Short communication A DFT overview of high-valent iron, cobalt and nickel tetraamidomacrocyclic ligand (TAML) complexes: The end of innocence? Marrigje Marianne Conradie a,b , Jeanet Conradie a,b , Abhik Ghosh a, * a Department of Chemistry, University of Tromsø, Breivika, N-9037 Tromsø, Norway b Department of Chemistry, University of the Free State, 9300 Bloemfontein, South Africa Received 10 December 2005; received in revised form 10 January 2006; accepted 10 January 2006 Available online 28 February 2006 Abstract Amidato-N ligands are normally viewed as classic, strongly r-donating, innocent ligands. However, when coordinated to high-valent transition metal centers, tetraamidomacrocyclic ligands are often substantially noninnocent, i.e., exhibit radical character involving the amido p-systems. Even the so-called MAC * ligand, generally considered to be an innocent ligand, is noninnocent in several of its known complexes. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Noninnocence; High-valent; TAML; Tetraamidomacrocyclic; Deprotonated amide Deprotonated amide (amidato-N) ligands are well- known in metal–peptide complexes [1] and are known to occur in several metallobiomolecules including bleomycin (Fe) [2,3], nitrile hydratase (Fe) [4], and the PrP prion pro- tein [5,6]. A deprotonated amide is generally viewed as a quintessential strong-field r-donating ligand and the pres- ence of one or more amidato-N ligands in the metal coor- dination sphere largely dominate the nature of the ligand field in the metallobiomolecules named above. Surpris- ingly, except in the case of bleomycin [3], transition metal–amidato-N interactions and bonding remain largely unexplored via quantum chemical, specifically DFT, calcu- lations. In this communication, we report a first DFT overview of several crystallographically characterized, paramagnetic, high-valent transition metal tetraamidomac- rocyclic ligand (TAML) complexes, reported over the last 15 years or so [7,8]. We will specifically focus on complexes based on the most innocent of these ligands, namely the so- called MAC * 4 ligand [7,8], shown in Fig. 1, rather than on ones where a pair of adjacent amidato nitrogens are derived from an oxidation-prone phenylenediamine moi- ety. In particular, we will critically examine the question of innocence or otherwise of amidato-N ligands, when bonded to high-valent transition metal centers, making considerable use of spin density computer graphics. Throughout this study, we used spin-unrestricted DFT calculations with full symmetry-unconstrained geometry optimizations, the VWN local functional, PW91 gradient corrections for both exchange and correlation [9–11], all- electron triple-f plus polarization Slater-type basis sets, a fine mesh for numerical integrations of matrix elements, and tight criteria for geometry optimizations, all as implemented in the ADF 2004 program system [12]. These methods have been documented to provide a gen- erally good description of high-valent metalloporphyrins and related compounds [13,14]. Figs. 2–9 depict the var- ious optimized structures, Mulliken spin populations, and spin density plots. Table 1 compares selected optimized versus crystallographic geometry parameters, while Table 2 lists Mulliken charges and spin populations in tabular form. In general, the agreement between calculated and 0162-0134/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2006.01.028 * Corresponding author. Tel.: +47 77644072; fax: +47 77644765. E-mail address: abhik@chem.uit.no (A. Ghosh). www.elsevier.com/locate/jinorgbio Journal of Inorganic Biochemistry 100 (2006) 620–626 JOURNAL OF Inorganic Biochemistry