Polyhedron Vol. I I, No. IS, pp. 239%2407, 1992 Printed in Great Britain 0277-5387/92 $5.00+.00 0 1992 Pergamon Press Ltd zyxwvutsr LIGAND FIELD PROPERTIES OF THE PEPTIDE NITROGEN: THE SHARP-LINE ELECTRONIC SPECTRUM OF Ml%- DI(3-AMINOPROPYL)AMINE(GLYCYLGLYCINATO-O,N,N’)- CHROMIUM(II1) JONG-HA CHOI* and PATRICK E. HOGGARDT Department of Chemistry, North Dakota State University, Fargo, ND 58105, U.S.A. (Received 13 January 1992 ; accepted 27 April 1992) Abstract-An Angular Overlap Model calculation was used to fit the electronic spectrum, particularly the sharp-line positions, of [Cr(dpt)(glygly)]ClO, - 0.5C4H6N202 - 2.5H20 [dpt = di(3-aminopropyl)amine, H2glygly = glycylglycine] at 77 K. An e, value of 502 cm- ’ for the peptide nitrogen indicates that it is a weak net a-donor to chromium(III), presumably because donation from the filled x-orbital and back-donation to the empty 7r-antibonding orbital are almost in balance. Although many transition metal complexes involv- ing a deprotonated peptide nitrogen have been reported, surprisingly little is known about the elec- tronic properties of the metal-nitrogen interaction. In particular, it cannot readily be said whether the peptide nitrogen should be a net n-donor or a rc- acceptor. Delocalization of the carbonyl double bond in the deprotonated peptide should lower the energy of the LUMO, compared to that in an isolated carbonyl. It thereby becomes more difficult to predict whether more electron density will be donated from the HOMO on the nitrogen to the metal, or more from the metal to the LUMO. One way to probe this is through the Angular Overlap Model (AOM) parameter, e,. When a suc- cessful analysis of the electronic spectrum can be performed, the ligand field can be resolved into g- and rr-components for each coordinated atom, * Permanent address : Department of Chemistry, Col- lege of Natural Science, Andong National University, Andong 760-749, Korea. 7 Author to whom correspondence should be addressed. denoted by e, and e,. ’ For a cylindrically symmetric ligating group, 3e,-4e, is equal to the ligand field strength, A or 10 Dq.’ If e, for the peptide ligand in a particular complex can be determined, a positive value can be interpreted as net donation to the metal, while a negative value implies donation in the opposite direction. Hydroxides, halides and car- boxylates are examples of n-donors, while N-coor- dinated nitrite and polypyridines are examples of n-acceptors. 3 The central metal can have a strong influence on the A-donor properties of ligands. In particular, a six-coordinate, approximately orthoaxial complex with a filled tb subshell should have a limited capacity to accept n-electron density from, the ligands. In this paper we report an analysis of the elec- tronic spectrum of [Cr(dpt)(glygly)]ClO,, where dpt = di(3-aminopropyl)amine and H2glygly = glycylglycine. The [Cr(dpt)(glygly)]+ complex was chosen to study the properties of the pep- tide nitrogen, in preference to [Cr(glygly)2]-, in order to keep the number of n-interacting coor- dinated atoms to a minimum (two). The three amine groups of the dpt ligand should have no significant n-interaction with the metal, while the d3 con- figuration of the chromium(II1) poses no hindrance to either donation or acceptance of ligand n-elec- trons. There is still the problem of distinguishing the effects of the coordinated carboxylate oxygen of the glycylglycine from those of the peptide nitrogen. 2399