TABLE 8.3 Average Bond Enthalpies (kJ/mol) Single Bonds C-H 413 N-H 391 О—н 463 F-F 155 348 N-N 163 0-0 146 293 201 0-F 190 CI-F 253 C-0 358 N-F 272 0-CI 203 Cl-CI 242 C-F 485 N-CI 200 0-I 234 C-CI 328 N-Br 243 Br-F 237 C-Br 276 S-H 339 Br-CI 218 C-I 240 Н-Н 436 S-F 327 Br-Br 193 C-S 259 Н—F 567 S-CI 253 Н—СІ 431 S-Br 218 I-CI 208 Si-H 323 Н—Br 366 S-S 266 I-Br 175 Si-Si 226 Н-I 299 I-I 151 Si-C 301 Si-O 368 Si-CI 464 Multiple Bonds 614 N=N 418 0=0 495 839 N=N 941 615 607 523 C=N 891 S=S 418 799 1072

The sulfate ion can be represented with four S—O bonds orwith two S—O and two S=O bonds. (a) Which representation is better from the stand point of formal charges? (b) What is theshape of the sulfate ion, and what hybrid orbitals of S are postu-lated for the σ bonding? (c) In view of the answer to part (b),what orbitals of S must be used for the π bonds? What orbitalsof O? (d) Draw a diagram to show how one atomic orbital fromS and one from O overlap to form a π bond.

A benzene molecule can be modeled as six carbon atoms arrangedin a regular hexagon in a plane. At each carbon atom, one of threeradicals NH2, COOH, or OH can be attached. How many suchcompounds are possible? (Make no distinction between single anddouble bonds between the atoms.)

Does a sulfer-oxygen bond have a significant dipole moment?

Antibonding molecular orbitals can be used to make bondsto other atoms in a molecule. For example, metal atomscan use appropriate d orbitals to overlap with the π*2p orbitalsof the carbon monoxide molecule. This is calledd-π backbonding. (a) Draw a coordinate axis system inwhich the y-axis is vertical in the plane of the paper andthe x-axis horizontal. Write “M” at the origin to denote ametal atom. (b) Now, on the x-axis to the right of M, drawthe Lewis structure of a CO molecule, with the carbon nearest the M. The CO bond axis should be on the x-axis.(c) Draw the CO π*2p orbital, with phases (see the “CloserLook” box on phases) in the plane of the paper. Two lobesshould be pointing toward M. (d) Now draw the dxy orbitalof M, with phases. Can you see how they will overlap withthe π*2p orbital of CO? (e) What kind of bond is being madewith the orbitals between M and C, σ or π ? (f) Predict whatwill happen to the strength of the CO bond in a metal–COcomplex compared to CO alone.