6 The polyatomic molecules (B)

Transcription

1 6 The polyatomic molecules () 6. lectron-deficient multi-center bonds 6.. oranes and their relatives its seqular equation is : :, : bond : three center two electron β β β β a b a orbital s hybridization sp i. 6 : 3 valence electrons forms - bond ) ( ) ( ) ( - bonding ) (anti ' - bonding ) (anti ) ( (bonding ) 3 3 c c β β > +

2 3 Three center two electron -- bond: ii. 5 9 lectron-deficient multi-center bonds β β β β β β ) ( 3 seqular equation 3 a b a solve β β center-electron bonds

3 hemical bonds in oranes (a) Single bond - - (b) 3center-electron bonds (c) Other polycenter-polyelectron bond lectron-deficient multi-center bonds 6.. For n n+m s open structure (including to nido and arachno) n equals to number of - bonds Its stationary condition: x m-s t n-s y (s-m)/ p sets of styx p isomers The topological structure of 6 0 n n+m (William Lipscomb) Nobel prize,976 s t y x (3 isomers) (40) (33) (40)

4 Structure of oranes lectron-deficient multi-center bonds 6..3 For orohydride ions n n - s closo structure and carboranes n n n+n +m n n - (closo) 4-4 ( tetrahedral) 6-6 (octahedral) 8-8 (dodecahedral) - (icosahedral) lectron-deficient multi-center bonds

5 polyhedra with n vertices loso series -formula n n - Total valence electrons (V) 3n (from ) +n (from ) + (negative charge) 4n+ ach unit uses electrons. ence skeletal or framework electrons (NF4n+-nn+) There is little tendency to add + and form neutral species. The closo species are, in effect, the anions of quite strong acids. Structures are those of the appropriate polyhedra with n vertices.

6 MOs in closo series Wade s rules n+ bonding and nonbonding MOs n- antibonding Notes: - σ orbital and its electrons are not taken into account lectron-deficient multi-center bonds Structure of arboranes,5-3 5,- 4 6,6-4 6, isomers of 0 (with hydrogen omitted) lectron-deficient multi-center bonds

7 Nido series formula n (n+4) Total valence electrons(v) 3n () + n() + 4 (extra and/or negative charges) 4n +4 Framework electrons (NF) n+4 (n+ pairs). The structure of the nido compound is based on the closo polyhedron with one more vertex than the nido compound. Arachno series formula n (n+6) Total valence electrons(v) 4n +6 Framework electrons (NF) n+4 (n+ pairs). The structure of the nido compound is based on the closo polyhedron with two more vertex than the nido compound. lectron-deficient multi-center bonds 6..4 other deficient electron compounds lectron-deficient multi-center bonds oron group Al Ga In Tl 3 0º Al 3 70º.4 Å.94Å Al 3 Gaseity: monomer Solid state: polymer 3 3 3

8 Alkali metals and alkali earth metals [Li( 3 )] 4 Li e 3 e( 3 ) e e 3 3 e e e 3 e e e e dimer trimer polymer 6. hemical bonds in the coordination compounds

9 oordination compounds compounds composed of a metal atom or ion and one or more ligands. Ligands usually donate electrons to the metal Includes organometallic compounds New theories arose to describe bonding. Valence bond, crystal field, and ligand field. O O O O N N O O O O ethylenediaminetetraacetate (DTA 4 ) (hexadentate)

10 6.. oordination polyhedron molecualr Ag(N 3 ) +.N. hybridization type sp Symmetry geometry linear ul 3-3 sp D 3h triangular Ni(O) 4 4 sp 3 T d tetrahedral Ptl 4-4 dsp D 4h square planar Fe(O) dsp 3 d sp D 3h 4v Trigonal bipyramidal square pyramid FeF d sp 3 O h octahedral others 8 D 4h tetragonal 8 0 D 4d Anti-square pyramid icapped square antiprism I h icosahedral 6.. rystal Field Model focuses on the energies of the d orbitals. Assumptions. Ligands are negative point charges.. Metal-ligand bonding is entirely ionic. strong-field (low-spin): large splitting of d orbitals weak-field (high-spin): small splitting of d orbitals

11 rystal-field Theory A. rystal field splitting rystal-field Theory A. rystal field splitting d z d x-y e g crystal field splitting, Δ 3d d xy d xz d yz t g "naked" metal coordination complex

12 A. rystal field splitting. spectrochemical series N > NO > en > N 3 > O > O 4 > O > F >l > r > I strongest bond largest Δ absorbs appears weakest Ni( O) + 6 <R G Ni(N 3 ) + O Ni(en) + 3 G V strongest Ni(N) 4 V Y ROYGV lower weakest bond smallest Δ higher Δ hν ~visible region A. rystal field splitting. charge on metal greater charge larger Δ (ligands held more closely, interact more strongly with d orbitals) absorbs appears Fe( O) + 6 R G Fe( O) 3+ 6 V RO ROYGV lower higher Surface plasma --- UV-Vis spectroscopy

13 II. rystal-field Theory. Magnetic properties e.g., Fe(N 3 ) 6 + (Fe + d 6 ) e g t g low-spin complex (maximum pairing) diamagnetic found experimentally or e g t g high-spin complex (minimum pairing) paramagnetic II. rystal-field Theory. Magnetic properties ompetition between: crystal field splitting (Δ) electron pairing energy (P) when Δ < P high-spin complex when Δ > P low-spin complex Generally: d, d, d 3 : always high-spin d 4, d 6 : high-spin with ligands O low-spin with ligands > O d 5 : high-spin with all ligands except N d 7 -d 0 always low-spin

14 II. rystal-field Theory. Magnetic properties e.g., [r( O) 6 ] 3+ e.g., [Ni(N 3 ) 6 ] + e.g., [Fe( O) 6 ] 3+ vs [Fe(N) 6 ] σ ligands and σ bond 3 4 ategories of central metal valence orbitals: 5 6 σ group: s, p x,p y,p z, d x -y, d z π group: d xy,d xz,d yz

15 6..3 σ ligands and σ bond ϕ4s ± ( σ+ σ + σ3 + σ4 + σ5 + σ6 ) 6 ϕ4px ± ( σ σ4 ) 3 ϕ4py ± ( σ σ5 ) 4 ϕ4pz ± ( σ3 σ6 ) 5 ϕ ± 3 ( σ3+ σ dz 6 σ σ σ4 σ5 ) 3 6 ϕ ± 3dx y ( 4 5 ) σ σ + σ σ

16 nergetic diagram of σ molecular orbitals 6..4 π onding A π-donor ligand donates electrons to the metal center in an interaction that involves a filled ligand orbital and an empty metal orbital. l-, r-, and I- donates p electrons to the metal center A π-acceptor ligand accepts electrons from the metal center in an interaction that involves a filled metal orbital and an empty ligand orbital. O, N, NO, and alkenes accept electrons into their vacant anti-bonding MO s. π-acceptor ligands can stabilize low oxidation state metal complexes.

17 Ni(O) 4 [Ptl 3 ( 4 )] - Ni O π* bond σ bond (d xy, d xz, d yz ) l l l Pt O OMO Pt π bonding orbital O LUMO π anti-bonding orbital d - π conjugation O bond made weakened electron rule A low oxidation state organometallic complex contains π- acceptor ligands and the metal center tends to acquire 8 electrons in its valence shell. Rules: Treat ligands as neutral entities. The number of valence electrons for a zero-valent metal center is equal to the group number. g: r (group 6) in r(o) 6, Fe (group 8) in Fe(O) 5, and Ni (group 0) in Ni(O) 4

18 8-electron rule Many ligands donate more than electron. -electron donor: (in any bonding mode), and terminal l, r, I,R (e.g. Ralkyl or Ph) or RO ; -electron donor: O, PR 3, P(OR) 3, R R (η -alkene), R : (carbene) 3-electron donor: η (allyl radical), R (carbyne), μ- l, μ- r, μ- I, μ- R P ; 4-electron donor: η 4 diene, η 4 4 R 4 (cyclo-butadienes); 5-electron donor: η 5 5 5, μ 3 -l, μ 3 -r, μ 3 -I, μ 3 -R P ; 6-electron donor: η 6 6 6, η Me; - or 3-electron donor: NO xample: Ferrocene 6.3 Prediction of structural features of inorganic iono-covalent compounds with tetrahedral anion complexes

