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~~
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WALTERA. WOLF Eisenhower College
Seneca Falls. NY 13148
Rules for Coordination Number of Metal Ions R. Thomas Myers Kent State University Kent. OH 44242
The student in general chemistry does not meet with a very large number of coordination compounds, nor with a great variety of ligands. Rut there are a fair numher of cumpiexes encountered, especially in qualitative analysis. A few simple rules will suffice to rememher the coordination numher, and geometry, of the vast majority of complexes of metal cations encountered. 1) We first note that ions having octets in the outermost shell. or havine" a filled dln confieuratiun outermost such as Cu+, Ag+ and Au+ are spherical. There is no directional nreference for lieands. and no great preference for a certain number of ligan& ~ & oftenthannot e they have a coordination number of two, with the linands arranged linearly, as far apart as possible. When the concentrati& of ligands is higher. - then these cations can acquire three ligands, arranged in a trigonal plane, again as far apart as possible. By the same token Zn2+, Cd2+, and Hg2+ are also spherical. They most usually have a coordination numher of four. The ligands are arranged tetrahedrally. In lower concentrations of ligands these cations can have three ligands in a trigonal plane. 2) Sn2+ and Ph2+, with n s q n the outer shell, would he expected to he spherical. Instead they tend to join with three ligands, and the unshared pair of electrons occupies the tet-
rahedral fourth position. Therefore, ex., S n C K and PhClaare pyramidal. 3) We find that the metal cations which are not spherical generally are more consistent in their coordination number and geometry. Ni2+,Pd2+,Pt2+,Cu2+,and Ag2+usually have a coordination numher of four, with the ligands arranged in a square-planar arrangement. Of these cations, Ni2+ is must variable. It is square planar in Ni(CN)42-, but tetrahedral in NiCld2-. I t is sometimes octahedral, as in Ni(NHa),j2+ and Ni(en)s2+. 4) Having learned these rules for a relatively few cations, an all-encompassing rule applies to the remainder. All cations not in the rules ahove can he expected to have a coordination number of six, with the ligands arranged octahedrally. These rules are not intended for a course in coordination chemistry, hut apply only to the ions generally encountered in general chemistry including qualitative analysis.
Compact Corrections J o h n Henderson of Jackson Community College, Jackson Mich., has identified a minor, but potentially confusing error in the column of July, 1980 (p. 506). In the Compact "An 11lustration to Demonstrate the Smallness of Molecules" by D. W. Kingston, the relationship between cm%nd m3 was given as (1000)3cm3/1 m3. instead of the correct one. ( l O O ) ~ m i / l m". If the incorrect figure is used in the calculation, one comes out with the much less impressive result of 390 molecules of John Doe in the glass of water.
Volume 58
Number 9
S e ~ t e m b e r1981
681
