US EPA

1355086 

Journal Article 

EPR STUDIES OF 1/1 COMPLEXES OF RHODIUM(II) AND COBALT(II) PORPHYRINS WITH SIGMA-DONOR AND PI-ACCEPTOR LIGANDS - ORIGINS OF RHODIUM(II) METALLORADICAL REACTIVITY 

Wayland, BB; Sherry, AE; Bunn, AG 

1993 

Yes 

Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 

115 

17 

7675-7684 

WOS:A1993LW56000011 

Tetrakis(2,4,6-trimethylphenyl)porphyrinato)rhodium(II)
((TMP)Rh) and tetrakis(2,4,6-triisopropylphenyl)-porphyrinato) rhodium(II) ((TTiPP)Rh) occur as
low-spin d7 complexes with EPR parameters associated with a (d(xy))2-(d(xz,yz))4(d(z)2)1 ground
configuration. (por)Rh(II) species typically react as metalloradicals with a wide variety of
substrates to give diamagnetic products; however, the use of increased porphyrin or substrate
steric demands has permitted observation of paramagnetic 1:1 adducts. EPR spectra were used in
examining features related to the electronic structure for a series of 1:1 five-coordinate
complexes with nitrogen, phosphorus, arsenic, and carbon donor ligands. Each of the five-
coordinate species is a low-spin d7 complex with the unpaired electron occupying a d(z)2 MO ((d
(xy)d(xz)d(yz))6-(d(z)2)1 ground configuration). The ligands form 1:1 adducts that have effective
axial symmetry with the exceptions of CO, which has as a bent Rh-CO unit, and ethene, which has a
symmetrical pi complex structure. Donor atom spin densities are estimated from ligand hyperfine
coupling. Rhodium-103 hyperfine coupling is used in evaluating the rhodium 4d(z)2 (0.67) and 5s
(0.02) spin density in the ethene pi complex, which combined with the total ethene C2p (0.278)
and C2s (0.008) spin densities, determined from the ligand hyperfine coupling, accounts for most
of the unpaired electron. Estimates of the d(z)2 to d(xz) and d(yz) energy separations for the
series of 1:1 complexes illustrate the elevation of d(z)2 by sigma donor ligands and the combined
lowering of d(xz), d(yz), and d(z)2 elevation by ligands with pi acceptor ability. Adduct
formation of (por)Rh. with pi acceptor ligands results in substantially larger d orbital energy
separations and ligand spin densities than the strictly sigma donor ammine ligands. The d orbital
energy separations and ligand spin densities of (por)Rh.L complexes are significantly greater
than those observed for the corresponding (por)Co-L complexes, which is also reflected in the
greatly enhanced scope of reactivity manifested by (por)Rh. species compared with the analogous
Co(II) derivatives. Unusual structural and reactivity properties of the (por)Rh.CO and (por)
Rh.C2H4 complexes are briefly discussed and contrasted with those of the corresponding (por)Co.L
complexes.