Editing Calcium (section)

===Chemical properties===
[[File:Ca(aq)6 improved image.tif|thumb|left|Structure of the polymeric [Ca(H{{sub|2}}O){{sub|6}}]{{sup|2+}} center in hydrated calcium chloride, illustrating the high coordination number typical for calcium complexes.]]

The chemistry of calcium is that of a typical heavy alkaline earth metal. For example, calcium spontaneously reacts with water more quickly than magnesium and less quickly than strontium to produce [[calcium hydroxide]] and hydrogen gas. It also reacts with the [[oxygen]] and [[nitrogen]] in air to form a mixture of [[calcium oxide]] and [[calcium nitride]].<ref name="CRC">C. R. Hammond ''The elements'' (pp. 4–35) in {{RubberBible86th}}</ref> When finely divided, it spontaneously burns in air to produce the nitride. Bulk calcium is less reactive: it quickly forms a hydration coating in moist air, but below 30% [[relative humidity]] it may be stored indefinitely at room temperature.{{sfn|Hluchan|Pomerantz|2005|p=483}}

Besides the simple oxide CaO, [[calcium peroxide]], CaO{{sub|2}}, can be made by direct oxidation of calcium metal under a high pressure of oxygen, and there is some evidence for a yellow [[superoxide]] Ca(O{{sub|2}}){{sub|2}}.{{sfn|Greenwood |Earnshaw|1997|p=119}}Calcium hydroxide, Ca(OH){{sub|2}}, is a strong base, though not as strong as the hydroxides of strontium, barium or the alkali metals.{{sfn|Greenwood |Earnshaw|1997|p=121}} All four dihalides of calcium are known.{{sfn|Greenwood|Earnshaw|1997|p = 117}} [[Calcium carbonate]] (CaCO{{sub|3}}) and [[calcium sulfate]] (CaSO{{sub|4}}) are particularly abundant minerals.{{sfn|Greenwood|Earnshaw|1997|pp = 122-15}} Like strontium and barium, as well as the alkali metals and the divalent [[lanthanide]]s [[europium]] and [[ytterbium]], calcium metal dissolves directly in liquid [[ammonia]] to give a dark blue solution.{{sfn|Greenwood|Earnshaw|1997|p=112}}

Due to the large size of the calcium ion (Ca{{sup|2+}}), high coordination numbers are common, up to 24 in some [[intermetallic compound]]s such as CaZn{{sub|13}}.{{sfn|Greenwood|Earnshaw|1997|p = 115}} Calcium is readily complexed by oxygen [[chelate]]s such as [[ethylenediaminetetraacetic acid|EDTA]] and [[polyphosphate]]s, which are useful in [[analytic chemistry]] and removing calcium ions from [[hard water]]. In the absence of [[steric hindrance]], smaller group 2 cations tend to form stronger complexes, but when large [[polydentate]] [[macrocycle]]s are involved the trend is reversed.{{sfn|Greenwood|Earnshaw|1997|pp = 122-15}}

Though calcium is in the same group as magnesium and [[organomagnesium compound]]s are very widely used throughout chemistry, organocalcium compounds are not similarly widespread because they are more difficult to make and more reactive, though they have recently been investigated as possible [[catalyst]]s.<ref>{{cite journal|last1=Harder|first1=S.|last2=Feil|first2=F.|last3=Knoll|first3=K.|year=2001|title=Novel Calcium Half-Sandwich Complexes for the Living and Stereoselective Polymerization of Styrene|journal=Angew. Chem. Int. Ed.|volume=40|issue=22|pages=4261–64|doi=10.1002/1521-3773(20011119)40:22<4261::AID-ANIE4261>3.0.CO;2-J|pmid=29712082}}</ref><ref>{{cite journal|last1=Crimmin|first1=Mark R.|last2=Casely|first2=Ian J.|last3=Hill|first3=Michael S.|title=Calcium-Mediated Intramolecular Hydroamination Catalysis|journal=[[Journal of the American Chemical Society]]|year=2005|volume=127|issue=7|pages=2042–43|doi=10.1021/ja043576n|pmid=15713071|bibcode=2005JAChS.127.2042C }}</ref><ref>{{cite journal|last1=Jenter|first1=Jelena|last2=Köppe|first2=Ralf|last3=Roesky|first3=Peter W.|title=2,5-Bis{''N''-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl Complexes of the Heavy Alkaline Earth Metals: Synthesis, Structures, and Hydroamination Catalysis|journal=Organometallics|year=2011|volume=30|issue=6|pages=1404–13|doi=10.1021/om100937c}}</ref><ref>{{cite journal|last1=Arrowsmith|first1=Merle|last2=Crimmin|first2=Mark R.|last3=Barrett|first3=Anthony G. M.|last4=Hill|first4=Michael S.|last5=Kociok-Köhn|first5=Gabriele|last6=Procopiou|first6=Panayiotis A.|title=Cation Charge Density and Precatalyst Selection in Group 2-Catalyzed Aminoalkene Hydroamination|journal=Organometallics|year=2011|volume=30|issue=6|pages=1493–1506|doi=10.1021/om101063m}}</ref><ref>{{cite journal|last1=Penafiel|first1=J.|last2=Maron|first2=L.|last3=Harder|first3=S.|year=2014|title=Early Main Group Metal Catalysis: How Important is the Metal?|journal=Angew. Chem. Int. Ed.|volume=54|issue=1|pages=201–06|doi=10.1002/anie.201408814|pmid=25376952|url=https://pure.rug.nl/ws/files/83571601/Early_Main_Group_Metal_Catalysis_How_Important_is_the_Metal.pdf }}</ref> Organocalcium compounds tend to be more similar to organoytterbium compounds due to the similar [[ionic radius|ionic radii]] of Yb{{sup|2+}} (102&nbsp;pm) and Ca{{sup|2+}} (100&nbsp;pm).{{sfn|Greenwood|Earnshaw|1997|pp = 136-37}}

Most of these compounds can only be prepared at low temperatures; bulky ligands tend to favour stability. For example, calcium di[[cyclopentadienyl]], Ca(C{{sub|5}}H{{sub|5}}){{sub|2}}, must be made by directly reacting calcium metal with [[mercurocene]] or [[cyclopentadiene]] itself; replacing the C{{sub|5}}H{{sub|5}} ligand with the bulkier C{{sub|5}}(CH{{sub|3}}){{sub|5}} ligand on the other hand increases the compound's solubility, volatility, and kinetic stability.{{sfn|Greenwood|Earnshaw|1997|pp = 122-15}}