19 %O T and %O [3] and %O [4] 0 ume-rothery s 8 - N rule (930/3) Rule rationalizes observed structural features of (post-transition) main group elements. y forming the correct number of shared bonds with its neighbors each atom succeeds to complete its octet. The number of bonds of an element is 8 - N where N is its column number in the periodic table (only for 4 N 8). lement structures which obey ume- Rothery s 8 - N rule N 4 4 bonds N 5 3 bonds N 6 bonds D, Si, Ge, α-sn P, As, Sb, i S, Se, Te N 7 bond N 8 0 bonds F, l, r, I e, Ne, Ar, Kr, Xe, Rn

20 Generalized 8 - N rule Pearson (964), ulliger & Mooser (965) 8 - V A AA - / (n/m) for m A n V A : Number of valence electrons par anion V A < 8, AA>0, 0 Polyanionic val. comp. V A 8, AA0, 0 Normal valence compound V A > 8, AA0, >0 Polycationic val. comp. AA: Average number of A-A bonds per anion ; Average number of - bonds per cations and/or electrons used for inert-electron pairs on cations

21 Polyanionic valence compound () K 6 Pd + Se 0 : V A 8/0, AA8/5 K 6 Pd[Se 5 ] 4 AA AA 4/3 AA 6/4 AA 8/5 N A/M /( - AA) N A/M : Average number of atoms in a non-cyclic charged anion molecule Polyanionic valence compounds () LaAs : V A 6.5; AA 3/; N A/M 4 La ^[As 4 ] LT, La 4^[As 3 ] ^[As 5 ] T ste 4 : V A 6.5; AA 7/4; N A/M 8 s ^[Te 8 ] Th S 5 : V A 7.6; AA /5 Th ^[S ][S] 3 Sr 5 Si 3 : V A 7.33; AA /3 Sr 5^[S ][S]

22 Polycationic valence compounds gl : V A 9; [g-g]l l 3 : V A 8.33; [-]l 6 SiAs : V A 9; [Si-Si]As 6.4 Transition-metal cluster compounds 6.4. Metal-metal bond L n M-ML n, L n MML n, L n M ML n, so called cluster Re l 8 - Re, d 5 s Re-Re.4Å (.76 Å in Re crystal) l l 3.3 Å Re 3+, d 4, dsp hybridization( σ bond), remain four d and one p orbital σ ( d d ) π ( d π ( d δ ( d z xz yz xy d d d z xz yz xy ) ) ) Quadruple ond 4 σ π δ Analog: Mo (O R) 4 and r (O R) 4 [Rel8] -

23 6.4 Transition-metal cluster compounds 6.4. Metal-metal bond L n M-ML n, L n MML n, L n M ML n, so called cluster (O) 3 Ir Ir(O) 3 Ir (O) Ir 3 (O) 3 Ir 4 (O) 6.4. luster geometry i. Structural polyhedron

24 ii. lectron counting b (8n g) b: bond valence (total number of metal-metal bonds) n: number of metal atoms g: total electrons in valence shell, including all In the case that main group atoms are included: b (8n + 8n g)

25 iii. ond valence and the cluster geometry b (8n g) Tri-nuclear compounds Metal cluster compounds Os 3 (O) 9 (μ 3 -S) g 50 b M-M/pm Os-Os, 8.3 Os Os Os Mn Fe(O) 4 Fe 3 (O) Mn-Fe, 8.5 Fe-Fe, 8.5 Mn Fe Mn Fe Fe Fe Os 3 (O) Os-Os, 8.5 Os [Mo 3 (μ 3 -O)(μ -O) 3 F 9 )] 5- Re 3 (μ -l) 3 (SiMe 3 ) OsOs, 68.0 MoMo, 50. Re Re, 38.7 Os Mo Re Mo Os Mo Re Re

26 Tetranuclear compounds Ir 4 (O) Re 4 (O) 6 - Os 4 (O) 6 g4*9+*60 g4*7+6*+6 g4*9+6*64 b/(8*4-60)6 b/(8*4-6)5 b/(8*4-64)4 Pentanuclear compounds exanuclear compounds

27 Multi-nuclear (N>6) compounds 6.5 arbon clusters and nanotubes. Fullerences 50 l 0

28 . arbon nanotubes 00 nm µm 6.6 ydrogen onding.85å 0.95Å * ydrogen bonding in DNA