Thermochemistry, singlet–triplet gap and crystal structure of (tetramethylethylenediamine)nickelbis(acetylacetonate) [(TMEDA)Ni(acac)2]

[(TMEDA)Ni(acac)2] was synthesized by the coordination of TMEDA to Ni(acac)2. Its crystal structure shows a slightly distorted octahedral geometry with Ni–N bond lengths of 2.152(2) and 2.1655(17) Å and Ni–O bond distances between 2.0219(14) and 2.0383(16), respectively. Density functional theory calculations (Becke3LYP/6-31G(d)) confirm that the ground state of this octahedral coordinated Ni(II) is a triplet favored by 22.6 kcal/mol over the singlet electron configuration. The thermochemistry of the coordination reaction is exergonic and shows an overall free energy difference of −10.6 kcal/mol. According to our computational results the reaction can be considered a two step process. First TMEDA coordinates to the Ni(acac)2 via one nitrogen atom yielding a five-coordinate intermediate (during this step the electron configuration changes from singlet to the triplet state), followed by the formation of the octahedral complex. Comparisons to related X-ray structures are given.