The structure of orthorhombic Ca-6.24(6)Mg-3.76(6)Sn-7 contains two types of chains built from edge- or face-sharing trigonal prisms of Ca and Mg atoms that are centered by Sn atoms. In one type, bonded zigzag (1)(infinity)[Sn-5(12-)] chains are built of interconnected square-planar (D-2h) Sn-5 units. Alternating chains of condensed trigonal prismatic cations centered by isolated Sn atoms are interbonded to the first type through Sn-Ca,Mg interactions. The chain unit is two electrons richer than that for a simple Zintl-Klemm formalism for 1-, 2-, and 4-bonded Sn (4b-Sn-0) because of the square-planar tin bonding. Band structure calculations within the extended Huckel formalism indicate the infinite tin chain alone is closed shell, but a metallic behavior (via open pi-bonding) is produced by cation mixing in the full structure. Local bonding within square-planar Sn-5 consists of normal s, p(x), p(y) (sigma) bonding, a single 4c-2e nonbonding MO on the outer tins, and a p(z)(2) lone (pi) pair. This new intermediate degree of hypervalency makes the unit isoelectronic with XeF42+. Property studies on powdered samples reveal a good metallic conduction but a small diamagnetism that is not uncharacteristic of heavy p-elements and their compounds.