Lecture 35 Isolobal analogy The isolobal analogy allows to relate and compare organic, inorganic and organometallic compounds on one uniform basis. What in common have CH3, NH2, OH, F, Cl, Co(CO)4 and CpMo(CO)3 ? These “fragments” or “building blocks” can replace each other in more complex structures. For example, all the groups above can combine to form an ordinary bond: CH3-CH3, NH2-NH2, HO-OH, F-F, Cl-Cl, (CO)4Co-Co(CO)4, CpMo(CO)3-Mo(CO)3Cp CH3-NH2, CH3-OH, CH3-F, CH3-Cl, CH3-Co(CO)4, CH3-Mo(CO)3Cp NH2-OH, NH2-F, NH2-Cl, NH2-Co(CO)4, NH2-Mo(CO)3Cp etc. “Fragments” or “building blocks” that can replace each other in complex structures are isolobal. The definition: Two “fragments” are called isolobal if their frontier orbitals: i) are same in number, ii) have similar symmetry, iii) are of approximately the same energy, and iv) have the same number of electrons on them. All examples above represent the case of one singly occupied (SO) “working” orbital: The definition above is not always easy to use. Simpler ways to identify isolobal groups exist.

2) Groups isolobal with CH3 (one SO working frontier orbital) Electron-equivalent groups miss the same number of electrons necessary for the central atom to reach the stable electronic configuration (8, 16, or 18 e’s, etc.). Electron-equivalent groups are always isolobal. Isolobal groups # of e’s in the valence shell Stable electron configuration # of missing e’s (“holes”) CH3 4+3=7 8 1 NH2 5+2=7 OH 6+1=7 F 7 Co(CO)4 9+4(2)=17 18 CpMoI(CO)3 5+6+3(2)=17 Transition metal derived fragments MLn isolobal with CH3 d1-ML8 d3-ML7 d5-ML6 d7-ML5 d9-ML4 (C8H8)2La Cp2V(CNR) CpMn(CO)3+ [Co(NH3)5]2+ Co(CO)4

3) Groups isolobal with triplet CH2 (two working SO frontier orbitals) Groups that are electron-equivalent with the triplet methylene of (a1)1(b2)1 configuration have two electrons missing in the valence shell of the central atom: Combination of two fragments isolobal with CH2 gives “dimers” with double bond between the fragments like in CH2=CH2: CH2=NH, CH2=O, CH2=Os(CO)4, CpRh(CO)(=CH2) etc. “Trimers”: Isolobal groups # of e’s in the valence shell Stable electron configuration # of missing e’s (“holes”) CH2 4+2=6 8 2 NH 5+1=6 O 6 Ni(CO)3 10+3(2)=16 18 Os(CO)4 8+4(2)=16 CpRh(CO) 6+8+2=16

4) Groups isolobal with CH (three working SO frontier orbitals) CH of (s)1(p)2 configuration is isolobal with N, O+, Co(CO)3, Re(CO)4, NiCp, CpW(CO)2 (and W(OMe)3!) etc. Some combinations of the fragments above: CH≡N, CH≡O+, N≡O+, Cp(CO)2W≡WCp(CO)2 Group # of e’s in the valence shell Stable electron configuration # of missing e’s CH 4+1=5 8 3 N 5 O+ 6-1=5 Co(CO)3 9+3(2)=15 18 Re(CO)4 7+4(2)=15 CpWI(CO)2 5+6+ 2(2)=15

5) Non-electron-equivalent isolobal groups CH3+ is isolobal with singlet CH2, Cr(CO)5, CpMn(CO)2, PtCl3-, Au(PPh3)+, (all have one empty “working” orbital) All these species can react in the similar manner with donors of an electron pair: CH3+ + NH3  CH3-NH3+ Cr(CO)5 + NH3  H3N-Cr(CO)5 PtCl3- + NH3  H3N-PtCl3- Au(PPh3)+ + NH3  H3N-Au(PPh3)+ More applications of the isolobal analogy: In boron cages isolobal groups can replace each other: BH, Si, Ge, Sn, CH+, NH2+, Os(CO)3 etc. In organometallic chemistry:

6) Isolobal ligands Ligands can also be isolobal analogues of each other and thus can replace each other in metal complexes. The bonding modes between the metal and any of the isolobal ligands will remain the same: