(1995; 53 pages)
6. COORDINATION COMPOUNDS
Non-cyclic linear structures
6.1 According to current usage (1), in a non-cyclic structure, the symbol of the central atom is placed on the left and is followed by the ionic ligands and then by the neutral ligands. Polyatomic ligands are placed in parentheses, with the atom linked to the central atom on the left. If several identical ligands are attached to the central atom, their number is indicated as a subscript to the right. In each class of ligands, the symbols of the linking atoms, and then of any other atoms, are shown in alphabetical order. The complete formula of the coordination entity (neutral group or complex ion) is placed in square brackets.
6.2 The individual charges usually carried by the central atom and the ligands are not normally shown; they may, however, be shown in structural formulae when it is difficult to show all the coordination links.
6.3 If the entire structure consists of ions, the positive ions are placed on the left and the negative ions on the right, the number of each being indicated as a subscript to the right. No spaces should be left between representations of ionic species within the formula of a coordination compound. If the charge of a coordination entity needs to be specified, it is placed outside the square bracket as a right superscript:
6.4 The rings follow the conventions for cyclic compounds. Where possible, the metal atom is placed in the centre of the group. Square brackets are placed round every coordination entity containing one or more rings, even if the charge is zero.
6.5 “Sandwich” structures are shown with the rings connected to the central atom by a line starting from inside the cycle and passing through one side.
6.6 Benzene rings and condensed benzene systems in “sandwich” compounds are drawn with alternating single and double bonds. Pentagonal and heptagonal rings are shown with a circle inside:
6.7 The stereochemistry of mononuclear complexes is expressed by means of special descriptors. The first of these is the “system indicator” formed from an abbreviation for the central atom geometry and the coordination number.
6.8 T-4: tetrahedral complexes. Described by the chirality symbol (R) or (S), they are shown in the same way as chiral carbon atoms, a broken line denoting a bond projecting behind the plane of the paper and a filled wedge one projecting in front of that plane:
6.9 SP-4: square planar complexes. The four coordination links are shown in the plane of the paper:
6.10 TBPY-5: trigonal bipyramidal complexes. The reference axis is shown in the plane of the paper; of the three other ligands, one is assumed also to be in the plane of the paper, one in front of it and the other behind it:
6.11 SPY-5: square pyramidal complexes. The reference axis with its lone coordinating atom is shown in the plane of the paper and four coordination links are assumed to be in a plane perpendicular to the reference axis, two in front and two behind the plane of the paper:
6.12 OC-6: octahedral complexes. Two coordination links are shown as the axis in the plane of the paper and four are assumed to be in a plane perpendicular to the reference axis, two in front and two behind the plane of the paper:
6.13 PBPY-7: pentagonal bipyramidal complexes. Two ligands are shown attached to the extremities of an axis in the plane of the paper; the five other coordination links are shown as their projection on to the plane perpendicular to this axis: