linear combination of atomic orbitals on an atom (goldbook-IUPAC), to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties



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Hybrid orbitals • sp



• [Ag(CN)2]-



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Hybrid orbitals • sp2 • [Cu(CN)3]2-



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Hybrid orbitals • sp3 • [CoCl4]2-



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Example 4: [NiCl4]2–, tetrahedral Ni2+ [Ar] 3d8



3d



4s



4p 4sp3 paramagnetic



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Hybrid orbitals • dsp2 • [Cu(NH3)4]2+



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Example 3: [PtCl4]2–, diamagnetic Pt2+ [Xe] 4f14 5d8 5d



6s



6p



dsp2 square planar



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Hybrid orbitals • sp3d • [SnCl5]2-



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Hybrid orbitals • sp3d2 • [Fe(H2O)6]2+



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Example 1: [Co(NH3)6]3+ Co [Ar] 3d7 4s2 Co3+ [Ar] 3d6 Inner orbital complex



3d



4s



4p



4d



d2sp3 octahedral Diamagnetic complexes sb/vbt setia budi/inor3/vbt



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Example 2: [CoF6]3– Co [Ar] 3d7 4s2 Co3+ [Ar] 3d6 Outer orbital complex



3d



4s



4p



4d



4sp3d2 octahedral Paramagnetic complexes sb/vbt setia budi/inor3/vbt



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Electroneutrality principle • • •



Electroneutrality principle expresses the tendency of all pure substances to carry a net charge of zero Complexes would be most stable when the electronegativity of the ligand was such that the metal achieved a condition of essentially zero net electrical charge. One difficulty with the VB assumption of electron donation from ligands to metal ions is the build up of formal negative charge on the metal



An example: [CoL6 ]2+, a complex of Co(II) • The six ligands share twelve electrons with the metal atom, thereby contributing to the formal charge on the metal a total of – 6, which is only partially canceled by the metal's ionic charge of + 2 • From a formal charge point of view, the cobalt acquires a net – 4 charge. Pauling pointed out why metals would not in fact exist with such unfavorable negative charges • the bonding electrons will not be shared equally between the metal and ligands due to the high electronegative of donor atoms on ligands (e.g. N, O and the halogens)



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Electroneutrality principle Explain briefly of the following fact: • [Be(H2O)4 ]2+ is stable, while [Be(H2O)6]2+ is not. • [AI(H2O)6 ]3+ is more stable than [AI(NH3)6]3+.



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Back bonding Opposite to the electroneutrality principle, • Many complexes in which the metal exists in a low oxidation state and yet is bonded to an element of fairly low electronegativity. • CO is the prominent ligand for such complex • When central atom is bound to atom carbon in the ligand, the source of stability in these complexes is the capacity of the carbon monoxide ligand to accept a "back donation" of electron density from the metal atom (described in term of resonance)



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Magnetic properties of Complexes • •



• • • •



In 1845, Michael Faraday classified substances as diamagnetic and paramagnetic Diamagnetic effect is generated by paired ē. When any diamagnetic complex is placed in an external magnetic field, there is an induced circulation of ē producing a net magnetic moment aligned in opposition to the applied field.



Paramagnetism is generated by unpaired ē. The spin and orbital motions of these ē give rise to permanent molecular magnetic moments that tend to align themselves with an applied field. Paramagnetic effect is much larger than diamagnetic effect. Consequently it cancels any repulsion between an applied field and paired ē. sb/vbt



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Magnetic susceptibility balance



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Magnetic properties of Complexes • Dalam atom, elektron bergerak mengorbit inti atom dan berotasi di sekitar sumbu sumbu rotasinya • Apabila 2 elektron berpasangan maka 1 elektron melakukan rotasi berlawanan arah dengan putaran jarumnya dan pasanganya beortasi dengan arah sebaliknya • Rotasi yang arahnya berlawanan jarum jam akan menghasilkan momen magnet yang arahnya ke bawah dan berlaku sebaliknya untuk elektron pasangannya • Momen magnet yang dihasilkan adalah sama besar, sehingga momen magnet pasangan elektron adalah nol sb/vbt setia budi/inor3/vbt



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• Momen magnetik terukur yang dimiliki oleh suatu kompleks disebut momen magnetik efektif (µe) • µe yang dimiliki suatu komplek merupakan hasil interaksi dari momen magnetik yang ditimbulkan akibat gerak mengorbit dan gerak rotasi dari elektron dalam atom • µe = 0, diamagnetik (ditolak oleh medan magnetik eksternal) • µe > 0, paramagnetik (ditarik oleh medan magnetik eksternal) • Momen magnetik yang dihasilkan oleh gerak mengorbit elektron sangat kecil dibandingkan dengan momen magnetik dari gerak rotasi • Momen magnetik yang hanya ditimbulkan dari gerak rotasi disebut momen magnetk spin (µs) dengan satuan Magneton Bohr (BM) µs = [n(n+2)]1/2 n = jumlah elektron tak berpasangan pada atom pusat kompleks sb/vbt setia budi/inor3/vbt



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Kelemahan Teori Ikatan Valensi • Tidak dapat menjelaskan warna atau spektra senyawa kompleks • Tidak dapat menjelaskan kestabilan senyawa kompleks • Tidak dapat menjelaskan bentuk geometri dari [Cu(NH3)4]2+ • dll



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