Charge density in Cu(glygly)(OH2)(2)center dot H2O at 10 K and the reproducibility of atomic orbital populations

An extensive accurate X-ray data set for diaqua(glycylglycinato) copper(II) hydrate, Cu(glygly)(OH2)(2).H2O, at 10 K is reported. The molecular struct ure is very accurately defined. The Cu(glygly)(OH2)(2) unit approximates to a square pyramid with the apical water molecule only loosely bound. The da ta have been analysed by multipole and valence orbital population models an d the topology of the electron density has been examined for chemical bondi ng features. There are two independent molecules of the complex in the unit cell, and the agreement between their bonding features is used as a measur e of the validity of obtaining quantitative valence-related parameters from X-ray data. The agreement is quite reasonable. Atomic charges evaluated by the Stockholder and monopole population methods do not conform with chemic al expectations. Those from the sum of valence orbital populations and the atom-in-molecules approach are more reasonable but contain anomalies. The ' hole' in the 3d(10) shell corresponding to the 3d(9) configuration, predict ed on the grounds of ligand field theory, is seen as a much lowered (1.16 e ) population of 3d(z2) relative to the other 3d orbitals (av. 1.65 e). Popu lations of valence hybrid orbitals conform with chemical expectations and, for the (sp(2))(sigma) hybridised carbon atoms of C=O groups, relocation of electrons from a p(pi) orbital to those sigma hybrids is seen. Bond critic al points have been located between all the appropriate non-hydrogen atom p airs. The electron densities at these points show values typical for mixed ionic/covalent character in the Cu-O and Cu-N interactions and of covalent character in the C-C, O-C and N-C bonds. The Laplacian of the density at th e bond critical points also behaves in a fashion which conforms with those bond types, although the scatter in the results is greater. There is little evidence of the copper atom having influenced the electron densities of th e bonds between light atoms within the glycylglycinato moiety.